Effect of oral infection of La Crosse virus on survival and fecundity of native Ochlerotatus triseriatus and invasive Stegomyia albopicta

Arboviruses can have benign, deleterious, or beneficial effects on the vector. We tested the hypothesis that oral infection with La Crosse virus (LACV) will have little to no effect on mosquito longevity and fecundity, a prediction of low virulence selected in a system with frequent vertical transmission. We tested the effects of infection in native Ochlerotatus triseriatus Say and invasive Stegomyia albopicta Skuse (Diptera: Culicidae). We artificially fed adult female mosquitoes of each species with either LACV-infected or uninfected bovine blood and determined adult longevity and fecundity. For females fed LACV-infected blood, bodies and legs, respectively, were separately homogenized and assayed by quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR) to determine the LACV infection and dissemination rates. Ochlerotatus triseriatus had a higher infection and dissemination rate than St. albopicta. For both species, female size had no effect on infection status. Infection status also had no effect on longevity or fecundity for both species. We suggest that the high frequency of vertical transmission may have selected for strains of the virus with low virulence in two vectors, in spite of their different evolutionary histories with the virus.

Effect of mixed infections of Sindbis and La Crosse viruses on replication of each virus in vitro

Mixed viral infections are ubiquitous in natural populations and may have significant but unpredictable biological and epidemiological consequences. These infections may be acquired simultaneously (coinfection) or result from two single infections at different time intervals (superinfection). This study investigated the effect of mixed infections of two mosquito-borne viruses, Sindbis (SINV) and La Crosse (LACV), on the replication of each virus in vertebrate baby hamster kidney (BHK) and invertebrate Aedes albopictus (C6/36) cell lines. SINV and LACV were inoculated onto confluent monolayers of each cell type either singly, simultaneously, or with a 2-h delay between single inoculations and titers for each virus were quantified by qRT-PCR. The titers for both viruses were significantly higher in BHK compared to C6/36 cells. Mixed infections suppressed replication of both viruses in BHK cells except for one superinfection treatment where inoculation of SINV ahead of LACV did not result in a reduction in SINV titer relative to single infection treatment. In C6/36 cells mixed infections had no effect on LACV replication, while coinfection enhanced SINV replication. These results demonstrate the potential for mixed viral infections to modify arbovirus transmission and pathogenesis.

The importance of oxidases in the tolerance of deciduous leaf infusions by Aedes (Stegomyia) aegypti and Aedes (Stegomyia) albopictus (Diptera: Culicidae)

Aedes (Stegomyia) aegypti (L.) and Aedes (Stegomyia) albopictus (Skuse) larvae rely on oxidases to reduce toxicity of water soluble toxins from some senescent tree leaf infusions. The mortality of third instar Ae. aegypti larvae in live oak and pin oak leaf infusions increased significantly in the presence of piperonyl butoxide (PBO), a broad inhibitor of cytochrome P450s (CYPs). In contrast, PBO treatment did not increase mortality in water controls or infusions of northern red oak or sugar maple leaf infusions for Ae. aegypti larvae. A similar pattern was observed for Ae. albopictus larvae, that is, an increase in mortality when CYPs were inhibited in live oak leaf infusions and no increase in sugar maple leaf infusions or water controls. However, the fresh live oak leaf infusion (5 d old) was the most toxic infusion to Ae. aegypti, but appeared less toxic to Ae. albopictus than the older infusions. A direct comparison of survival between the two Aedes species revealed Ae. aegypti exhibited a greater mortality than Ae. albopictus in PBO-treated live oak leaf infusions. These findings suggest that toxic components of some leaf litter in larval habitats may impose cryptic energy costs (detoxification).

Pesticide-induced release from competition among competing Aedes aegypti and Aedes albopictus (Diptera: Culicidae)

Competitive interactions between mosquitoes Aedes aegypti (L.) and Aedes albopictus (Skuse) may depend on environmental conditions. Pesticides may alleviate density-dependent competition for limited food, and a differential species response to sublethal concentrations may modify interspecific competition. We tested the hypothesis that exposure to malathion alters interspecific resource competition between these two species. In the absence of malathion, Ae. aegypti survivorship and per capita rate of population change were negatively affected by increasing densities of Ae. albopictus. However, the asymmetrical negative effect ofAe. albopictus on Ae. aegypti was eliminated in the presence of malathion. In addition, the presence of malathion resulted in shorter development time compared with the controls. The relative importance of pesticide-mediated coexistence in nature has not been evaluated, so its role in mediating coexistence is unclear; however, these findings underscore the potential of environmental concentrations of malathion, and perhaps other pesticides to facilitate coexistence between species.

Ecology and behavior of Anopheles arabiensis in relation to agricultural practices in central Kenya

Ecological changes associated with anthropogenic ecosystem disturbances can influence human risk of exposure to malaria and other vector-borne infectious diseases. This study in Mwea, Kenya, investigated the pattern of insecticide use in irrigated and nonirrigated agroecosystems and association with the density, survival, and blood-feeding behavior of the malaria vector Anopheles arabiensis. The parity rates of adult An. arabiensis from randomly selected houses were determined by examining their ovaries for tracheal distension, and polymerase chain reaction was used to identify the host blood meals. In addition, structured questionnaires were used to generate data on insecticide use. Anopheles arabiensis densities were highest in irrigated rice agroecosystems, intermediate in irrigated French beans agroecosystems, and lowest in the nonirrigated agroecosystem. Anopheles arabiensis adult survivorship was significantly lower in irrigated rice agroecosystems than in irrigated French beans agroecosystems. The human blood index (HBI) was significantly higher in the nonirrigated agroecosystem compared to irrigated agroecosystems. Moreover, there was marked variation in HBI among villages in irrigated agroecosystems with significantly lower HBI in Kangichiri and Mathangauta compared to Kiuria, Karima, and Kangai. The proportion of mosquitoes with mixed blood meals varied among villages ranging from 0.25 in Kangichiri to 0.83 in Kiuria. Sumithion, dimethoate, and alpha cypermethrin were the most commonly used insecticides. The 1st was used mostly in irrigated rice agroecosystems, and the last 2 were used mostly in irrigated French beans agroecosystems. These findings indicate that agricultural practices may influence the ecology and behavior of malaria vectors and ultimately the risk of malaria transmission.

Effect of leaf type and pesticide exposure on abundance of bacterial taxa in mosquito larval habitats

Lentic freshwater systems including those inhabited by aquatic stages of mosquitoes derive most of their carbon inputs from terrestrial organic matter mainly leaf litter. The leaf litter is colonized by microbial communities that provide the resource base for mosquito larvae. While the microbial biomass associated with different leaf species in container aquatic habitats is well documented, the taxonomic composition of these microbes and their response to common environmental stressors is poorly understood. We used indoor aquatic microcosms to determine the abundances of major taxonomic groups of bacteria in leaf litters from seven plant species and their responses to low concentrations of four pesticides with different modes of action on the target organisms; permethrin, malathion, atrazine and glyphosate. We tested the hypotheses that leaf species support different quantities of major taxonomic groups of bacteria and that exposure to pesticides at environmentally relevant concentrations alters bacterial abundance and community structure in mosquito larval habitats. We found support for both hypotheses suggesting that leaf litter identity and chemical contamination may alter the quality and quantity of mosquito food base (microbial communities) in larval habitats. The effect of pesticides on microbial communities varied significantly among leaf types, suggesting that the impact of pesticides on natural microbial communities may be highly complex and difficult to predict. Collectively, these findings demonstrate the potential for detritus composition within mosquito larval habitats and exposure to pesticides to influence the quality of mosquito larval habitats.

Larval rearing temperature influences the effect of malathion on Aedes aegypti life history traits and immune responses

The effects of anthropogenic chemical contaminants on aquatic organisms are largely influenced by underlying environmental conditions. This study evaluated how larval rearing temperature influences the impact of malathion on the fitness of the yellow fever mosquito Aedes aegypti. Larvae were exposed to water control, and low (0.03mg/L) or high (0.05mg/L) malathion dose at 20°C, 25°C and 30°C and emergence rate, time to emergence, female fecundity and expression of genes encoding two antimicrobial peptides (defensin, cecropin) and an iron-binding protein (transferrin) quantified. High malathion dose at 25°C and 30°C resulted in significantly lower emergence rates compared to control and low malathion dose but this effect was not observed at 20°C. Female time to emergence was inversely proportional to temperature and was significantly shorter in high malathion dose than in control and low malathion dose at 25°C and 30°C but not at 20°C. Regardless of temperature treatment, females from high malathion dose were significantly larger and laid more eggs than their counterparts in control and low malathion dose. Relative to the controls, two immune genes were significantly over-expressed in adult females from malathion-exposed treatments at 20°C (defensin and cecropin) and 25°C (defensin and transferrin) and one gene (defensin) was significantly under-expressed at 30°C. These findings suggest that larval rearing temperature can modify the effect of malathion on fitness traits in mosquitoes.

Effect of larval density and Sindbis virus infection on immune responses in Aedes aegypti

Stressful environmental conditions during mosquito larval development may enhance susceptibility of adult mosquitoes to viral pathogens. Although anti-viral defense system in mosquitoes remains uncertain, stress-related enhancement of mosquito susceptibility to viral pathogens may be due to alteration of signaling pathways such as the Toll and immune deficiency (IMD) pathways. To test the influence of larval density and Sindbis virus (SINV) infection on mosquito Toll/Imd pathways, 100 or 200 Aedes aegypti larvae were reared in 2L of live oak (Quercus virginiana) leaf infusion and the adults were fed on SINV-infected (treatments) or non-infected (controls) bovine blood. SINV infection status and expression of genes encoding three antimicrobial peptides (cecropin, defensin, diptericin), an iron-binding protein (transferrin), and four regulators of Toll/Imd pathways (caspar, cactus, Rel1A, Rel2) were quantified by RT-qPCR at 7 and 14days post blood meals. Irrespective of larval density, females incubated for 14days after an infectious blood meal had significantly higher SINV titers compared to females from low density treatments that were incubated for 7days. For both larval densities and time intervals, there was significant down-regulation of the Toll/Imd regulator genes in SINV-infected mosquitoes compared to controls. At day 7 post-infection, there was significant down-regulation of cecropin, defensin, diptericin and transferrin in SINV-infected mosquitoes at low larval density but this effect was only observed for diptericin at high larval density. These genes remained suppressed on day 14, except cecropin which was significantly up-regulated at both larval densities, and transferrin which was similar to controls at low larval density. We conclude that SINV infection suppresses Toll/Imd pathways, but high larval density enables SINV to attain maximum titers in Ae. aegypti much earlier compared to low density treatments despite the up-regulation of cecropin.

The role of Anopheles arabiensis and Anopheles coustani in indoor and outdoor malaria transmission in Taveta District, Kenya

BACKGROUND

The scaling up of malaria vector control efforts in Africa has resulted in changing the malaria vectorial systems across different ecological settings. In view of the ongoing trends in vector population dynamics, abundance, species composition and parasite infectiousness, there is a need to understand vector distribution and their contribution to malaria transmission to facilitate future planning of control strategies. We studied indoor and outdoor malaria transmission dynamics and vector population variability of Anopheles mosquitoes in Taveta district along the Kenyan Coast.

METHODS

Anopheles mosquitoes were collected indoors and outdoors in 4 ecologically different villages using CDC light traps (both indoor and outdoor) and aspiration method (day resting indoors; DRI) methods. Mosquitoes were examined for infection with P. falciparum sporozoites and blood feeding preferences using enzyme linked immunosorbent assay (ELISA). The An. gambiae and An. funestus complexes were identified by PCR technique to determine the sibling species composition.

RESULTS

A total of 4,004 Anopheles mosquitoes were collected consisting of 34.9%% (n = 1,397) An. gambiae s.1., 28.1% (n = 1,124) An. funestus s.l., 33.5% (n = 1,340) An. coustani and 3.6% (n = 143) An. pharoensis. A total of 14,654 culicine mosquitoes were collected, mainly Cx. quinquefasciatus. Of the total Anopheles collected, 3,729 were tested for P. falciparum sporozoite infection. The sporozoite transmission was found to be occurring both indoors and outdoors. The overall sporozoite infectivity was 0.68% (n = 2,486) indoors and 1.29% (n = 1,243) outdoors. Indoor and outdoor sporozoite infectivity and the vectorial systems varied across the 4 ecological villages. Entomological inoculation rates for the 4 villages indicate that there was site-to-site variation. In the 4 villages, Mwarusa had the highest EIRs with An. arabiensis, An. funestus and An. coustani contributing to 23.91, 11.96 and 23.91 infectious bites per person per year ib/p/year respectively. In Kiwalwa and Njoro outdoor EIR was significantly higher than indoors.

CONCLUSIONS

This study shows that malaria transmission is occurring both indoors and outdoors. The main vectors are An. arabiensis, An. funestus and An. coustani indoors while An. coustani is playing a major role in outdoor transmission. Effective malaria control programmes, should therefore include tools that target both indoor and outdoor transmission.

Shifts in malaria vector species composition and transmission dynamics along the Kenyan coast over the past 20 years

BACKGROUND

Over the past 20 years, numerous studies have investigated the ecology and behaviour of malaria vectors and Plasmodium falciparum malaria transmission on the coast of Kenya. Substantial progress has been made to control vector populations and reduce high malaria prevalence and severe disease. The goal of this paper was to examine trends over the past 20 years in Anopheles species composition, density, blood-feeding behaviour, and P. falciparum sporozoite transmission along the coast of Kenya.

METHODS

Using data collected from 1990 to 2010, vector density, species composition, blood-feeding patterns, and malaria transmission intensity was examined along the Kenyan coast. Mosquitoes were identified to species, based on morphological characteristics and DNA extracted from Anopheles gambiae for amplification. Using negative binomial generalized estimating equations, mosquito abundance over the period were modelled while adjusting for season. A multiple logistic regression model was used to analyse the sporozoite rates.

RESULTS

Results show that in some areas along the Kenyan coast, Anopheles arabiensis and Anopheles merus have replaced An. gambiae sensu stricto (s.s.) and Anopheles funestus as the major mosquito species. Further, there has been a shift from human to animal feeding for both An. gambiae sensu lato (s.l.) (99% to 16%) and An. funestus (100% to 3%), and P. falciparum sporozoite rates have significantly declined over the last 20 years, with the lowest sporozoite rates being observed in 2007 (0.19%) and 2008 (0.34%). There has been, on average, a significant reduction in the abundance of An. gambiae s.l. over the years (IRR = 0.94, 95% CI 0.90-0.98), with the density standing at low levels of an average 0.006 mosquitoes/house in the year 2010.

CONCLUSION

Reductions in the densities of the major malaria vectors and a shift from human to animal feeding have contributed to the decreased burden of malaria along the Kenyan coast. Vector species composition remains heterogeneous but in many areas An. arabiensis has replaced An. gambiae as the major malaria vector. This has important implications for malaria epidemiology and control given that this vector predominately rests and feeds on humans outdoors. Strategies for vector control need to continue focusing on tools for protecting residents inside houses but additionally employ outdoor control tools because these are essential for further reducing the levels of malaria transmission.

Effects of nutrition and density in Culex pipiens

Mosquito larvae face numerous biotic and abiotic challenges that affect their development and survivorship, as well as adult fitness. We conducted two experiments under semi-natural conditions to evaluate the effects of intraspecific competition, nutrient limitation and sub-lethal doses of malathion on individual life history traits in adult Culex pipiens (Diptera: Culicidae). In the first experiment, larvae of Cx. pipiens were reared at different intraspecific densities and exposed to sub-lethal doses of malathion. In the second experiment, different intraspecific densities of Cx. pipiens larvae were reared under conditions of low or high larval nutrients, and subsequent adults were fed on either water or 10% sucrose solution. Malathion treatment had relatively minor effects compared with density, which had significant negative effects on development rate, survivorship to adulthood, body size (wing length) and longevity. As larval density increased, a sex ratio distortion in survivorship to adulthood emerged, in which a bias towards males was apparent. Nutrient-rich larval environments alleviated, in part, the effects of increasing density and extended the lifespan of mosquitoes fed on water and 10% sucrose. Density-dependent alterations in adult longevity attributable to the larval environment are complex and show contrasting results depending on interactions with other environmental factors. This study suggests that larval resource availability and competition influence Cx. pipiens population growth correlates and have lasting effects on traits that relate to a mosquito's ability to vector pathogens.

Influence of leaf detritus type on production and longevity of container-breeding mosquitoes

Freshwater ecosystems are positioned at low levels in the landscape and receive large inputs of diverse plant-based detritus, a major source of energy for consumers in aquatic ecosystems. We conducted field experiments in Urbana, IL to determine the independent and combined effects of leaves of common tree species including the northern red oak (Quercus rubra L.), sugar maple (Acer saccharum Marshall), and common hackberry (Celtis occidentalis L.) on the performance of container-dwelling mosquitoes, especially Culex restuans Theobald (Diptera: Culicidae). We tested the hypothesis that leaf species have asymmetric effects on adult mosquito production and longevity. Hackberry followed by combined leaf treatments and maple produced the greatest number of pupae, whereas oak leaves produced the fewest. Leaf treatments had no significant effects on adult female sizes but female longevity was significantly lower in oak leaf treatments compared with the other leaf treatments. These findings support the hypothesis that leaf species identity influences the performance of container-dwelling mosquitoes with potential consequences for the transmission of infectious diseases.

Temperature and density-dependent effects of larval environment on Aedes aegypti competence for an alphavirus

Mosquito larvae experience multiple environmental stressors that may modify how subsequent adults interact with pathogens. We evaluated the effect of larval rearing temperature and intraspecific larval competition on adult mosquito immunity and vector competence for Sindbis virus (SINV). Aedes aegypti larvae were reared at two intraspecific densities (150 and 300 larvae) at 20° C and 30° C and the adults were fed artificially on citrated bovine blood containing 10(5) plaque forming units of SINV. Expression of cecropin, defensin, and transferrin was also evaluated in one- and five-day-old female adults. There was a direct relationship between larval density and SINV infection and dissemination rates at low temperature (20° C) and an inverse relationship between larval density and SINV infection rate at high temperature (30° C). Cecropin was only expressed in five-day-old adults that were raised at high temperature as larvae and was 20-fold over-expressed at low compared to high density treatments. Defensin and transferrin were under-expressed in one-day-old adults and over-expressed in five-day-old adults in all competition-temperature combinations relative to low density treatments at 20° C. These findings suggest that interaction between biotic and abiotic conditions of the larval environment may alter adult mosquito immunity resulting in enhanced vector competence for arboviruses.

Temperature-mediated differential expression of immune and stress-related genes in Aedes aegypti larvae

The mechanisms by which natural populations of vector mosquitoes cope with daily and seasonal fluctuations in temperature are poorly understood. We examined the effect of water temperature on expression of stress and immune-related genes in Aedes aegypti larvae. Aedes aegypti 3rd instars were exposed for 24 h to one of 7 constant temperatures (10 degrees C, 15 degrees C, 20 degrees C, 25 degrees C [control], 32 degrees C, 36 degrees C, or 40 degrees C) and expression of antimicrobial peptides (cecropin, defensin), transferrin, and heat shock proteins (HSP70 and HSP83) quantified by real-time reverse-transcriptase polymerase chain reaction. Cecropin, defensin, and transferrin were overexpressed at 36 degrees C and underexpressed at 15 degrees C and 32 degrees C. HSP83 was overexpressed at 10 degrees C and 40 degrees C and underexpressed at 20 degrees C, while HSP70 was underexpressed at 15 degrees C, 32 degrees C, and 36 degrees C. These findings suggest that antimicrobial peptides can serve as biomarkers of thermal stress and that HSP83 may buffer mosquito larvae against extreme temperatures.

Relationship between leaf litter identity, expression of cytochrome P450 genes and life history traits of Aedes aegypti and Aedes albopictus

The role of toxic component of leaf litter in mediating the outcome of mosquito species interactions is not well documented. To examine the effect of leaf litter toxins on mosquito performance and interspecific interactions, we reared monospecific and heterospecific cultures of Aedes aegypti L. and Aedes albopictus Skuse larvae in microcosms with one of five leaf species and measured the expression of five cytochrome P450 genes and life history traits of the two mosquito species. For both mosquito species, survival to adulthood was significantly higher in black alder, black walnut, and cypress infusion compared to sugar maple and eastern white pine infusion. In pine but not in other leaf treatments, the presence of A. albopictus had significant positive effects on A. aegypti wing length and development time to adulthood. A. albopictus from heterospecific cultures were larger than those from monospecific cultures and were smaller and took longer to develop in pine and sugar maple infusions than in the other infusions. Up regulation of CYP6Z6 and CYP9M9 in A. aegypti and A. albopictus respectively appeared to be closely associated with the deleterious effects of sugar maple infusion on mosquito performance as was the down regulation of CYP6N12 (in A. aegypti) and lack of induction of CYP6Z6 and CYP9M9 (in A. aegypti and A. albopictus respectively) in pine infusion. Results suggest that metabolic capabilities that enable the two species to tolerate natural xenobiotics are associated with a fitness cost.

Plasmodium falciparum transmission and aridity: a Kenyan experience from the dry lands of Baringo and its implications for Anopheles arabiensis control

Background

The ecology of malaria vectors particularly in semi-arid areas of Africa is poorly understood. Accurate knowledge on this subject will boost current efforts to reduce the burden of malaria in sub-Saharan Africa. The objective of this study was to describe the dynamics of malaria transmission in two model semi-arid sites (Kamarimar and Tirion) in Baringo in Kenya.

Methods

Adult mosquitoes were collected indoors by pyrethrum spray collections (PSC) and outdoors by Centers for Disease Control (CDC) light traps and identified to species by morphological characteristics. Sibling species of Anopheles gambiae complex were further characterized by rDNA. PCR and enzyme-linked immuno-sorbent assays (ELISA) were used to test for Plasmodium falciparum circumsporozoite proteins and host blood meal sources respectively.

Results

Anopheles arabiensis was not only the most dominant mosquito species in both study sites but also the only sibling species of An. gambiae s.l. present in the area. Other species identified in the study area were Anopheles funestus, Anopheles pharoensis and Anopheles coustani. For Kamarimar but not Tirion, the human blood index (HBI) for light trap samples was significantly higher than for PSC samples (Kamarimar, 0.63 and 0.11, Tirion, 0.48 and 0.43). The HBI for light trap samples was significantly higher in Kamarimar than in Tirion while that of PSC samples was significantly higher in Tirion than in Kamarimar. Entomological inoculation rates (EIR) were only detected for one month in Kamarimar and 3 months in Tirion. The number of houses in a homestead, number of people sleeping in the house, quality of the house, presence or absence of domestic animals, and distance to the animal shelter and the nearest larval habitat were significant predictors of An. arabiensis occurrence.

Conclusion

Malaria transmission in the study area is seasonal with An. arabiensis as the dominant vector. The fact this species feeds readily on humans and domestic animals suggest that zooprophylaxis may be a plausible malaria control strategy in semi-arid areas of Africa. The results also suggest that certain household characteristics may increase the risk of malaria transmission.

Larval environmental stress alters Aedes aegypti competence for Sindbis virus

OBJECTIVE

To evaluate how stress at the larval stage alters adult mosquito performance and susceptibility to viral infection.

METHODS

We used a model system consisting of Sindbis virus (SINV) and the yellow fever mosquito Aedes aegypti. Larvae were either reared under optimal conditions (control) or exposed to one of four types of stressors; suboptimal nutrients, starvation, elevated temperature, and a low dose of the insecticide malathion and adult females were fed SINV infectious blood meal. Differential expressions of stress, immune-specific and detoxification genes was measured in fourth instar larvae (HSP70, HSP83, cecropin, defensin, transferrin and CYP6Z6) and 3-day-old females (cecropin, defensin, transferrin) to identify plausible molecular mechanisms associated with mosquito response to stress.

RESULTS

There were stress-specific variations in mosquito performance (survival, development time, female size), but all stressors had a consistent effect of significantly increasing susceptibility to viral infection and dissemination relative to the controls. Three genes were up-regulated in fourth instar larvae exposed to temperature stress (cecropin, defensin and CYP6Z6) compared to single genes in suboptimal nutrient (cecropin) and malathion (transferrin) stress treatments and down-regulation of all the six genes in starvation treatments. In adult samples, transferrin was up-regulated in all but starvation treatments while defensin was up-regulated in starvation and temperature stress treatments.

CONCLUSIONS

Stress during larval development may cause alterations in adult mosquito phenotype and immunity that can increase their susceptibility to pathogens.

Larval environmental temperature and insecticide exposure alter Aedes aegypti competence for arboviruses

Temperature is a key factor influencing mosquito growth and development and is also known to affect insecticide efficacy. We evaluated the effects of larval rearing temperature and exposure to insecticides on adult mosquito fitness and competence for arboviral infection using Sindbis virus (SINV). We exposed newly hatched larvae of Aedes aegypti to an environmentally realistic level of insecticide malathion at 20°C and 30°C and allowed the resulting adults to feed on SINV-infected blood meal. Exposure to malathion significantly reduced survival to adulthood. Statistically significant interactions between temperature and malathion were observed for body size, estimated population growth, and SINV infection and dissemination. Malathion-exposed Ae. aegypti cohorts had significantly higher population growth at 20°C than at 30°C. Body size decreased with higher temperature and malathion-exposed females were larger than unexposed females at 20°C but not at 30°C. Viral infection and dissemination increased with larval rearing temperature and were higher in malathion-exposed than unexposed females at 30°C but not at 20°C. These results show that environmental factors, including those factors used in controlling mosquitoes, experienced by immature stages have latent effects that continue to adulthood and alter vector competence to arboviruses.

Can pesticides and larval competition alter susceptibility of Aedes mosquitoes (Diptera: Culicidae) to arbovirus infection?

Density-dependent processes such as larval competition may be important regulatory factors among some mosquito species. The application of pesticides used for control may alter these density-dependent interactions with consequences for the number of survivors and associated sublethal and chronic effects on these individuals. We examined how intraspecific competition among larvae and low concentrations of malathion alter Aedes aegypti L. and Aedes albopictus Skuse adult life history traits and competence for arboviruses using Sindbis virus as a model system. Larvae were reared at densities of 150 and 300 larvae per container and in the absence or presence of 0.04 parts per million of malathion, before surviving females were exposed to an infectious blood meal containing 10(5) plaque-forming units/ml Sindbis virus. For both species, competition and the presence of malathion reduced survival to adulthood. The presence of malathion eliminated the negative effects of competition that resulted in lengthened development time and smaller-sized adults. For Ae. aegypti, but not Ae. albopictus, high competition conditions and the presence of malathion independently and not interactively led to an increase in virus dissemination from the midgut. Our results suggest that larval competition and chemical contaminants may influence disease transmission directly by altering adult mosquito fitness and indirectly by altering vector interactions with arboviruses.

Effect of temperature and insecticide stress on life-history traits of Culex restuans and Aedes albopictus (Diptera: Culicidae)

The chronic effects of exposure of Culex restuans (Theobald) and Aedes albopictus (Skuse) (Diptera: Culicidae) to low concentrations of malathion were examined by exposing larvae of the two species to four malathion doses at 20, 25, and 30 degrees C and maintaining the resulting adults at 25 degrees C. For both species, a significant temperature by malathion interaction on survival was found. Greater temperatures at the highest malathion dosage had significantly lower survivorship than in other treatments, but this effect was not observed at 20 degrees C. These results suggest that low temperature may buffer against some of the negative effects of malathion. For both species, temperature but not malathion had significant effects on female developmental time to adulthood and adult longevity. Temperature also affected adult female size for Ae. albopictus but not Cx. restuans. Ae. albopictus females developed faster as temperature increased, lived longer when larvae were maintained at 30 degrees C than at 20 degrees C, and were larger when larvae were maintained at 25 degrees C than at 20 or 30 degrees C. Cx. restuans females developed faster at 25 and 30 degrees C than at 20 degrees C and lived longer at 25 degrees C than at 20 or 30 degrees C. The estimated finite rate of increase (lambda') for Cx. restuans was significantly lower at 20 degrees C than at 25 and 30 degrees C, whereas that of Ae. albopictus was significantly influenced by an interaction between temperature and malathion with significantly lower lambda' at 20 degrees C than at 25 and 30 degrees C for all malathion treatments except 0.014 mg liter(-1). Understanding how pesticides interact with abiotic environmental conditions will contribute to management decisions about vector control practices.

Dry season ecology of Anopheles gambiae complex mosquitoes at larval habitats in two traditionally semi-arid villages in Baringo, Kenya

BACKGROUND

Pre-adult stages of malaria vectors in semi-arid areas are confronted with highly variable and challenging climatic conditions. The objective of this study was to determine which larval habitat types are most productive in terms of larval densities in the dry and wet seasons within semi-arid environments, and how vector species productivity is partitioned over time.

METHODS

Larval habitats were mapped and larvae sampled longitudinally using standard dipping techniques. Larvae were identified to species level morphologically using taxonomic keys and to sub-species by polymerase chain reaction (PCR) methods. Physical characteristics of larval habitats, including water depth, turbidity, and presence of floating and emergent vegetation were recorded. Water depth was measured using a metal ruler. Turbidity, pH, conductivity, dissolved oxygen, temperatures salinity and total dissolved solids (TDS) were measured in the field using the hand-held water chemistry meters.

RESULTS

Mean larval densities were higher in the dry season than during the wet season but the differences in density were not statistically significant (F = 0.04, df = 1, p = 0.8501). Significantly higher densities of larvae were collected in habitats that were shaded and holding turbid, temporary and still water. Presence of emergent or floating vegetation, habitat depth, habitat size and habitat distance to the nearest house did not significantly affect larval density in both villages. There was a weakly positive relationship between larval density and salinity (r = 0.19, p < 0.05), conductivity (r = 0.05, p = 0.45) and total dissolved solids (r = 0.17, p < 0.05). However, the relationship between water temperature and larval density was weakly negative (r = 0.15, p = 0.35). All statistical tests were significant at alpha = 0.05.

CONCLUSION

Breeding of malaria vector mosquitoes in Baringo is driven by predominantly human-made and permanent breeding sites in which Anopheles arabiensis and Anopheles funestus breed at a low level throughout the year. Permanent water sources available during the dry season serve as inocula by providing "larval seed" to freshly formed rain-fed habitats during the rainy season. The highly localized and focal nature of breeding sites in these semi-desert environments provides a good opportunity for targeted larval control since the habitats are few, well-defined and easily traceable.

The effects of resource type and ratio on competition with Aedes albopictus and Culex pipiens (Diptera:Culicidae)

The introduction of Aedes albopictus (Skuse) in the United States has been associated with declines in abundance of resident mosquito species, presumably because of resource competition, as larvae of Ae. albopictus have been illustrated as superior competitors under certain resource conditions. We evaluated the hypothesis that varying the type and ratio of two food resources (Foxtail grass: American elm) alters the competitive outcome of Ae. albopictus and Culex pipiens (L.). We measured survivorship, development time, size, and adult longevity, and estimated the population growth index (A') of populations raised both alone and in equal number with the interspecific competitor, across five ratios of the two food resources. Competition was asymmetric with Ae. albopictus, the superior competitor across all resource treatments; however, the competitive advantage Ae. albopictus had over Cx. pipiens was reduced as grass became the predominant resource. With elm as the predominant resource, the population growth index (A') for both Ae. albopictus and Cx. pipiens was lower in intraspecific and interspecific competition treatments, respectively. The treatments also impacted adult life history, as life spans of both Ae. albopictus and Cx. pipiens varied when they emerged from larval conditions with different resource and competition treatments. We discuss the possible differences in the two species' efficiencies in exploiting the two resource types. Despite some resource conditions alleviating the competitive effects of Ae. albopictus on Cx. pipiens, competition remained asymmetric; thus, additional mechanisms are likely operating under field conditions when the two species coexist.

Interaction of a pesticide and larval competition on life history traits of Culex pipiens

Mosquito larval development occurs in aquatic habitats that are directly or indirectly exposed to chemical contaminants. Little is known about how interaction of these chemicals with other biotic and abiotic stressors impact mosquito populations. We used two levels of nutrient (low and high) and four larval densities (10, 20, 30, 40) to examine the effects of low concentrations of insecticide malathion, on Culex pipiens L. mosquitoes experiencing stress from larval competition. Addition of malathion at the high nutrient condition enhanced survival with increasing larval densities, but this effect was not observed at low nutrient condition. Males exposed to malathion were significantly larger than those from control treatments while the effect of malathion on size of females varied with larval density and the level of nutrients. Larval exposure to malathion and low nutrient resulted in significantly larger females with increases in larval densities compared with other treatments. The effect of malathion on male longevity varied with larval density and amount of nutrients. At higher densities, male longevity was consistently higher in low nutrient than in high nutrient conditions and addition of malathion in high nutrient treatment increased male longevity at the highest density. These effects are most likely attributable to release from competition among survivors after mortality from malathion and density-dependent effects. We conclude that biotic conditions of the larval habitat can alter the impact of low concentrations of chemical contaminants on mosquito populations in ways that may influence the pattern of disease transmission and the outcome of vector control efforts.

Anopheles larval abundance and diversity in three rice agro-village complexes Mwea irrigation scheme, central Kenya

BACKGROUND

The diversity and abundance of Anopheles larvae has significant influence on the resulting adult mosquito population and hence the dynamics of malaria transmission. Studies were conducted to examine larval habitat dynamics and ecological factors affecting survivorship of aquatic stages of malaria vectors in three agro-ecological settings in Mwea, Kenya.

METHODS

Three villages were selected based on rice husbandry and water management practices. Aquatic habitats in the 3 villages representing planned rice cultivation (Mbui Njeru), unplanned rice cultivation (Kiamachiri) and non-irrigated (Murinduko) agro-ecosystems were sampled every 2 weeks to generate stage-specific estimates of mosquito larval densities, relative abundance and diversity. Records of distance to the nearest homestead, vegetation coverage, surface debris, turbidity, habitat stability, habitat type, rice growth stage, number of rice tillers and percent Azolla cover were taken for each habitat.

RESULTS

Captures of early, late instars and pupae accounted for 78.2%, 10.9% and 10.8% of the total Anopheles immatures sampled (n = 29,252), respectively. There were significant differences in larval abundance between 3 agro-ecosystems. The village with 'planned' rice cultivation had relatively lower Anopheles larval densities compared to the villages where 'unplanned' or non-irrigated. Similarly, species composition and richness was higher in the two villages with either 'unplanned' or limited rice cultivation, an indication of the importance of land use patterns on diversity of larval habitat types. Rice fields and associated canals were the most productive habitat types while water pools and puddles were important for short periods during the rainy season. Multiple logistic regression analysis showed that presence of other invertebrates, percentage Azolla cover, distance to nearest homestead, depth and water turbidity were the best predictors for Anopheles mosquito larval abundance.

CONCLUSION

These results suggest that agricultural practices have significant influence on mosquito species diversity and abundance and that certain habitat characteristics favor production of malaria vectors. These factors should be considered when implementing larval control strategies which should be targeted based on habitat productivity and water management.

Population structure of Anopheles gambiae along the Kenyan coast

In the tropics, Anopheles mosquito abundance is greatest during the wet season and decline significantly during the dry season as larval habitats shrink. Population size fluctuations between wet and dry seasons may lead to variation in distribution of specific alleles within natural Anopheles populations, and a possible effect on the population genetic structure. We used 11 microsatellite markers to examine the effect of seasonality on population genetic structure of Anopheles gambiae s.s. at two sites along the Kenyan coast. All loci were highly polymorphic with the total number of alleles for pooled samples ranging from 7 (locus ND36) to 21 (locus AG2H46). Significant estimates of genetic differentiation between sites and seasons were observed suggesting the existence of spatio-temporal subpopulation structuring. Genetic bottleneck analysis showed no indication of excess heterozygosity in any of the populations. These findings suggest that along the Kenyan coast, seasonality and site specific ecological factors can alter the genetic structure of A. gambiae s.s. populations.

Interspecies predation between Anopheles gambiae s.s. and Culex quinquefasciatus larvae

Interaction of aquatic stages of coexisting mosquito species may have significant influence on resulting adult mosquito populations. We used two coexisting species, Anopheles gambiae s.s. and Culex quinquefasciatus to investigate whether third instars of one species consumed first instars of the other. First instars of one species were readily consumed by a third instar of the other species irrespective food quantity. DNA of Cx. quinquefasciatus was detected in the eight An. gambiae s.s. third instars presumed to have consumed at least one Cx. quinquefasciatus first instar. Likewise, DNA of An. gambiae s.s. was detected in five of eight Cx. quinquefasciatus third instars presumed to have consumed at least one An. gambiae s.s. first instar. A small number of dead first instars was found in the controls indicating that some larvae in the treatment group may have been consumed after they had died. These findings suggest that intraguild predation between the two species may be common in nature and that it is a facultative process that is not induced by food shortage. The findings further suggest that polymerase chain reaction could be a useful technique in the study of this phenomenon in natural habitats.

Population genetic structure of Anopheles arabiensis (Diptera: Culicidae) in a rice growing area of central Kenya

Studies were conducted to examine the population genetic structure of Anopheles arabiensis (Patton) in Mwea Rice Irrigation Scheme and surrounding areas in Central Kenya, under different agricultural systems. This study was motivated by observed differences in malaria transmission indices of An. arabiensis within the scheme compared with adjacent nonirrigated areas. Agricultural practices can modify local microclimate and influence the number and diversity of larval habitats and in so doing may occasion subpopulation differentiation. Thirty samples from each of the three study sites were genotyped at eight microsatellite loci. Seven microsatellite loci showed high polymorphism but revealed no genetic differentiation (FST = 0.006, P = 0.312) and high gene flow (Nm = 29-101) among the three populations. Genetic bottleneck analysis showed no indication of excess heterozygosity in any of the populations. There was high frequency of rare alleles, suggesting that An. arabiensis in the study area has a high potential of responding to selective pressures from environmental changes and vector control efforts. These findings imply that An. arabiensis in the study area occurs as a single, continuous panmictic population with great ability to adapt to human-imposed selective pressures.

A heteroskedastic error covariance matrix estimator using a first-order conditional autoregressive Markov simulation for deriving asympotical efficient estimates from ecological sampled Anopheles arabiensis aquatic habitat covariates

Background

Autoregressive regression coefficients for Anopheles arabiensis aquatic habitat models are usually assessed using global error techniques and are reported as error covariance matrices. A global statistic, however, will summarize error estimates from multiple habitat locations. This makes it difficult to identify where there are clusters of An. arabiensis aquatic habitats of acceptable prediction. It is therefore useful to conduct some form of spatial error analysis to detect clusters of An. arabiensis aquatic habitats based on uncertainty residuals from individual sampled habitats. In this research, a method of error estimation for spatial simulation models was demonstrated using autocorrelation indices and eigenfunction spatial filters to distinguish among the effects of parameter uncertainty on a stochastic simulation of ecological sampled Anopheles aquatic habitat covariates. A test for diagnostic checking error residuals in an An. arabiensis aquatic habitat model may enable intervention efforts targeting productive habitats clusters, based on larval/pupal productivity, by using the asymptotic distribution of parameter estimates from a residual autocovariance matrix. The models considered in this research extends a normal regression analysis previously considered in the literature.

Methods

Field and remote-sampled data were collected during July 2006 to December 2007 in Karima rice-village complex in Mwea, Kenya. SAS 9.1.4^® was used to explore univariate statistics, correlations, distributions, and to generate global autocorrelation statistics from the ecological sampled datasets. A local autocorrelation index was also generated using spatial covariance parameters (i.e., Moran's Indices) in a SAS/GIS^® database. The Moran's statistic was decomposed into orthogonal and uncorrelated synthetic map pattern components using a Poisson model with a gamma-distributed mean (i.e. negative binomial regression). The eigenfunction values from the spatial configuration matrices were then used to define expectations for prior distributions using a Markov chain Monte Carlo (MCMC) algorithm. A set of posterior means were defined in WinBUGS 1.4.3^®. After the model had converged, samples from the conditional distributions were used to summarize the posterior distribution of the parameters. Thereafter, a spatial residual trend analyses was used to evaluate variance uncertainty propagation in the model using an autocovariance error matrix.

Results

By specifying coefficient estimates in a Bayesian framework, the covariate number of tillers was found to be a significant predictor, positively associated with An. arabiensis aquatic habitats. The spatial filter models accounted for approximately 19% redundant locational information in the ecological sampled An. arabiensis aquatic habitat data. In the residual error estimation model there was significant positive autocorrelation (i.e., clustering of habitats in geographic space) based on log-transformed larval/pupal data and the sampled covariate depth of habitat.

Conclusion

An autocorrelation error covariance matrix and a spatial filter analyses can prioritize mosquito control strategies by providing a computationally attractive and feasible description of variance uncertainty estimates for correctly identifying clusters of prolific An. arabiensis aquatic habitats based on larval/pupal productivity.

Spatial distribution, blood feeding pattern, and role of Anopheles funestus complex in malaria transmission in central Kenya

Studies were conducted to determine the role of sibling species of Anopheles funestus complex in malaria transmission in three agro-ecosystems in central Kenya. Mosquitoes were sampled indoors and outdoors, and rDNA PCR was successfully used to identify 340 specimens. Anopheles parensis (91.8%), A. funestus (6.8%), and Anopheles leesoni (1.5%) were the three sibling species identified. A. parensis was the dominant species at all study sites, while 22 of 23 A. funestus were collected in the non-irrigated study site. None of the 362 specimens tested was positive for Plasmodium falciparum circumsporozoite proteins by enzyme-linked immunosorbent assay. The most common blood-meal sources (mixed blood meals included) for A. parensis were goat (54.0%), human (47.6%), and bovine (39.7%), while the few A. funestus s.s. samples had fed mostly on humans. The human blood index (HBI) for A. parensis (mixed blood meals included) in the non-irrigated agro-ecosystem was 0.93 and significantly higher than 0.33 in planned rice agro-ecosystem. The few samples of A. funestus s.s. and A. funestus s.l. also showed a trend of higher HBI in the non-irrigated agro-ecosystem. We conclude that agricultural practices have significant influence on distribution and blood feeding behavior of A. funestus complex. Although none of the species was implicated with malaria transmission, these results may partly explain why non-irrigated agro-ecosystems are associated with higher risk of malaria transmission by this species compared to irrigated agro-ecosystems.

Seasonal mosquito larval abundance and composition in Kibwezi, lower eastern Kenya

BACKGROUND & OBJECTIVES

Changes in weather patterns especially rainfall affects the distribution and densities of mosquitoes. The objective of this study was to describe mosquito aquatic habitats, to determine larval abundance, species composition, and habitat types found in Kasayani village of Kibwezi division.

METHODS

A cross-sectional survey of mosquito larval habitats was conducted in Kasayani village in Kibwezi division to determine species composition, larval abundance, and habitat types found in this village. This survey was conducted during the rainy season in November and December 2006 and during the dry season in February and March 2007. Larvae were collected using the standard dipping technique and a total of 24 habitats were sampled. The primary habitats identified were water reservoir tanks, puddles, temporary pools, and tyre tracks.

RESULTS

A total of 2660 mosquito larvae were collected of which 2140 (80.45%) were culicines, 503 (18.91%) were Anopheles and 17 (0.64%) were pupae. For culicines, 1787 (83.5%) were categorized as early instars and 353 (16.5%) were as late instars while in the Anopheles, 425 (84.49%) were classified as early instars and 78 (15.51%) were late instars. Morphological identification of the III and IV instar larvae by use of microscopy yielded 16.24% (n = 70) Anopheles gambiae complex, 1.16% (n=5) An. funestus, 0.70% (n=3) An. coustani, 42.46% (n=183) Culex quinquefasciatus, 6.26% (n = 27) Cx. duttoni, and 33.18% (n=143) Ae. aegypti. Puddles, tyre tracks and pools had highly turbid water while water reservoir tanks had clear water. Anopheles gambiae and Cx. quinquefasciatus were found in all habitat categories while Ae. aegypti were found only in water storage tanks.

INTERPRETATION & CONCLUSION

The mosquito larval densities indicate that the inhabitants of this village are at risk of mosquito-borne diseases including malaria, which is one of the greatest causes of morbidity and mortality in this area. Furthermore, mosquito control measures targeting both the mosquito immatures and adults should be enhanced especially during the rainy season to ensure maximum protection of the inhabitants.

Spatiotemporal dynamics of immature culicines (subfamily Culicinae) and their larval habitats in Mwea Rice Scheme, Kenya

An ecological study was conducted at three study sites in Mwea Rice Scheme, Kenya to identify the diverse aquatic habitats in which culicine mosquitoes thrived and to explore the best strategies for mosquito control in the area. During the 11-month study period, ten habitat categories and 11 culicine species mainly dominated by Culex quinquefasciatus (72.0%) and Culex annulioris (17.9%) were identified from pupae and late instars larval samples. Two of the 11 culicine species, Ficalbia (Mimomyia) plumosa and Uranotaenia spp., have not been reported previously in the study area. Rurumi had more habitat types than either of the other study sites but the least number of mosquito species. In contrast, Karima had the least number of habitat types but significantly higher density of early instars than the other study sites. The relative abundance of late instars and pupae did not vary significantly among study sites. The contribution of different habitat types to larval production varied markedly between seasons and among study sites. Paddies and canals were perennial contributors of culicine mosquito larvae while the other habitat types were important mainly during the wet season. Some habitat types such as ditches, seeps, marshes, and fishpond were absent in some study sites but of great significance in other study sites. C. quinquefasciatus was positively associated with turbidity at all study sites and also negatively associated with emergent vegetation and distance to the nearest homestead in Karima, emergent vegetation in Kiuria, and other aquatic invertebrates in Rurumi. C. annulioris was positively associated with emergent vegetation at all study sites and also with depth in Kiuria. These findings indicate that besides rice fields and associated habitats, a diversity of other aquatic habitats contribute to culicine adult mosquito production in the study area and that environmental factors that determine the occurrence of a particular mosquito species may vary significantly even among areas of similar land use. This information is critical when designing and implementing mosquito larval control programs.

Diversity of riceland mosquitoes and factors affecting their occurrence and distribution in Mwea, Kenya

Knowledge of mosquito species diversity, occurrence, and distribution is an essential component of vector ecology and a guiding principle to formulation and implementation of integrated vector management programs. A 12-month entomological survey was conducted to determine the diversity of riceland mosquitoes and factors affecting their occurrence and distribution at 3 sites targeted for malaria vector control in Mwea, Kenya. Adult mosquitoes were sampled indoors by pyrethrum spray catch and outdoors by the Centers for Disease Control and Prevention light traps. Mosquitoes were then morphologically identified to species using taxonomic keys. The characteristics of houses sampled for indoor resting mosquitoes, including number of people sleeping in each house the night preceding collection, presence of bed nets, location of the house, size of eaves, wall type, presence of cattle and distance of the house to the cowshed, and proximity to larval habitats, were recorded. Of the 191,378 mosquitoes collected, 95% were identified morphologically to species and comprised 25 species from 5 genera. Common species included Anopheles arabiensis (53.5%), Culex quinquefasciatus (35.5%), An. pharoensis (4.7%), An. coustani (2.5%), and An. funestus (1.6%). Shannon's species diversity and evenness indices did not differ significantly among the 3 study sites. There was a marked house-to-house variation in the average number of mosquitoes captured. The number of people sleeping in the house the night preceding collection, size of eaves, distance to the cowshed, and the nearest larval habitat were significant predictors of occurrence of either or both An. arabiensis and Cx. quinquefasciatus. The peak abundance of An. arabiensis coincided with land preparation and the first few weeks after transplanting of rice seedlings, and that of Cx. quinquefasciatus coincided with land preparation, late stage of rice development, and short rains. After transplanting of rice seedlings, the populations of Cx. quinquefasciatus were collected more outdoors than indoors, suggesting a shift from endophily to exophily. These results demonstrate that irrigated rice cultivation has a strong impact on mosquito species occurrence, distribution, abundance, and behavior, and that certain house characteristics increase the degree of human-vector contact.

Environmental factors associated with the distribution of Anopheles arabiensis and Culex quinquefasciatus in a rice agro-ecosystem in Mwea, Kenya

Studies were conducted between May and June, 2006 to investigate the environmental factors affecting the distribution of An. arabiensis Patton and Culex quinquefasciatus Say in Mwea, Kenya. The sampling unit comprised all non-paddy aquatic habitats and ten randomly selected paddies and canals located within a 200 m radius from the periphery of the study site. Thirteen physico-chemical variables were recorded for each sampling site in each sampling occasion and a sample of mosquito larvae and other aquatic invertebrates collected. The non-paddy aquatic habitats identified included pools and marshes. Morphological identification of 1,974 mosquito larvae yielded four species dominated by Cx. quinquefasciatus (73.2%) and An. arabiensis (25.0%). Pools were associated with significantly higher Cx. quinquefasciatus larval abundance and less diversity of other aquatic invertebrates compared with other habitat types. In contrast, the abundance of An. arabiensis did not differ significantly among habitat types. Culex quinquefasciatus habitats had higher water conductivity and exhibited a higher abundance of other aquatic invertebrates than An. arabiensis habitats. Chi-square analysis indicated that the two species were more likely to coexist in the same habitats than would be expected by chance alone. Anopheles arabiensis larvae were positively associated with dissolved oxygen and adults of family Haliplidae and negatively associated with emergent vegetation and Heptageniidae larvae. Culex quinquefasciatus larvae were positively associated with dissolved oxygen, total dissolved solids, Chironomidae larvae, and Microvelidae adults and negatively associated with emergent vegetation. These findings suggest that both biotic and abiotic factors play a significant role in niche partitioning among Cx. quinquefasciatus and An. arabiensis, a factor that should be considered when designing an integrated vector control program.

Contribution of different aquatic habitats to adult Anopheles arabiensis and Culex quinquefasciatus (Diptera: Culicidae) production in a rice agroecosystem in Mwea, Kenya

Studies were conducted to determine the contribution of diverse larval habitats to adult Anopheles arabiensis Patton and Culex quinquefasciatus Say production in a rice land agro-ecosystem in Mwea, Kenya. Two sizes of cages were placed in different habitat types to investigate the influence of non-mosquito invertebrates on larval mortalities and the contribution of each habitat type to mosquito productivities, respectively. These emergence traps had fine netting material covers to prevent adult mosquitoes from ovipositing in the area covered by the trap and immature mosquitoes from entering the cages. The emergence of Anopheles arabiensis in seeps, tire tracks, temporary pools, and paddies was 10.53%, 17.31%, 12.50%, and 2.14%, respectively, while the corresponding values for Cx. quinquefasciatus were 16.85% in tire tracks, 8.39% in temporary pools, and 5.65% in the paddies from 0.125 m3 cages during the study. Cages measuring 1 m3 were placed in different habitat types which included paddy, swamp, marsh, ditch, pool, and seep to determine larval habitat productivity. An. arabiensis was the predominant anopheline species (98.0%, n = 232), although a few Anopheles coustani Laveran (2.0%, n = 5) emerged from the habitats. The productivity for An. arabiensis larvae was 6.0 mosquitoes per m2 for the temporary pools, 5.5 for paddy, 5.4 for marsh, 2.7 for ditch, and 0.6 for seep. The Cx. quinquefasciatus larval habitat productivity was 47.8 mosquitoes per m2 for paddies, 35.7 for ditches, 11.1 for marshes, 4.2 for seeps, 2.4 for swamps, and 1.0 for temporary pools. Pools, paddy, and marsh habitat types were the most productive larval habitats for An. arabiensis while paddy, ditch, and marsh were the most productive larval habitats for Cx. quinquefasciatus. The most common non-mosquito invertebrate composition in the cages included Dytiscidae, Notonectidae, Belostomatidae, and Ephemerellidae, and their presence negatively affected the number of emergent mosquitoes from the cages. In conclusion, freshly formed habitats are the most productive aquatic habitats, while old and more permanent habitats are the least productive due to natural regulation of mosquito immatures.

Hydrological modeling of geophysical parameters of arboviral and protozoan disease vectors in Internally Displaced People camps in Gulu, Uganda

Background

The aim of this study was to determine if remotely sensed data and Digital Elevation Model (DEM) can test relationships between Culex quinquefasciatus and Anopheles gambiae s.l. larval habitats and environmental parameters within Internally Displaced People (IDP) campgrounds in Gulu, Uganda. A total of 65 georeferenced aquatic habitats in various IDP camps were studied to compare the larval abundance of Cx. quinquefasciatus and An. gambiae s.l. The aquatic habitat dataset were overlaid onto Land Use Land Cover (LULC) maps retrieved from Landsat imagery with 150 m × 150 m grid cells stratified by levels of drainage. The LULC change was estimated over a period of 14 years. Poisson regression analyses and Moran's I statistics were used to model relationships between larval abundance and environmental predictors. Individual larval habitat data were further evaluated in terms of their covariations with spatial autocorrelation by regressing them on candidate spatial filter eigenvectors. Multispectral QuickBird imagery classification and DEM-based GIS methods were generated to evaluate stream flow direction and accumulation for identification of immature Cx. quinquefasciatus and An. gambiae s.l. and abundance.

Results

The main LULC change in urban Gulu IDP camps was non-urban to urban, which included about 71.5 % of the land cover. The regression models indicate that counts of An. gambiae s.l. larvae were associated with shade while Cx. quinquefasciatus were associated with floating vegetation. Moran's I and the General G statistics for mosquito density by species and instars, identified significant clusters of high densities of Anopheles; larvae, however, Culex are not consistently clustered. A stepwise negative binomial regression decomposed the immature An. gambiae s.l. data into empirical orthogonal bases. The data suggest the presence of roughly 11% to 28 % redundant information in the larval count samples. The DEM suggest a positive correlation for Culex (0.24) while for Anopheles there was a negative correlation (-0.23) for a local model distance to stream.

Conclusion

These data demonstrate that optical remote sensing; geostatistics and DEMs can be used to identify parameters associated with Culex and Anopheles aquatic habitats.

Distribution of mosquito larvae within the paddy and its implication in larvicidal application in Mwea rice irrigation scheme, Central Kenya

Distribution of mosquito larvae in inundated rice fields is poorly known despite its profound implications in implementation of vector control programs. Based on oviposition behavior of gravid females and biotic and abiotic conditions of the rice field, distribution of mosquito larvae within the paddy may vary greatly. As a guide to implementation of mosquito vector control program targeting the aquatic stages in the rice fields in Mwea, studies were conducted to determine the distribution of mosquito larvae within the paddy. Twenty-eight cages measuring 50 cm3 were distributed randomly within the paddy during the transplanting stage of the rice growth cycle, and were examined twice per week up to the flowering stage to determine mosquito oviposition pattern. A total of 17,218 mosquito larvae were collected at the periphery and a further 17,570 at the center of the paddy. These comprised 7,461 larvae from the genus Anopheles and 27,327 from genus Culex. The number of pupae collected at the periphery was 1,004 and 1.5 times greater than the number collected at the center. Significantly higher counts of Anopheles larvae were collected at the center (1.00 +/- 0.11) than at the periphery (0.55 +/- 0.05) of the paddy during transplanting stage, but the difference was not significant during the tillering stage. In contrast, significantly higher numbers of Culex larvae were collected from the periphery (3.09 +/- 0.39) than at the center (2.81 +/- 0.24) of the paddy. More pupae were also collected at the center than at the periphery of the paddy. These findings indicate the distribution of Anopheles and Culex larvae in rice fields to be nonrandom; however, for successful achievement of an integrated vector control program targeting the diverse mosquito fauna occurring in rice fields, there is need to target the whole paddy for larvicidal application.

Host choice and multiple blood feeding behaviour of malaria vectors and other anophelines in Mwea rice scheme, Kenya

Background

Studies were conducted between April 2004 and February 2006 to determine the blood-feeding pattern of Anopheles mosquitoes in Mwea Kenya.

Methods

Samples were collected indoors by pyrethrum spay catch and outdoors by Centers for Disease Control light traps and processed for blood meal analysis by an Enzyme-linked Immunosorbent Assay.

Results

A total of 3,333 blood-fed Anopheles mosquitoes representing four Anopheles species were collected and 2,796 of the samples were assayed, with Anopheles arabiensis comprising 76.2% (n = 2,542) followed in decreasing order by Anopheles coustani 8.9% (n = 297), Anopheles pharoensis 8.2% (n = 272) and Anopheles funestus 6.7% (n = 222). All mosquito species had a high preference for bovine (range 56.3–71.4%) over human (range 1.1–23.9%) or goat (0.1–2.2%) blood meals. Some individuals from all the four species were found to contain mixed blood meals. The bovine blood index (BBI) for An. arabiensis was significantly higher for populations collected indoors (71.8%), than populations collected outdoors (41.3%), but the human blood index (HBI) did not differ significantly between the two populations. In contrast, BBI for indoor collected An. funestus (51.4%) was significantly lower than for outdoor collected populations (78.0%) and the HBI was significantly higher indoors (28.7%) than outdoors (2.4%). Anthropophily of An. funestus was lowest within the rice scheme, moderate in unplanned rice agro-ecosystem, and highest within the non-irrigated agro-ecosystem. Anthropophily of An. arabiensis was significantly higher in the non-irrigated agro-ecosystem than in the other agro-ecosystems.

Conclusion

These findings suggest that rice cultivation has an effect on host choice by Anopheles mosquitoes. The study further indicate that zooprophylaxis may be a potential strategy for malaria control, but there is need to assess how domestic animals may influence arboviruses epidemiology before adapting the strategy.

Effect of rice cultivation on malaria transmission in central Kenya

A 12-month field study was conducted between April 2004 and March 2005 to determine the association between irrigated rice cultivation and malaria transmission in Mwea, Kenya. Adult mosquitoes were collected indoors twice per month in three villages representing non-irrigated, planned, and unplanned rice agro-ecosystems and screened for blood meal sources and Plasmodium falciparum circumsporozoite proteins. Anopheles arabiensis Patton and An. funestus Giles comprised 98.0% and 1.9%, respectively, of the 39,609 female anophelines collected. Other species including An. pharoensis Theobald, An. maculipalpis Giles, An. pretoriensis Theobald, An. coustani Laveran, and An. rufipes Gough comprised the remaining 0.1%. The density of An. arabiensis was highest in the planned rice village and lowest in the non-irrigated village and that of An. funestus was significantly higher in the non-irrigated village than in irrigated ones. The human blood index (HBI) for An. arabiensis was significantly higher in the non-irrigated village compared with irrigated villages. For An. funestus, the HBI for each village differed significantly from the others, being highest in the non-irrigated village and lowest in the planned rice village. The sporozoite rate and annual entomologic inoculation rate (EIR) for An. arabiensis was 1.1% and 3.0 infective bites per person, respectively with no significant difference among villages. Sporozoite positive An. funestus were detected only in planned rice and non-irrigated villages. Overall, 3.0% of An. funestus samples tested positive for Plasmodium falciparum sporozoites. The annual EIR of 2.21 for this species in the non-irrigated village was significantly higher than 0.08 for the planned rice village. We conclude that at least in Mwea Kenya, irrigated rice cultivation may reduce the risk of malaria transmission by An. funestus but has no effect on malaria transmission by An. arabiensis. The zoophilic tendency of malaria vectors in irrigated areas accounts partly for low malaria transmission rates despite the presence of higher vector densities, highlighting the potential of zooprophylaxis in malaria control.

Environmental covariates of Anopheles arabiensis in a rice agroecosystem in Mwea, Central Kenya

Water quality of aquatic habitats is an important determinant of female mosquito oviposition and successful larval development. This study examined the influence of environmental covariates on Anopheles arabiensis mosquito abundance in the Mwea Irrigation Scheme, Central Province of Kenya, prior to implementation of a malaria vector control program. Experimental rice plots were used to examine the environmental covariates responsible for regulating abundance and diversity of the aquatic stages of malaria vectors. Mosquito larval sampling and water quality analysis were done weekly from the flooding stage to the rice maturation stage. Sampling for mosquito larvae was conducted using standard dipping technique. During each larval collection, environmental covariates such as pH, temperature, conductivity, salinity, dissolved oxygen, water depth, and rice stage were measured. Anopheles arabiensis larval density was highest between 1 wk before transplanting and 4 wk after transplanting with peaks at weeks 0, 3, and 8. The fluctuation in values of the various environmental covariates showed characteristic patterns in different rice growth phases depending on the changes taking place due to the agronomic practices. Using a backward linear regression model, the factors that were found to be associated with abundance of An. arabiensis larvae at any of the rice growing phases included the following: dissolved oxygen, pH, turbidity, water depth, rice height, number of rice tillers, salinity, conductivity, and temperature. The environmental covariates associated with abundance of An. arabiensis were associated with early vegetative stage of the rice growth. For effective control of developmental stages of mosquito larvae, the application of larvicides should be done at the vegetative stage and the larvicides should persist until the beginning of the reproductive stage of the rice.

Laboratory studies on the effect of inorganic fertilizers on survival and development of immature Culex quinquefasciatus (Diptera: Culicidae)

BACKGROUND & OBJECTIVES

Studies were conducted to determine the effect of ammonium sulfate (AM) and muriate of potash (MOP) fertilizers on survival and development of immature stages of Culex quinquefasciatus Say, a major vector of Bancroftian filariasis in Africa.

METHODS

Twenty I instar larvae each were added in four doses of each fertilizer dissolved in one litre of deionised water and in one litre of deionised water as a control in replicates of five. The larvae were monitored every morning throughout their life and the numbers of each instar surviving were recorded. The experiments were discontinued when all the larvae had died or emerged into adults.

RESULTS

An analysis of variance test and Tukey's HSD test revealed a significant impact of fertilizers on survival and development of aquatic stages of Cx. quinquefasciatus. Ammonium sulfate accounted for up to 40% mortality rate, and one week delay in development time and this effect was both instar and dose dependent. None of the MOP dosages had significant impact on survival of immatures of Cx. quinquefasciatus and only the higher dosages showed significant impact on development time but in significantly lower magnitudes compared with similar dosages of ammonium sulfate.

INTERPRETATION & CONCLUSION

These findings demonstrate the toxic effect of fertilizers on immature stages of Cx. quinquefasciatus contrary to field observations.

Remote and field level quantification of vegetation covariates for malaria mapping in three rice agro-village complexes in Central Kenya

Background

We examined algorithms for malaria mapping using the impact of reflectance calibration uncertainties on the accuracies of three vegetation indices (VI)'s derived from QuickBird data in three rice agro-village complexes Mwea, Kenya. We also generated inferential statistics from field sampled vegetation covariates for identifying riceland Anopheles arabiensis during the crop season. All aquatic habitats in the study sites were stratified based on levels of rice stages; flooded, land preparation, post-transplanting, tillering, flowering/maturation and post-harvest/fallow. A set of uncertainty propagation equations were designed to model the propagation of calibration uncertainties using the red channel (band 3: 0.63 to 0.69 μm) and the near infra-red (NIR) channel (band 4: 0.76 to 0.90 μm) to generate the Normalized Difference Vegetation Index (NDVI) and the Soil Adjusted Vegetation Index (SAVI). The Atmospheric Resistant Vegetation Index (ARVI) was also evaluated incorporating the QuickBird blue band (Band 1: 0.45 to 0.52 μm) to normalize atmospheric effects. In order to determine local clustering of riceland habitats Gi*(d) statistics were generated from the ground-based and remotely-sensed ecological databases. Additionally, all riceland habitats were visually examined using the spectral reflectance of vegetation land cover for identification of highly productive riceland Anopheles oviposition sites.

Results

The resultant VI uncertainties did not vary from surface reflectance or atmospheric conditions. Logistic regression analyses of all field sampled covariates revealed emergent vegetation was negatively associated with mosquito larvae at the three study sites. In addition, floating vegetation (-ve) was significantly associated with immature mosquitoes in Rurumi and Kiuria (-ve); while, turbidity was also important in Kiuria. All spatial models exhibit positive autocorrelation; similar numbers of log-counts tend to cluster in geographic space. The spectral reflectance from riceland habitats, examined using the remote and field stratification, revealed post-transplanting and tillering rice stages were most frequently associated with high larval abundance and distribution.

Conclusion

NDVI, SAVI and ARVI generated from QuickBird data and field sampled vegetation covariates modeled cannot identify highly productive riceland An. arabiensis aquatic habitats. However, combining spectral reflectance of riceland habitats from QuickBird and field sampled data can develop and implement an Integrated Vector Management (IVM) program based on larval productivity.

Influence of biological and physicochemical characteristics of larval habitats on the body size of Anopheles gambiae mosquitoes (Diptera: Culicidae) along the Kenyan coast

BACKGROUND & OBJECTIVES

The number and productivity of larval habitats ultimately determine the density of adult mosquitoes. The biological and physicochemical conditions at the larval habitat affect larval development hence affecting the adult body size. The influence of biological and physicochemical characteristics on the body size of Anopheles gambiae was assessed in Jaribuni village, Kilifi district along the Kenyan Coast.

METHODS

Ten cages measuring 1 x 1 x 1 m (1 m3) with a netting material were placed in 10 different aquatic habitats, which were positive for anopheline mosquito larvae. Emergent mosquitoes were collected daily by aspiration and the wing lengths were determined by microscopy. In the habitats, physicochemical parameters were assessed: pH, surface debris, algae and emergent plants, turbidity, substrate, nitrate, ammonia, phosphate and chlorophyll a content.

RESULTS

A total of 685 anopheline and culicine mosquitoes were collected from the emergent cages. Only female mosquitoes were considered in this study. Among the Anopheles spp, 202 were An. gambiae s.s., eight An. arabiensis, two An. funestus, whereas the Culex spp was composed of 214 Cx. quinquefasciatus, 10 Cx. tigripes, eight Cx. annulioris and one Cx. cumminsii. The mean wing length of the female An. gambiae s.s. mosquitoes was 3.02 mm (n=157), while that of An. arabiensis was 3.09 mm (n=9). There were no associations between the wing lengths and the environmental and chemical parameters, except for a positive correlation between wing length of An. gambiae and chlorophyll a content (r = 0.622). The day on which the mosquitoes emerged was not significant for the anopheline (p = 0.324) or culicine mosquitoes (p = 0.374), because the mosquito emerged from the cages on a daily basis.

INTERPRETATION & CONCLUSION

In conclusion, there was variability in production of emergent mosquitoes from different habitats, which means that there should be targeted control on these habitats based on productivity.

Evaluation of four sampling techniques for surveillance of Culex quinquefasciatus (Diptera: Culicidae) and other mosquitoes in African rice agroecosystems

Field studies were conducted in a rice, Oryza sativa L., agroecosystem in Mwea Kenya to compare the efficiency of CO2-baited Centers for Disease Control (CDC) light traps against nonbaited CDC light traps and gravid traps against oviposition traps in outdoor collection of Culex quinquefasciatus Say (Diptera: Culicidae) and other mosquitoes. Collectively, 21 mosquito species from the genera Culex, Anopheles, Mansonia, Ficalbia, and Aedes were captured during the 10-wk study period. Cx. quinquefasciatus was the predominant species in all trap types with proportions ranging from 57% in the nonbaited CDC light traps to 95% in the gravid traps. Significantly higher numbers of Cx. quinquefasciatus and Culex annulioris Theobald were collected in the CO2-baited CDC light traps than in the nonbaited CDC light traps, but the numbers of other mosquito species, including malaria vectors Anopheles arabiensis Patton and Anopheles funestus Giles did not differ significantly between the two trap types. More Cx. quinquefasciatus females were collected in grass infusion-baited gravid traps than egg rafts of this species in oviposition traps containing the same infusion. Although most mosquitoes captured in CO,-baited and nonbaited CDC light traps were unfed, most of those collected in gravid traps were gravid. From these findings, it is concluded that at least in the rice-growing area of Mwea Kenya, CO2-baited CDC light traps in conjunction with gravid traps can be used in monitoring of Cx. quinquefasciatus both for control and disease surveillance.

Spatial distribution and habitat characterisation of Anopheles larvae along the Kenyan coast

BACKGROUND & OBJECTIVES

A study was conducted to characterise larval habitats and to determine spatial heterogeneity of the Anopheles mosquito larvae. The study was conducted from May to June 1999 in nine villages along the Kenyan coast.

METHODS

Aquatic habitats were sampled by use of standard dipping technique. The habitats were characterised based on size, pH, distance to the nearest house, coverage of canopy, surface debris, algae and emergent plants, turbidity, substrate, and habitat type.

RESULTS

A total of 110 aquatic habitats like stream pools (n=10); puddles (n=65); tire tracks (n=5); ponds (n=5) and swamps (n=25) were sampled in nine villages located in three districts of the Kenyan coast. A total of 7,263 Anopheles mosquito larvae were collected, 63.9% were early instars and 36.1% were late instars. Morphological identification of the III and IV instar larvae by use of microscopy yielded 90.66% (n=2377) Anopheles gambiae Complex, 0.88% (n=23) An. funestus, An. coustani 7.63% (n=200), An. rivulorum 0.42% (n=11), An. pharoensis 0.19% (n=5), An. swahilicus 0.08% (n=2), An. wilsoni 0.04% (n=1) and 0.11% (n=3) were unidentified. A subset of the An. gambiae Complex larvae identified morphologically, was further analysed using rDNA-PCR technique resulting in 68.22% (n=1290) An. gambiae s.s., 7.93% (n=150) An. arabiensis and 23.85% (n=451) An. merus. Multiple logistic regression model showed that emergent plants (p = 0.019), and floating debris (p = 0.038) were the best predictors of An. gambiae larval abundance in these habitats.

INTERPRETATION & CONCLUSION

Habitat type, floating debris and emergent plants were found to be the key factors determining the presence of Anopheles larvae in the habitats. For effective larval control, the type of habitat should be considered and most productive habitat type be given a priority in the mosquito abatement programme.

Mosquito species succession and physicochemical factors affecting their abundance in rice fields in Mwea, Kenya

The succession of mosquito species and abiotic factors affecting their distribution and abundance in rice (Oryza spp.) fields was investigated over a 16-wk rice growing cycle covering the period between January and May 2006. Fifteen experimental rice plots were sampled for mosquito larvae and characterized based on rice height, number of tillers, floating vegetation cover, water depth, water temperature, turbidity, salinity, pH, dissolved oxygen, total dissolved solids, and conductivity. Microscopic identification of 3,025 larvae yielded nine mosquito species predominated by Anopheles arabiensis Patton (45.0%), Culex quinquefasciatus Say (35.8%), Anopheles pharoensis Theobald (9.0%) and Ficalbia splendens Theobald (7.1%). Other species, including Anopheles rufipes Gough, Anopheles coustani Laveran, Anonopheles maculipalpis Giles, Culex annulioris Theobald, and Culex poicilipes Theobald made up 3.1% of the total collection. Anopheles gambiae s.l., Cx. quinquefasciatus, and An. pharoensis occurred throughout the cycle, but they were more abundant up to 4 wk posttransplanting with peaks after fertilizer application. As rice plants became established, three groups of mosquitoes were recognized: the first groups included An. rufipes, Fl. splendens, and Cx. annulioris, which occurred throughout much of the second half of the rice cycle, whereas the second group included Cx. poicilipes, which was found in the middle of the rice cycle. An. coustani and An. maculipalpis formed the third group occurring toward the end of the cycle. Dissolved oxygen, number of tillers, and rice height were negatively associated with the abundance ofAn. arabiensis and Cx. quinquefasciatus larvae. In addition, Cx. quinquefasciatus also was associated with water depth (-ve) and turbidity (+ve). Abundance of An. pharoensis larvae was significantly associated with water temperature (+ve), the number of tillers (-ve), and rice height (-ve), whereas Fl. splendens was significantly associated with the number of tillers (+ve). The results demonstrate a complex nature of the interactions between some of the factors in the ecosystem and mosquito species abundance and calls for time-dependent and species-specific mosquito control operations.

Larval habitat dynamics and diversity of Culex mosquitoes in rice agro-ecosystem in Mwea, Kenya

Introduction of irrigation projects in developing nations has often been blamed for aggravating the problem of mosquito-borne diseases by creating ideal larval habitats for vector mosquitoes. However, whereas several studies have demonstrated the relationship between malaria vectors and irrigation, little work has been done on culicine mosquitoes despite their potential in transmission of filariasis and arboviruses and their significant biting nuisance in these areas. This study examined the diversity of Culex mosquito fauna and their larval habitats at two sites (Murinduko and Kiamachiri) in Mwea, Kenya over a 12-month period. The habitat types present at each site within a 200-meter radius around the study village, including randomly selected paddies and canals, were sampled every two weeks to examine the relationship between vegetation cover, water depth, turbidity, and Culex larval counts. Ten culicine species belonging to four genera were identified, with 73.1% of the total collection comprising of Culex duttoni and Cx. quinquefasciatus. Other species collected included Cx. annulioris, Cx. poicilipes, Cx. cinereus, Cx. tigripes, Cx. trifilatus, Aedes spp., Coquilettidia fuscopennata, and Ficalbia splendens. Murinduko was more diverse than Kiamachiri in terms of species richness (10 versus 7 species) and larval habitat diversity (11 versus 8 habitat types). Paddies, canals, and rain pools were the most diverse habitats in terms of species richness, and ditches, rock pools, and tree holes were the least diverse. Principal component and correlation analyses showed a strong association between three Culex species and the measured habitat characteristics. Culex poicilipes was strongly associated with floating vegetation, Cx. annulioris with clean water containing emergent vegetation, and Cx. quinquefasciatus was associated with turbid water. Seasonal changes in larval counts in water reservoirs and pool and ditch habitats were closely associated with rainfall. These findings provide important information on larval habitat preference for different Culex species, which will be useful in designing and implementation of larval control operations.

Survival of immature Anopheles arabiensis (Diptera: Culicidae) in aquatic habitats in Mwea rice irrigation scheme, central Kenya

Background

The survivorship and distribution of Anopheles arabiensis larvae and pupae was examined in a rice agro-ecosystem in Mwea Irrigation Scheme, central Kenya, from August 2005 to April 2006, prior to implementation of larval control programme.

Methods

Horizontal life tables were constructed for immatures in semi-field condition. The time spent in the various immature stages was determined and survival established. Vertical life tables were obtained from five paddies sampled by standard dipping technique.

Results

Pre-adult developmental time for An. arabiensis in the trays in the experimental set up in the screen house was 11.85 days from eclosion to emergence. The mean duration of each instar stage was estimated to be 1.40 days for first instars, 2.90 days for second instars, 1.85 days for third instars, 3.80 days for fourth instars and 1.90 days for pupae. A total of 590 individuals emerged into adults, giving an overall survivorship from L1 to adult emergence of 69.4%. A total of 4,956 An. arabiensis immatures were collected in 1,400 dips throughout the sampling period. Of these, 55.9% were collected during the tillering stage, 42.5% during the transplanting period and 1.6% during the land preparation stage. There was a significant difference in the An. arabiensis larval densities among the five stages. Also there was significant variation in immature stage composition for each day's collection in each paddy. These results indicate that the survival of the immatures was higher in some paddies than others. The mortality rate during the transplanting was 99.9% and at tillering was 96.6%, while the overall mortality was 98.3%.

Conclusion

The survival of An. arabiensis immatures was better during the tillering stage of rice growth. Further the survival of immatures in rice fields is influenced by the rice agronomic activities including addition of nitrogenous fertilizers and pesticides. For effective integrated vector management, the application of larvicides should target An. arabiensis larvae at the tillering stage (early vegetative stage of rice) when their survival in the aquatic habitats is high to significantly reduce them and the larvicides should be long-lasting to have a significant impact on the malaria vector productivity on the habitats.