Effects of larvicidal and larval nutritional stresses on Anopheles gambiae development, survival and competence for Plasmodium falciparum

Anopheles aquatic development kinetic and adults' longevity through different seasons in laboratory and semi-field conditions in Burkina Faso

ABSTRACT: BACKGROUND: Anopheles mosquitoes are ectothermic and involved in numerous pathogen transmissions. Their life history traits are influenced by several environmental factors such as temperature, relative humidity and photoperiodicity. Despite extensive investigations of these environmental conditions on vector population ecology, their impact on the different life stages of Anopheles at different seasons in the year remains poorly explored. This study reports the potential impact of these abiotic factors on the immature and adult stages of Anopheles gambiae sensu lato during different seasons.

METHODS

Environmental conditions were simulated in the laboratory using incubators to mimic the environmental conditions of two important periods of the year in Burkina Faso: the peak of rainy season (August) and the onset of dry season (December). Eggs from wild An. coluzzii and An. gambiae s.l. were reared separately under each environmental condition. For Anopheles coluzzii or An. gambiae s.l., eggs were equally divided into two groups assigned to the two experimental conditions. Four replicates were carried out for this experiment. Then, egg hatching rate, pupation rate, larval development time, larva-to-pupae development time, adult emergence dynamics and longevity of Anopheles were evaluated. Also, pupae-to-adult development time from wild L3 and L4 Anopheles larvae was estimated under semi-field conditions in December.

RESULTS

A better egg hatching rate was recorded overall with conditions mimicking the onset of the dry season compared to the peak of the rainy season. Larval development time and longevity of An. gambiae s.l. female were significantly longer at the onset of the dry season compared than at the peak of the rainy season. Adult emergence was spread over 48 and 96 h at the peak of the rainy season and onset of dry season conditions respectively. This 96h duration in the controlled conditions of December was also observed in the semi-field conditions in December.

CONCLUSIONS

The impact of temperature and relative humidity on immature stages and longevity of An. gambiae s.l. adult females differed under both conditions. These findings contribute to a better understanding of vector population dynamics throughout different seasons of the year and may facilitate tailoring of control strategies.

Development of circulating isolates of Plasmodium falciparum is accelerated in Anopheles vectors with reduced reproductive output

Anopheles gambiae and its sibling species Anopheles coluzzii are the most efficient vectors of the malaria parasite Plasmodium falciparum. When females of these species feed on an infected human host, oogenesis and parasite development proceed concurrently, but interactions between these processes are not fully understood. Using multiple natural P. falciparum isolates from Burkina Faso, we show that in both vectors, impairing steroid hormone signaling to disrupt oogenesis leads to accelerated oocyst growth and in a manner that appears to depend on both parasite and mosquito genotype. Consistently, we find that egg numbers are negatively linked to oocyst size, a metric for the rate of oocyst development. Oocyst growth rates are also strongly accelerated in females that are in a pre-gravid state, i.e. that fail to develop eggs after an initial blood meal. Overall, these findings advance our understanding of mosquito-parasite interactions that influence P. falciparum development in malaria-endemic regions.

Mosquito larvae exposed to a sublethal dose of photosensitive insecticides have altered juvenile development but unaffected adult life history traits

BACKGROUND

Larvicides are critical for the control of mosquito-borne diseases. However, even sublethal exposure to a larvicide can alter development and life history traits, which can then affect population density and disease transmission dynamics. Photosensitive insecticides (PSIs) are a promising class of larvicide that are toxic when ingested and activated by light. We investigated whether the time of day when exposure occurs, or the process of pupation, affects larval susceptibility to PSI phototoxicity in the mosquito Anopheles gambiae, and whether sublethal exposure to PSIs alters life history traits.

METHODS

Larvae were treated with lethal concentrations of the PSIs methylene blue (MB) and rose bengal (RB), and larval survival was measured at various times of day. Additionally, larvae were exposed to two concentrations of each PSI that resulted in low and medium mortality, and the life history traits of the surviving larvae were measured.

RESULTS

Pupation, which predominantly occurs in the evening, protected larvae from PSI toxicity, but the toxicity of PSIs against larvae that had yet to pupate was unaffected by time of day. Larval exposure to a sublethal concentration of MB, but not RB, shortened the time to pupation. However, larval exposure to a sublethal concentration of RB, but not MB, increased pupal mortality. Neither PSI had a meaningful effect on the time to eclosion, adult longevity, or adult melanization potential.

CONCLUSIONS

PSIs are lethal larvicides. Sublethal PSI exposure alters mosquito development, but does not affect adult life history traits.

Nutritional stress compromises mosquito fitness and antiviral immunity, while enhancing dengue virus infection susceptibility

Diet-induced nutritional stress can influence pathogen transmission potential in mosquitoes by impacting life history traits, infection susceptibility, and immunity. To investigate these effects, we manipulate mosquito diets at larval and adult stages, creating two nutritional levels (low and normal), and expose adults to dengue virus (DENV). We observe that egg number is reduced by nutritional stress at both stages and viral exposure separately and jointly, while the likelihood of laying eggs is exclusively influenced by adult nutritional stress. Adult nutritional stress alone shortens survival, while any pairwise combination between both-stage stress and viral exposure have a synergistic effect. Additionally, adult nutritional stress increases susceptibility to DENV infection, while larval nutritional stress likely has a similar effect operating via smaller body size. Furthermore, adult nutritional stress negatively impacts viral titers in infected mosquitoes; however, some survive and show increased titers over time. The immune response to DENV infection is overall suppressed by larval and adult nutritional stress, with specific genes related to Toll, JAK-STAT, and Imd immune signaling pathways, and antimicrobial peptides being downregulated. Our findings underscore the importance of nutritional stress in shaping mosquito traits, infection outcomes, and immune responses, all of which impact the vectorial capacity for DENV transmission.

Impact of different diets on the survival, pupation, and adult emergence of Culex pipiens biotype molestus larvae, and infectability with the insect-specific Culex Y virus

The current rapidly advancing climate change will affect the transmission of arthropod-borne viruses (arboviruses), mainly through changes in vector populations. Mosquitos of the Culex pipiens complex play a particularly prominent role in virus transmission in central Europe. Factors that contribute to the vector population density and the ability of those vectors to transmit viral pathogens (vector competence) can include nutrition during the larval stages. To test the influence of larval diet on larval survival and adult emergence, as well as vector competence, several diets varying in their nutritional composition were compared using a newly established assay. We tested the effects of 17 diets or diet combinations on the fitness of third-instar larvae of Culex pipiens biotype molestus. Larval survival rates at day 7 ranged from 43.33% to 94.44%. We then selected 3 of the 17 diets (Tetra Pleco, as the routine feed; JBL NovoTab, as the significantly inferior feed; and KG, as the significantly superior feed) and tested the effect of these diets, in combination with Culex Y virus infection, on larval survival rate. All Culex Y virus-infected larvae showed significantly lower larval survival, as well as low pupation and adult emergence rates. However, none of the tested diets in our study had a significant impact on larval survival in combination with viral infection. Furthermore, we were able to correlate several water quality parameters, such as phosphate, nitrate, and ammonium concentration, electrical conductivity, and low O2 saturations, with reduced larval survival. Thus, we were able to demonstrate that Culex Y virus could be a suitable agent to reduce mosquito population density by reducing larval density, pupation rate, and adult emergence rate. When combined with certain water quality parameters, these effects can be further enhanced, leading to a reduced mosquito population density, and reduce the cycle of transmission. Furthermore, we demonstrate, for the first time, the infection of larvae of the mosquito Culex pipiens biotype molestus with a viral pathogen.

Changes in Container-Breeding Mosquito Diversity and Abundance Along an Urbanization Gradient are Associated With Dominance of Arboviral Vectors

Environmental conditions associated with urbanization are likely to influence the composition and abundance of mosquito (Diptera, Culicidae) assemblages through effects on juvenile stages, with important consequences for human disease risk. We present six years (2011-2016) of weekly juvenile mosquito data from distributed standardized ovitraps and evaluate how variation in impervious cover and temperature affect the composition and abundance of container-breeding mosquito species in Maryland, USA. Species richness and evenness were lowest at sites with high impervious cover (>60% in 100-m buffer). However, peak diversity was recorded at sites with intermediate impervious cover (28-35%). Four species were observed at all sites, including two recent invasives (Aedes albopictus Skuse, Ae. japonicus Theobald), an established resident (Culex pipiens L), and one native (Cx. restuans Theobald). All four are viral vectors in zoonotic or human transmission cycles. Temperature was a positive predictor of weekly larval abundance during the growing season for each species, as well as a positive predictor of rapid pupal development. Despite being observed at all sites, each species responded differently to impervious cover. Abundance of Ae. albopictus larvae was positively associated with impervious cover, emphasizing that this medically-important vector not only persists in the warmer, impervious urban landscape but is positively associated with it. Positive temperature effects in our models of larval abundance and pupae occurrence in container habitats suggest that these four vector species are likely to continue to be present and abundant in temperate cities under future temperature scenarios.

Implications of Sublethal Insecticide Exposure and the Development of Resistance on Mosquito Physiology, Behavior, and Pathogen Transmission

Simple Summary

Mosquitoes are one of the greatest threats to human lives; they transmit a wide range of pathogens, including viruses that cause lethal diseases. Mosquitoes are found in both aquatic (as larvae or pupae) and terrestrial (as adults) environments during their complex life cycle. For decades, insecticides have been systematically used on mosquitoes with the aim to reduce their population. Little is known about how the stress resulting from the exposure of mosquitoes to insecticides impacts the tri-partite relationship between the mosquitoes, their vertebrate hosts, and the pathogens they transmit. In this work, we review existing experimental evidence to obtain a broad picture on the potential effects of the (sub)lethal exposure of hematophagous mosquitoes to different insecticides. We have focused on studies that have advanced our understanding of their physiological and behavioral responses (including the mechanisms behind insecticide resistance) and the spread of pathogens by these vectors—understudied but critically important issues for epidemiology. Studying these exposure-related effects is of paramount importance for predicting how they respond to insecticide exposure and whether this exposure makes them more or less likely to transmit pathogens.

Abstract

For many decades, insecticides have been used to control mosquito populations in their larval and adult stages. Although changes in the population genetics, physiology, and behavior of mosquitoes exposed to lethal and sublethal doses of insecticides are expected, the relationships between these changes and their abilities to transmit pathogens remain unclear. Thus, we conducted a comprehensive review on the sublethal effects of insecticides and their contributions to insecticide resistance in mosquitoes, with the main focus on pyrethroids. We discuss the direct and acute effects of sublethal concentrations on individuals and populations, the changes in population genetics caused by the selection for resistance after insecticide exposure, and the major mechanisms underlying such resistance. Sublethal exposures negatively impact the individual’s performance by affecting their physiology and behavior and leaving them at a disadvantage when compared to unexposed organisms. How these sublethal effects could change mosquito population sizes and diversity so that pathogen transmission risks can be affected is less clear. Furthermore, despite the beneficial and acute aspects of lethality, exposure to higher insecticide concentrations clearly impacts the population genetics by selecting resistant individuals, which may bring further and complex interactions for mosquitoes, vertebrate hosts, and pathogens. Finally, we raise several hypotheses concerning how the here revised impacts of insecticides on mosquitoes could interplay with vector-mediated pathogens’ transmission.

The application of drones for mosquito larval habitat identification in rural environments: a practical approach for malaria control?

BACKGROUND

Spatio-temporal trends in mosquito-borne diseases are driven by the locations and seasonality of larval habitat. One method of disease control is to decrease the mosquito population by modifying larval habitat, known as larval source management (LSM). In malaria control, LSM is currently considered impractical in rural areas due to perceived difficulties in identifying target areas. High resolution drone mapping is being considered as a practical solution to address this barrier. In this paper, the authors' experiences of drone-led larval habitat identification in Malawi were used to assess the feasibility of this approach.

METHODS

Drone mapping and larval surveys were conducted in Kasungu district, Malawi between 2018 and 2020. Water bodies and aquatic vegetation were identified in the imagery using manual methods and geographical object-based image analysis (GeoOBIA) and the performances of the classifications were compared. Further, observations were documented on the practical aspects of capturing drone imagery for informing malaria control including cost, time, computing, and skills requirements. Larval sampling sites were characterized by biotic factors visible in drone imagery and generalized linear mixed models were used to determine their association with larval presence.

RESULTS

Imagery covering an area of 8.9 km² across eight sites was captured. Larval habitat characteristics were successfully identified using GeoOBIA on images captured by a standard camera (median accuracy = 98%) with no notable improvement observed after incorporating data from a near-infrared sensor. This approach however required greater processing time and technical skills compared to manual identification. Larval samples captured from 326 sites confirmed that drone-captured characteristics, including aquatic vegetation presence and type, were significantly associated with larval presence.

CONCLUSIONS

This study demonstrates the potential for drone-acquired imagery to support mosquito larval habitat identification in rural, malaria-endemic areas, although technical challenges were identified which may hinder the scale up of this approach. Potential solutions have however been identified, including strengthening linkages with the flourishing drone industry in countries such as Malawi. Further consultations are therefore needed between experts in the fields of drones, image analysis and vector control are needed to develop more detailed guidance on how this technology can be most effectively exploited in malaria control.

Implications of diet on mosquito life history traits and pathogen transmission

The environment, directly and indirectly, affects many mosquito traits in both the larval and adult stages. The availability of food resources is one of the key factors influencing these traits, although its role in mosquito fitness and pathogen transmission remains unclear. Larvae nutritional status determines their survivorship and growth, having also an impact on adult characteristics like longevity, body size, flight capacity or vector competence. During the adult stage, mosquito diet affects their survival rate, fecundity and host-seeking behaviour. It also affects mosquito susceptibility to infection, which may determine the vectorial capacity of mosquito populations. The aim of this review is to critically revise the current knowledge on the effects that both larval and adult quantity and quality of the diet have on mosquito life history traits, identifying the critical knowledge gaps and proposing future research lines. The quantity and quality of food available through their lifetime greatly determine adult body size, longevity or biting frequency, therefore affecting their competence for pathogen transmission. In addition, natural sugar sources for adult mosquitoes, i.e., specific plants providing high metabolic energy, might affect their host-seeking and vertebrate biting behaviour. However, most of the studies are carried out under laboratory conditions, highlighting the need for studies of feeding behaviour of mosquitoes under field conditions. This kind of studies will increase our knowledge of the impact of diets on pathogen transmission, helping to develop successful control plans for vector-borne diseases.

Differential effects of larval and adult nutrition on female survival, fecundity, and size of the yellow fever mosquito, Aedes aegypti

Background

The yellow fever mosquito, Aedes aegypti, is the principal vector of medically-important infectious viruses that cause severe illness such as dengue fever, yellow fever and Zika. The transmission potential of mosquitoes for these arboviruses is largely shaped by their life history traits, such as size, survival and fecundity. These life history traits, to some degree, depend on environmental conditions, such as larval and adult nutrition (e.g., nectar availability). Both these types of nutrition are known to affect the energetic reserves and life history traits of adults, but whether and how nutrition obtained during larval and adult stages have an interactive influence on mosquito life history traits remains largely unknown.

Results

Here, we experimentally manipulated mosquito diets to create two nutritional levels at larval and adult stages, that is, a high or low amount of larval food (HL or LL) during larval stage, and a good and poor adult food (GA or PA, represents normal or weak concentration of sucrose) during adult stage. We then compared the size, survival and fecundity of female mosquitoes reared from these nutritional regimes. We found that larval and adult nutrition affected size and survival, respectively, without interactions, while both larval and adult nutrition influenced fecundity. There was a positive relationship between fecundity and size. In addition, this positive relationship was not affected by nutrition.

Conclusions

These findings highlight how larval and adult nutrition differentially influence female mosquito life history traits, suggesting that studies evaluating nutritional effects on vectorial capacity traits should account for environmental variation across life stages.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12983-021-00395-z.

Trials of the Automated Particle Counter for laboratory rearing of mosquito larvae

As a means of obtaining reproducible and accurate numbers of larvae for laboratory rearing, we tested a large-particle flow-cytometer type device called the ‘Automated Particle Counter’ (APC). The APC is a gravity-fed, self-contained unit that detects changes in light intensity caused by larvae passing the detector in a water stream and controls dispensing by stopping the flow when the desired number has been reached. We determined the accuracy (number dispensed compared to the target value) and precision (distribution of number dispensed) of dispensing at a variety of counting sensitivity thresholds and larva throughput rates (larvae per second) using < 1-day old Anopheles gambiae and Aedes aegypti larvae. All measures were made using an APC algorithm called the ‘Smoothed Z-Score’ which allows the user to define how many standard deviations (Z scores) from the baseline light intensity a particle’s absorbance must exceed to register a count. We dispensed a target number of 100 An. gambiae larvae using Z scores from 2.5–8 and observed no difference among them in the numbers dispensed for scores from 2.5–6, however, scores of 7 and 8 under-counted (over-dispensed) larvae. Using a Z score ≤ 6, we determined the effect of throughput rate on the accuracy of the device to dispense An. gambiae larvae. For rates ≤ 98 larvae per second, the accuracy of dispensing a target of 100 larvae was - 2.29% ± 0.72 (95% CI of the mean) with a mode of 99 (49 of 348 samples). When using a Z score of 3.5 and rates ≤ 100 larvae per second, the accuracy of dispensing a target of 100 Ae. aegypti was - 2.43% ± 1.26 (95% CI of the mean) with a mode of 100 (6 of 42 samples). No effect on survival was observed on the number of An. gambiae first stage larvae that reached adulthood as a function of dispensing.

Differential side-effects of Bacillus thuringiensis bioinsecticide on non-target Drosophila flies

Bioinsecticides based on Bacillus thuringiensis (Bt) spores and toxins are increasingly popular alternative solutions to control insect pests, with potential impact of their accumulation in the environment on non-target organisms. Here, we tested the effects of chronic exposure to commercial Bt formulations (Bt var. kurstaki and israelensis) on eight non-target Drosophila species present in Bt-treated areas, including D. melanogaster (four strains). Doses up to those recommended for field application (~ 106 Colony Forming Unit (CFU)/g fly medium) did not impact fly development, while no fly emerged at ≥ 1000-fold this dose. Doses between 10- to 100-fold the recommended one increased developmental time and decreased adult emergence rates in a dose-dependent manner, with species-and strain-specific effect amplitudes. Focusing on D. melanogaster, development alterations were due to instar-dependent larval mortality, and the longevity and offspring number of adult flies exposed to bioinsecticide throughout their development were moderately influenced. Our data also suggest a synergy between the formulation compounds (spores, cleaved toxins, additives) might induce the bioinsecticide effects on larval development. Although recommended doses had no impact on non-target Drosophila species, misuse or local environmental accumulation of Bt bioinsecticides could have side-effects on fly populations with potential implications for their associated communities.

Effects of larval exposure to sublethal doses of Bacillus thuringiensis var. israelensis on body size, oviposition and survival of adult Anopheles coluzzii mosquitoes

Background

Application of the larvicide Bacillus thuringiensis var. israelensis (Bti) is a viable complementary strategy for malaria control. Efficacy of Bti is dose-dependent. There is a knowledge gap on the effects of larval exposure to sublethal Bti doses on emerging adult mosquitoes. The present study examined the effect of larval exposure to sublethal doses of Bti on the survival, body size and oviposition rate in adult Anopheles coluzzii.

Methods

Third-instar An. coluzzii larvae were exposed to control and sublethal Bti concentrations at LC₂₀, LC₅₀ and LC₇₀ for 48 h. Surviving larvae were reared to adults under standard colony conditions. Thirty randomly selected females from each treatment were placed in separate cages and allowed to blood feed. Twenty-five gravid females from the blood-feeding cages were randomly selected and transferred into new cages where they were provided with oviposition cups. Numbers of eggs laid in each cage and mortality of all adult mosquitoes were recorded daily. Wing lengths were measured of 570 mosquitoes as a proxy for body size.

Results

Exposure to LC₇₀Bti doses for 48 h as third-instar larvae reduced longevity of adult An. coluzzii mosquitoes. Time to death was 2.58 times shorter in females exposed to LC₇₀Bti when compared to the control females. Estimated mortality hazard rates were also higher in females exposed to the LC₅₀ and LC₂₀ treatments, but these differences were not statistically significant. The females exposed to LC₇₀ concentrations had 12% longer wings than the control group (P < 0.01). No differences in oviposition rate of the gravid females were observed between the treatments.

Conclusions

Exposure of An. coluzzii larvae to sublethal Bti doses reduces longevity of resultant adults and is associated with larger adult size and unclear effect on oviposition. These findings suggest that anopheline larval exposure to sublethal Bti doses, though not recommended, could reduce vectorial capacity for malaria vector populations by increasing mortality of resultant adults.

Linking Mosquito Ecology, Traits, Behavior, and Disease Transmission

Mosquitoes are considered to be the deadliest animals on Earth because the diseases they transmit claim at least a million human lives every year globally. Here, we discuss the scales at which the effects of ecological factors cascade to influence epidemiologically relevant behaviors of adult mosquitoes. In particular, we focused our review on the environmental conditions (coarse-scale variables) that shape the life-history traits of larvae and adult mosquitoes (fine-scale traits), and how these factors and their association, in turn, modulate adult behaviors to influence mosquito-borne disease transmission. Finally, we explore the integration of physical, physiological, and behavioral information into predictive models with epidemiological applications.

Consequences of larval competition and exposure to permethrin for the development of the rodent malaria Plasmodium berghei in the mosquito Anopheles gambiae

Background

Mosquitoes and other vectors are often exposed to sublethal doses of insecticides. Larvae can be exposed to the run-off of agricultural use, and adults can be irritated by insecticides used against them and move away before they have picked up a lethal dose. This sublethal exposure may affect the success of control of insect-borne diseases, for it may affect the competence of insects to transmit parasites, in particular if the insects are undernourished.

Methods

We assessed how exposure of larvae and adults to a sublethal dose of permethrin (a pyrethroid) and how larval competition for food affect several aspects of the vector competence of the mosquito Anopheles gambiae for the malaria parasite Plasmodium berghei. We infected mosquitoes with P. berghei and measured the longevity and the prevalence and intensity of infection to test for an effect of our treatments.

Results

Our general result was that the exposure to the insecticide helped mosquitoes deal with infection by malaria. Exposure of either larvae or adults decreased the likelihood that mosquitoes were infected by about 20%, but did not effect the parasite load. Exposure also increased the lifespan of infected mosquitoes, but only if they had been reared in competition. Larval competition had no effect on the prevalence of infection, but increased parasite load. These effects may be a consequence of the machinery governing oxidative stress, which underlies the responses of mosquitoes to insecticides, to food stress and to parasites.

Conclusions

We conclude that insecticide residues are likely to affect the ability of mosquitoes to carry and transmit pathogens such as malaria, irrespective of the stage at which they are exposed to the insecticide. Our results stress the need for further studies to consider sublethal doses in the context of vector ecology and vector-borne disease epidemiology.

Larval exposure to a pyrethroid insecticide and competition for food modulate the melanisation and antibacterial responses of adult Anopheles gambiae

The insecticides we use for agriculture and for vector control often arrive in water bodies, where mosquito larvae may be exposed to them. Not only will they then likely affect the development of the larvae, but their effects may carry over to the adults, potentially affecting their capacity at transmitting infectious diseases. Such an impact may be expected to be more severe when mosquitoes are undernourished. In this study, we investigated whether exposing larvae of the mosquito Anopheles gambiae to a sub-lethal dose of permethrin (a pyrethroid) and forcing them to compete for food would affect the immune response of the adults. We found that a low dose of permethrin increased the degree to which individually reared larvae melanised a negatively charged Sephadex bead and slowed the replication of injected Escherichia coli. However, if mosquitoes had been reared in groups of three (and thus had been forced to compete for food) permethrin had less impact on the efficacy of the immune responses. Our results show how larval stressors can affect the immune response of adults, and that the outcome of exposure to insecticides strongly depends on environmental conditions.

Larval predation in malaria vectors and its potential implication in malaria transmission: an overlooked ecosystem service?

The role of aquatic predators in controlling the anopheline aquatic stage has been known for decades. Recently, studies have highlighted that exposition to predation stress during aquatic development can have a profound impact on life-history traits (e.g. growth rate, fecundity and longevity) and consequently on the ability of adults to transmit human malaria parasites. In this study, we present a review aiming to contextualize the role of Anopheles larvae predators as an ecosystem factor interacting with the malaria pathogen through its vector, i.e. the female adult Anopheles. We first envisage the predator diversity that anopheline vectors are susceptible to encounter in their aquatic habitats. We then focus on mosquito-predator interactions with a special mention to anti-predator behaviors and prey adaptations developed to deal with the predation threat. Next, we address the direct and indirect effects of larval predation stress on mosquito populations and on individual life-history traits, which strongly suggest some carry-over effect of the impact of larval predation on vectorial capacity. The last part addresses the impact of human activities on larval predation. Concluding remarks highlight gaps in the knowledge of anopheline bio-ecology which may constitute avenues for researchers in the future.

Transstadial immune activation in a mosquito: Adults that emerge from infected larvae have stronger antibacterial activity in their hemocoel yet increased susceptibility to malaria infection

Larval and adult mosquitoes mount immune responses against pathogens that invade their hemocoel. Although it has been suggested that a correlation exists between immune processes across insect life stages, the influence that an infection in the hemocoel of a larva has on the immune system of the eclosed adult remains unknown. Here, we used Anopheles gambiae to test whether a larval infection influences the adult response to a subsequent bacterial or malaria parasite infection. We found that for both female and male mosquitoes, a larval infection enhances the efficiency of bacterial clearance following a secondary infection in the hemocoel of adults. The adults that emerge from infected larvae have more hemocytes than adults that emerge from naive or injured larvae, and individual hemocytes have greater phagocytic activity. Furthermore, mRNA abundance of immune genes-such as cecropin A, Lysozyme C1, Stat-A, and Tep1-is higher in adults that emerge from infected larvae. A larval infection, however, does not have a meaningful effect on the probability that female adults will survive a systemic bacterial infection, and increases the susceptibility of females to Plasmodium yoelii, as measured by oocyst prevalence and intensity in the midgut. Finally, immune proficiency varies by sex; females exhibit increased bacterial killing, have twice as many hemocytes, and more highly express immune genes. Together, these results show that a larval hemocoelic infection induces transstadial immune activation-possibly via transstadial immune priming-but that it confers both costs and benefits to the emerged adults.

Aedes aegypti larvae treated with spinosad produce adults with damaged midgut and reduced fecundity

The mosquito Aedes aegypti is the main vector of Dengue, Chikungunya, Zika, and yellow fever viruses, which are responsible for high human morbidity and mortality. The fight against these pathogens is mainly based on the control of the insect vector with the use of insecticides. Among insecticides, spinosad bioinsecticide is efficient against A. aegypti larvae and may be an alternative for vector control. Here, we investigate the sublethal effects of spinosad during midgut metamorphosis of A. aegypti females and its cumulative effects on blood acquisition capacity and fecundity in adults. We studied the midgut because it is an important model organ directly related to blood acquisition and digestion. Treatment of larvae with spinosad induced oxidative stress, apoptosis, and damage to the midgut cells at all stages of development and in adults. There was a reduction in the number of proliferating cells and the number of enteroendocrine cells in treated individuals. In addition, damage caused by spinosad led to a reduction in oviposition and egg viability of A. aegypti females. Finally, the exposure of mosquito larvae to sublethal concentrations of spinosad interfered with the development of the midgut, arresting the blood digestion and reproduction of adult females with blood digestion and reproduction difficulties.

The interaction between permethrin exposure and malaria infection affects the host-seeking behaviour of mosquitoes

Background

Insecticide-treated bed nets (ITNs) help to control malaria by mechanically impeding the biting of mosquitoes, by repelling and irritating them and by killing them. In contrast to spatial repellency, irritancy implies that mosquitoes contact the ITN and are exposed to at least a sub-lethal dose of insecticide, which impedes their further blood-seeking. This would weaken the transmission of malaria, if mosquitoes are infectious.

Methods

It was therefore tested whether sub-lethal exposure to permethrin impedes blood-feeding differently in uninfected mosquitoes and in mosquitoes carrying the non-transmissible stage (oocysts) or the infectious stage (sporozoites) of the malaria parasite Plasmodium berghei. In addition, as the degree of irritancy determines the dose of insecticide the mosquitoes may receive, the irritancy to permethrin of infected and uninfected mosquitoes was compared.

Results

In this laboratory setting, sub-lethal exposure to permethrin inhibited the blood-seeking behaviour of Anopheles gambiae mosquitoes for almost 48 h. Although infection by malaria did not affect the irritancy of the mosquitoes to permethrin at either the developmental stage or the infectious stage, both stages of infection shortened the duration of inhibition of blood-seeking.

Conclusions

The results suggest that the impact of ITNs may be weaker for malaria-infected than for uninfected mosquitoes. This will help to understand the global impact of ITNs on the transmission of malaria and gives a more complete picture of the effectiveness of that vector control measure.

Electronic supplementary material

The online version of this article (10.1186/s12936-019-2718-x) contains supplementary material, which is available to authorized users.

Impact of irradiation on the reproductive traits of field and laboratory An. arabiensis mosquitoes

Background

The sterile insect technique (SIT) aims at suppressing or decreasing insect pest populations by introducing sterile insects into wild populations. SIT requires the mass-production of insects and their sterilization through, for example, radiation. However, both mass-rearing and radiation can affect the life history traits of insects making them less competitive than their wild counterparts. In the malaria mosquito Anopheles arabiensis, some progress has been made to improve the mating competitiveness of mass-reared irradiated males. However, to date, no study has explored the relative effects of colonization and irradiation on important reproductive traits in this species. Such data may help to focus research efforts more precisely to improve current techniques.

Methods

Two strains of An. arabiensis originating from the same locality were used: one reared in the laboratory for five generations and the second collected as late larval instars in the field prior to experimentation. Pupae were irradiated with 95 Gy and some adult reproductive traits, including insemination rate, fecundity, oviposition behavior, fertility and male survivorship, were assessed in different mating combinations.

Results

Our study revealed the different effects of mosquito strain and irradiation on reproductive processes. The insemination rate was higher in field (67.3%) than in laboratory (54.9%) females and was negatively affected by both female and male irradiation (un-irradiated vs irradiated: 70.2 vs 51.3% in females; 67.7 vs 53.7% in males). Irradiated females did not produce eggs and egg prevalence was lower in the field strain (75.4%) than in the laboratory strain (83.9%). The hatching rate was higher in the field strain (88.7%) than in the laboratory strain (70.6%) as well as in un-irradiated mosquitoes (96.5%) than in irradiated ones (49%). Larval viability was higher in the field strain (96.2%) than in the laboratory strain (78.5%) and in un-irradiated mosquitoes (97.6%) than irradiated ones (52%). Finally, field males lived longer than laboratory males (25.1 vs 20.5 days, respectively).

Conclusions

Our results revealed that both irradiation and colonization alter reproductive traits. However, different developmental stages are not equally affected. It is necessary to consider as many fitness traits as possible to evaluate the efficacy of the sterile insect technique.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3228-3) contains supplementary material, which is available to authorized users.

Cost-effective larval diet mixtures for mass rearing of Anopheles arabiensis Patton (Diptera: Culicidae)

Background

Larval nutrition, particularly diet quality, is a key driver in providing sufficient numbers of high quality mosquitoes for biological control strategies such as the sterile insect technique. The diet currently available to mass rear Anopheles arabiensis, referred here to as the “IAEA diet”, is facing high costs and difficulties concerning the availability of the bovine liver powder component. To promote more affordable and sustainable mosquito production, the present study aimed to find alternative diet mixtures. Eight cheaper diet mixtures comprised of varying proportions of tuna meal (TM), bovine liver powder (BLP), brewer’s yeast (BY), and chickpea (CP) were developed and evaluated through a step by step assessment on An. arabiensis larvae and adult life history traits, in comparison to the IAEA diet which served as a basis and standard.

Results

Four mixtures were found to be effective regarding larval survival to pupation and to emergence, egg productivity, adult body size and longevity. These results suggest that these different diet mixtures have a similar nutritional value that support the optimal development of An. arabiensis larvae and enhance adult biological quality and production efficiency, and thus could be used for mass rearing.

Conclusions

Our study demonstrated that four different diet mixtures, 40 to 92% cheaper than the IAEA diet, can result in a positive assessment of the mosquitoes’ life history traits, indicating that this mosquito species can be effectively mass reared with a significant reduction in costs. The mixture comprised of TM + BY + CP is the preferred choice as it does not include BLP and thus reduces the cost by 92% compared to the IAEA diet.

Transstadial transmission of larval hemocoelic infection negatively affects development and adult female longevity in the mosquito Anopheles gambiae

During all life stages, mosquitoes are exposed to pathogens, and employ an immune system to resist or limit infection. Although much attention has been paid to how adult mosquitoes fight infection, little is known about how an infection during the larval stage affects the biology of the resultant adult. In this study, we investigated whether a bacterial infection in the hemocoel of the African malaria mosquito, Anopheles gambiae, is transstadially transmitted from larvae to adults (both females and males), and whether immune stimulation in the hemocoel as a larva alters development or biological traits of the adult. Specifically, larvae were injected in the hemocoel with either fluorescent microspheres or Escherichia coli, and the following traits were examined: transstadial transmission, larval development to adulthood, adult survival, and adult body size. Our results show that transstadial transmission of hemocoel contents occurs from larvae to pupae and from pupae to adults, but that bacterial prevalence and intensity varies with age. Injury, immune stimulation or infection decreases the proportion of larvae that undergo pupation and eclosion, infection decreases the longevity of adult females, and treatment has complex effects on the body size of the resultant adults. The present study adds larval hemocoelic infection to the known non-genetic factors that reduce overall fitness by negatively affecting development and adult biological traits that influence mosquito vector competence.

Potential for sublethal insecticide exposure to impact vector competence of Aedes albopictus (Diptera: Culicidae) for dengue and Zika viruses

Chikungunya, dengue, and Zika viruses (CHIKV, family Togaviridae, genus Alphavirus; DENV and ZIKV, family Flaviviridae, genus Flavivirus) are arboviruses that cause human epidemics. Due to the lack of vaccines for many mosquito-borne diseases, there is a need for mosquito control. In the US and other regions, residual barrier insecticide sprays are applied to foliage where female mosquitoes rest and/or sugar feed between blood meals. Residual sprays are an important control method for anthropogenic day-active sylvan mosquitoes such as Aedes albopictus (vector of CHIKV, DENV, and ZIKV) that are difficult to control using ultralow-volume sprays applied only at dusk or dawn when these mosquitoes are not active. In this exploratory study, we analyzed the extent to which ingestion of a sublethal dose of the active ingredient bifenthrin affected vector competence (i.e., infection, dissemination, and transmission) of Ae. albopictus for DENV and ZIKV. Two incubation periods (IPs; 7 and 14 d) were tested at 28°C for insecticide-fed and sugar-fed mosquitoes. We show that mosquitoes that were fed bifenthrin (0.128 µg/mL) mixed with sucrose solution exhibited significantly lower DENV infection rates and body titers after a 14-d IP. During the 7-d IP, one mosquito (sugar group) transmitted ZIKV. During the 14-d IP, 100% of mosquitoes showed body and leg ZIKV infections, and one mosquito (sugar+bifenthrin group) transmitted ZIKV. This is a preliminary communication, and more studies will be required to further investigate these findings. We expect the findings of this small-scale study to spur larger-scale investigations of the impacts of insecticides on mechanisms regulating vector competence, and exposure to other active ingredients, and aid in development of new insecticides.

Diet-Induced Nutritional Stress and Pathogen Interference in Wolbachia-Infected Aedes aegypti

The pathogen interference phenotype greatly restricts infection with dengue virus (DENV) and other pathogens in Wolbachia-infected Aedes aegypti, and is a vital component of Wolbachia-based mosquito control. Critically, the phenotype's causal mechanism is complex and poorly understood, with recent evidence suggesting that the cause may be species specific. To better understand this important phenotype, we investigated the role of diet-induced nutritional stress on interference against DENV and the avian malarial parasite Plasmodium gallinaceum in Wolbachia-infected Ae. aegypti, and on physiological processes linked to the phenotype. Wolbachia-infected mosquitoes were fed one of four different concentrations of sucrose, and then challenged with either P. gallinaceum or DENV. Interference against P. gallinaceum was significantly weakened by the change in diet however there was no effect on DENV interference. Immune gene expression and H2O2 levels have previously been linked to pathogen interference. These traits were assayed for mosquitoes on each diet using RT-qPCR and the Amplex Red Hydrogen Peroxide/Peroxidase Assay Kit, and it was observed that the change in diet did not significantly affect immune expression, but low carbohydrate levels led to a loss of ROS induction in Wolbachia-infected mosquitoes. Our data suggest that host nutrition may not influence DENV interference for Wolbachia-infected mosquitoes, but Plasmodium interference may be linked to both nutrition and oxidative stress. This pathogen-specific response to nutritional change highlights the complex nature of interactions between Wolbachia and pathogens in mosquitoes.

Larval food quantity affects the capacity of adult mosquitoes to transmit human malaria

Adult traits of holometabolous insects are shaped by conditions experienced during larval development, which might impact interactions between adult insect hosts and parasites. However, the ecology of larval insects that vector disease remains poorly understood. Here, we used Anopheles stephensi mosquitoes and the human malaria parasite Plasmodium falciparum, to investigate whether larval conditions affect the capacity of adult mosquitoes to transmit malaria. We reared larvae in two groups; one group received a standard laboratory rearing diet, whereas the other received a reduced diet. Emerging adult females were then provided an infectious blood meal. We assessed mosquito longevity, parasite development rate and prevalence of infectious mosquitoes over time. Reduced larval food led to increased adult mortality and caused a delay in parasite development and a slowing in the rate at which parasites invaded the mosquito salivary glands, extending the time it took for mosquitoes to become infectious. Together, these effects increased transmission potential of mosquitoes in the high food regime by 260-330%. Such effects have not, to our knowledge, been shown previously for human malaria and highlight the importance of improving knowledge of larval ecology to better understand vector-borne disease transmission dynamics.