Characterization of the swarming behavior of Anopheles coluzzii and Anopheles gambiae (Diptera: Culicidae) populations in a hybrid zone of Senegal

Anopheles gambiae and Anopheles coluzzii, often found in sympatry and synchronous, have undergone a premating reproductive isolation across their distribution range. However, in the Western coast of Africa, unexpected hybridization zones have been observed, and little is known about swarming behavior of these cryptic taxa. Here, we characterized the swarming behavior of An. coluzzii and An. gambiae to investigate its role in the high hybridization level in Senegal. The study was conducted in the south and central Senegal during the 2018 rainy season. Mating swarms of malaria vectors were surveyed at sunset and collected using an insect net. Meanwhile, indoor resting populations of malaria vectors were collected by pyrethrum spray catches. Upon collection, specimens were identified morphologically, and then members of the An. gambiae complex were identified at the species level by polymerase chain reaction (PCR). An. gambiae swarmed mainly over bare ground, whereas An. coluzzii were found swarming above various objects creating a dark-light contrast with the bare ground. The swarms height varied from 0.5 to 2.5 m. Swarming starting time was correlated with sunset whatever the months for both species, and generally lasted about 10 min. No mixed swarm of An. gambiae and An. coluzzii was found even in the high hybridization area. These results indicated a premating isolation between An. coluzzii and An. gambiae. However, the high hybridization rate in the sympatric area suggests that heterogamous mating is occurring, thus stressing the need for further extensive studies.

Swarming and mate selection in Anopheles gambiae mosquitoes (Diptera: Culicidae)

Treating both male and female Anopheles gambiae as if they are "boids" (a computer program that mimics flocking in birds) explains much of the swarming and mating behavior in this important group of malaria vectors. It is suggested that species specific swarm sites act as the mate recognition system in anophelines and it is proposed that virgin females respond to the swarm site per se rather than the swarm itself. Given the high operational sex ratio and the inability of any male to dominate all females within the swarm, it is considered that chance, rather than sexual selection, is the most important determinant of mating. The male being in the swarm may be a sufficiently strong signal to the female of his fitness, so that more elaborate sexual selection is unnecessary. The possibility of alternative mechanisms for mating may also exist but need to be investigated further. Given the importance of swarms as the isolating mechanism between species, emphasis should be placed on determining the characteristics of swarm sites and markers between them.

The Potential of a New Beauveria bassiana Isolate for Mosquito Larval Control

The African malaria mosquito, Anopheles gambiae Giles (Diptera: Culicidae), and the Asian tiger mosquito, Aedes albopictus Skuse (Diptera: Culicidae) are of public health concern due to their ability to transmit disease-causing parasites and pathogens. Current mosquito control strategies to prevent vector-borne diseases rely mainly on the use of chemicals. However, insecticide resistance in mosquito populations necessitates alternative control measures, including biologicals such as entomopathogenic fungi. Here we report the impact of a new Beauveria bassiana (Balsamo) Vuillemin (Hyprocreales: Cordycipitaeceae) isolate, isolated from field-collected Ae. albopictus larvae on mosquito survival and development. Larval infection bioassays using three B. bassiana conidial concentrations were performed on the second and third larval instars of An. gambiae and Ae. albopictus mosquitoes. Larvae were monitored daily for survival and development to pupae and adults. Our results show that B. bassiana MHK was more effective in killing An. gambiae than Ae. albopictus larvae. We further observed delays in development to pupae and adults in both mosquito species exposed the varying concentrations of B. bassiana as compared to the water control. In addition, larval exposure to B. bassiana reduced adult male and female survival in both mosquito species, further contributing to mosquito population control. Thus, this study identifies a new B. bassiana isolate as a possible biological control agent of two mosquito species of public health concern, increasing the arsenal for integrated mosquito control.

Spatial Distribution of PCR-Identified Species of Anopheles gambiae senu lato (Diptera: Culicidae) Across Three Eco-Vegetational Zones in Cross River State, Nigeria

Anopheles gambiae sensu lato complex (An. gambiae s.l.) describes a group of nine morphologically indistinguishable members that vary in their distribution, ability to transmit malaria, and susceptibility to pyrethroids. Here, we recorded the spatial patterns of PCR-identified An. gambiae s.l. complex species collected from four sites in Cross River State, Nigeria that represented three different ecological zones. Trapping was conducted between October 2015 and June 2016. Anopheles gambiae s.l. complex species identification was performed using species-specific primers followed by An. gambiae and An. coluzzii differentiation using the restriction fragment length polymorphism (RFLP) method. Bivariate and multivariate logistic regression models were used to identify ecological and seasonal variables closely associated with An. coluzzii and An. gambiae distribution. Out of 1,388 An. gambiae s.l. successfully amplified, 1,074 (77.4%) were An. coluzzii, 278 (20%) were An. gambiae, and 25 (1.8%) were hybrids (An. coluzzii/An. gambiae). A very small number of An. arabiensis (0.8%, n = 11) were also collected. Statistical analysis indicated that An. coluzzii is predominant in Guinea-savannah and tropical rainforest, and is highly associated with rainy seasons, while, An. gambiae is prevalent in mangrove swamp forest during dry seasons. Only 13 An. gambiae s.l. females were infected with Plasmodium falciparum (P. falciparum). The sporozoite infection rate was higher in mangrove swamp forest (53.8%, n = 7) than in rain forest (38.5%, n = 5) followed by Guinea-savannah (7.7%, n = 1) ecological zones. These results provide important insights for strategic planning of malaria control programs in Nigeria.

Interplay Between Oxytetracycline and the Homozygote kdr (L1014F) Resistance Genotype on Fecundity in Anopheles gambiae (Diptera: Culicidae) Mosquitoes

The insecticide resistance in Anopheles gambiae mosquitoes has remained the major threat for vector control programs but the fitness effects conferred by these mechanisms are poorly understood. To fill this knowledge gap, the present study aimed at testing the hypothesis that antibiotic oxytetracycline could have an interaction with insecticide resistance genotypes and consequently inhibit the fecundity in An. gambiae. Four strains of An. gambiae: Kisumu (susceptible), KisKdr (kdr (L1014F) resistant), AcerKis (ace-1 (G119S) resistant) and AcerKdrKis (both kdr (L1014F) and ace-1 (G119S) resistant) were used in this study. The different strains were allowed to bloodfeed on a rabbit previously treated with antibiotic oxytetracycline at a concentration of 39·10-5 M. Three days later, ovarian follicles were dissected from individual mosquito ovaries into physiological saline solution (0.9% NaCl) under a stereomicroscope and the eggs were counted. Fecundity was substantially lower in oxytetracycline-exposed KisKdr females when compared to that of the untreated individuals and oxytetracycline-exposed Kisumu females. The exposed AcerKis females displayed an increased fecundity compared to their nontreated counterparts whereas they had reduced fecundity compared to that of oxytetracycline-exposed Kisumu females. There was no substantial difference between the fecundity in the treated and untreated AcerKdrKis females. The oxytetracycline-exposed AcerKdrKis mosquitoes had an increased fecundity compared to that of the exposed Kisumu females. Our data indicate an indirect effect of oxytetracycline in reducing fecundity of An. gambiae mosquitoes carrying kdrR (L1014F) genotype. These findings could be useful for designing new integrated approaches for malaria vector control in endemic countries.

Phenotypic Insecticide Resistance in Anopheles gambiae (Diptera: Culicidae): Specific Characterization of Underlying Resistance Mechanisms Still Matters

An effective control of malaria vectors requires an extensive knowledge of mechanisms underlying the resistance-phenotypes developed by these vectors against insecticides. We investigated Anopheles gambiae mosquitoes from Benin and Togo for their intensity of insecticide resistance and we discussed the involvement of genotyped mechanisms in the resistance-phenotypes observed. Three- to five-day-old adult mosquitoes emerged from field and laboratory An. gambiae larvae were assayed using WHO tube intensity tests against various doses of deltamethrin: 1× (0.05%); 2× (0.1%); 5× (0.25%); 7.5× (0.375%) and those of pirimiphos-methyl: 0.5× (0.125%); 1× (0.25%). Members of An. gambiae complex were screened in field populations using polymerase chain reaction (PCR) assays. The presence of kdrR(1014F/1014S) and ace-1R(119S) mutations was also investigated using TaqMan and PCR-RFLP techniques, respectively. Anopheles gambiae from field were very resistant to deltamethrin, whereas KisKdr and AcerKdrKis strains displayed 100% mortality rates at 2× the diagnostic dose. In contrast, the field mosquitoes displayed a low resistance-intensity against 1× the diagnostic dose of pirimiphos-methyl, whereas AcerKis and AcerKdrKis strains showed susceptibility at 0.5× the diagnostic dose. Anopheles gambiae s.s., Anopheles coluzzii, and Anopheles arabiensis were identified. Allelic frequencies of kdrR (1014F) and ace-1R (119S) mutations in the field populations varied from 0.65 to 1 and 0 to 0.84, respectively. The field An. gambiae displayed high-resistance levels against deltamethrin and pirimiphos-methyl when compared with those of the laboratory An. gambiae-resistant strains. These results exhibit the complexity of underlying insecticide resistance mechanisms in these field malaria vectors.

A Malaria Transmission Model Predicts Holoendemic, Hyperendemic, and Hypoendemic Transmission Patterns Under Varied Seasonal Vector Dynamics

A model is developed of malaria (Plasmodium falciparum) transmission in vector (Anopheles gambiae) and human populations that include the capacity for both clinical and parasite suppressing immunity. This model is coupled with a population model for Anopheles gambiae that varies seasonal with temperature and larval habitat availability. At steady state, the model clearly distinguishes uns hypoendemic transmission patterns from stable hyperendemic and holoendemic patterns of transmission. The model further distinguishes hyperendemic from holoendemic disease based on seasonality of infection. For hyperendemic and holoendemic transmission, the model produces the relationship between entomological inoculation rate and disease prevalence observed in the field. It further produces expected rates of immunity and prevalence across all three endemic patterns. The model does not produce mesoendemic transmission patterns at steady state for any parameter choices, leading to the conclusion that mesoendemic patterns occur during transient states or as a result of factors not included in this study. The model shows that coupling the effect of varying larval habitat availability with the effects of clinical and parasite-suppressing immunity is enough to produce known patterns of malaria transmission.

High Levels of Knockdown Resistance in Anopheles coluzzii and Anopheles gambiae (Diptera: Culicidae), Major Malaria Vectors in Togo, West Africa: A 2011 Monitoring Report

A survey of susceptibility to DDT, deltamethrin, bendiocarb, and chlorpyrifos-methyl was conducted in five localities in 2011 in Togo, West Africa, to assess the insecticide resistance status of Anopheles gambiae s.l. (Diptera: Culicidae). Female populations of An. gambaie s.l. emerged from collected larvae (F0) were exposed to insecticide-impregnated papers using World Health Organization test kits for adult mosquitoes; the susceptible reference strain Kisumu was used as a control. Resistance to DDT and deltamethrin was observed within the mosquito populations tested. Anopheles gambiae s.s. and Anopheles coluzzii represented the only species recorded in the study sites. The frequency of knockdown resistance (kdr L1014F) mutation determined using polymerase chain reaction diagnostic tests was lower in An. gambiae than in An. coluzzii in all of the localities except Kolokopé. Further investigations of An. gambiae s.l. resistance are needed in Togo to help the National Malaria Control Programme in vector control decision making and implementation of resistance management strategy.

Survival Value and Sugar Access of Four East African Plant Species Attractive to a Laboratory Strain of Sympatric Anopheles gambiae (Diptera: Culicidae)

Mosquitoes derive energy from plant sugar, thereby promoting survival and reproduction. Its survival value to females plays a key role in the vectorial capacity of mosquito populations. Previous olfactometry assays of responsiveness demonstrated that Senna didymobotrya Fresenius, Parthenium hysterophorus, L. Senna occidentalis, (L) and Lantana camara L were among the most attractive plants for the Mbita strain of Anopheles gambiae s.s. Giles in eastern Africa. Here, we provide experimental evidence that three of these four species also provide varying but substantial amounts of sugar for mosquito survival, whereas a fourth does not. Rank order of survival of both sexes of mosquitoes housed with these plants was as follows: S. didymobotrya was highest, followed by S. occidentalis and L. camara, whereas survival on P. hysterophorus was only slightly better than on only water. A positive control group, housed with 10% sucrose, survived well but fell significantly short of those with S. didymobotrya. A causal connection between survival and sugar availability was established by exposing mosquitoes to plants overnight, and then testing them for the presence and amount of undigested fructose. Fructose positivity was most frequent in those exposed to L. camara, whereas greatest amounts of fructose were obtained from S. occidentalis and S. didymobotrya. Parthenium hysterophorus scored lowest in both categories. We conclude that attractiveness and sugar availability are often, but not always, concordant. It remains unclear why P. hysterophorus should be attractive if it offers little sugar and does not prolong survival. Furthermore, the cause behind the superior survival benefit of S. didymobotrya, compared with 10% sucrose, is unknown.