Measuring the Host-Seeking Ability of Aedes aegypti Destined for Field Release

Aedes albopictus colonies from different geographic origins differ in their sleep and activity levels but not in the time of peak activity

Mosquitoes occupy a wide range of habitats where they experience various environmental conditions. The ability of some species, such as the tiger mosquito, Aedes albopictus, to adapt to local conditions certainly contributes to their invasive success. Among traits that remain to be examined, mosquitoes' ability to time their activity with that of the local host population has been suggested to be of significant epidemiological importance. However, whether different populations display heritable differences in their chronotype has not been examined. Here, we compared laboratory strains originating from 8 populations from 3 continents, monitored their spontaneous locomotor activity patterns, and analyzed their sleep-like states. Overall, all strains showed conserved diurnal activity concentrated in the hours preceding the crepuscule. Similarly, they all showed increased sleep levels during the morning and night hours. However, we observed strain-specific differences in the activity levels at each phase of the day. We also observed differences in the fraction of time that each strain spends in a sleep-like state, explained by variations in the sleep architecture across strains. Human population density and the latitude of the site of geographic origin of the tested strain showed significant effects on sleep and activity patterns. Altogether, these results suggest that Ae. albopictus mosquitoes adapt to local environmental conditions via heritable adaptations of their chronotype.

Increased biting rate and decreased Wolbachia density in irradiated Aedes mosquitoes

Background

Releasing considerable numbers of radiation-sterilized males is a promising strategy to suppress mosquito vectors. However, releases may also include small percentages of biting females, which translate to non-negligible numbers when releases are large. Currently, the effects of irradiation on host-seeking and host-biting behaviors have not been exhaustively investigated. Information is also lacking regarding the effects of sterilizing treatment on the endosymbiotic bacterium Wolbachia, which is known to affect the vector competence of infected mosquitos.

Methods

To ascertain the effects of irradiation on females, the pupae of two Aedes albopictus strains, differing in their natural or artificial Wolbachia infection type, and Aedes aegypti—which is not infected by Wolbachia—were treated with various doses of X-rays and monitored for key fitness parameters and biting behavior over a period of 2 weeks. The effect of radiation on Wolbachia was investigated by quantitative polymerase chain reaction (qPCR) and fluorescence in situ hybridization (FISH) analysis.

Results

Partial Aedes albopictus female sterility was achieved at 28 Gy, but the number of weekly bites more than doubled compared to that of the controls. Radiation doses of 35 and 45 Gy completely inhibited progeny production but did not significantly affect the survival or flight ability of Ae. albopictus females and caused a tripling of the number of bites per female per week (compared to untreated controls). These results were also confirmed in Ae. aegypti after treatment at 50 Gy. Wolbachia density decreased significantly in 45-Gy-irradiated females, with the greatest decreases in the early irradiation group (26 ± 2-h-old pupae). Wolbachia density also decreased as adults aged. This trend was confirmed in ovaries but not in extra-ovarian tissues. FISH analysis showed a strongly reduced Wolbachia-specific fluorescence in the ovaries of 13 ± 1-day-old females.

Conclusions

These results suggest that, under sterile insect technique (SIT) programs, the vector capacity of a target population could increase with the frequency of the irradiated females co-released with the sterile males due to an increased biting rate. In the context of successful suppression, the related safety issues are expected to be generally negligible, but they should be conservatively evaluated when large-scale programs relying on imperfect sexing and high overflooding release ratios are run for long periods in areas endemic for arboviral diseases. Also, the effects of irradiation on the vector competence deserve further investigation.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05188-9.

Long-Range But Not Short-Range Attraction of Male Aedes aegypti (Diptera: Culicidae) Mosquitoes to Humans

Female Aedes aegypti (Linnaeus) mosquitoes integrate multiple sensory cues to locate human hosts for blood meals. Although male Ae. aegypti swarm around and land on humans in nature to mate, direct evidence of attraction to humans is limited. Male mosquito attraction to human host cues is often undetectable in confined laboratory assays, leading to a misconception that male mosquitoes are not attracted to humans. We used semifield experiments to demonstrate robust attraction of male Ae. aegypti to humans. Human-baited traps captured up to 25% of released males within 15 min, whereas control traps without humans as bait failed to capture males. Rapid attraction to humans was further demonstrated through videography. Males swarmed around and landed on human subjects, with no activity recorded in paired unbaited controls. Finally, we confirm the lack of discernible male attraction to humans in small laboratory cages. Our experiments demonstrate that both male and female Ae. aegypti show attraction to humans, but with clear sex-specific behavioral differences at short-range. Male mosquito attraction to humans is likely to be important for mating success in wild populations and its basis should be further explored. Our results highlight the importance of arena size and assay design for mosquito behavioral research. A better understanding of host cues that attract males could help us to improve mosquito surveillance and control.

Effect of BG-Lures on the Male Aedes (Diptera: Culicidae) Sound Trap Capture Rates

With global expansion of the two main vectors of dengue, Aedes aegypti (Linnaeus, Diptera: Culicidae) and Aedes albopictus (Skuse, Diptera: Culicidae), there is a need to further develop cost-effective and user-friendly surveillance tools to monitor the population dynamics of these species. The abundance of Ae. aegypti and Ae. Albopictus, and associated bycatch captured by Male Aedes Sound Traps (MASTs) and BG-Sentinel (BGS) traps that were unbaited or baited with BG-Lures were compared in Cairns, Australia and Madang, Papua New Guinea. Mean male Ae. aegypti and Ae. albopictus catch rates in MASTs did not significantly differ when deployed with BG-Lures. Similarly, males of both these species were not sampled at statistically different rates in BGS traps with or without BG-Lures. However, MASTs with BG-Lures caught significantly less male Ae. aegypti than BGS traps baited with BG-Lures in Cairns, and MASTs without BG-Lures caught significantly more male Ae. albopictus than BGS traps without BG-Lures in Madang. Additionally, BG-Lures significantly increased female Ae. aegypti catch rates in BGS traps in Cairns. Lastly, bycatch capture rates in BGS traps were not significantly influenced by the addition of the BG-Lures. While this study provides useful information regarding the surveillance of Ae. aegypti and Ae. albopictus in these locations, further development and investigation is required to successfully integrate an olfactory lure into the MAST system.

A diagnostic primer pair to distinguish between wMel and wAlbB Wolbachia infections

Detection of the Wolbachia endosymbiont in Aedes aegypti mosquitoes through real-time polymerase chain reaction assays is widely used during and after Wolbachia releases in dengue reduction trials involving the wMel and wAlbB strains. Although several different primer pairs have been applied in current successful Wolbachia releases, they cannot be used in a single assay to distinguish between these strains. Here, we developed a new diagnostic primer pair, wMwA, which can detect the wMel or wAlbB infection in the same assay. We also tested current Wolbachia primers and show that there is variation in their performance when they are used to assess the relative density of Wolbachia. The new wMwA primers provide an accurate and efficient estimate of the presence and density of both Wolbachia infections, with practical implications for Wolbachia estimates in field collected Ae. aegypti where Wolbachia releases have taken place.

Optimal release programs for dengue prevention using Aedes aegypti mosquitoes transinfected with wMel or wMelPop Wolbachia strains

In this paper, we propose a dengue transmission model of SIR(S)-SI type that accounts for two sex-structured mosquito populations: the wild mosquitoes (males and females that are Wolbachia-free), and those deliberately infected with either wMel or wMelPop strain of Wolbachia. This epidemiological model has four possible outcomes: with or without Wolbachia and with or without dengue. To reach the desired outcome, with Wolbachia and without dengue, we employ the dynamic optimization approach and then design optimal programs for releasing Wolbachia-carrying male and female mosquitoes. Our discussion is focused on advantages and drawbacks of two Wolbachia strains, wMelPop and wMel, that are recommended for dengue prevention and control. On the one hand, the wMel strain guarantees a faster population replacement, ensures durable Wolbachia persistence in the wild mosquito population, and requiters fewer releases. On the other hand, the wMelPop strain displays better results for averting dengue infections in the human population.

Wolbachia's Deleterious Impact on Aedes aegypti Egg Development: The Potential Role of Nutritional Parasitism

Simple Summary

Mosquito-borne viral diseases such as dengue, Zika and chikungunya cause a significant global health burden and are currently increasing in outbreak frequency and geographical reach. Wolbachia pipientis, an endosymbiotic bacterium, offers a solution to this. When Wolbachia is introduced into the main mosquito vector of these viruses, Aedes aegypti, it alters the mosquito’s reproductive biology, as well as reducing the ability of the mosquitoes to transmit viruses. These traits can be leveraged to reduce virus transmission within a community by mass releasing Wolbachia-infected mosquitoes. However, Wolbachia has some negative effects on Aedes aegypti fitness, particularly egg longevity, and the reason behind this remains ambiguous. Insect fitness is very important for the success for Wolbachia-biocontrol strategies as they rely on the released insects being competitive with the wild mosquito population. This review summarises the fitness effects of Wolbachia on Aedes aegypti and investigates the possible contribution of Wolbachia as a nutritional parasite in lowering host fitness. It proposes the next stages of research that can be conducted to address nutritional parasitism to aid in the expansion of Wolbachia-based disease management programs worldwide.

Abstract

The artificial introduction of the endosymbiotic bacterium, Wolbachia pipientis, into Aedes (Ae.) aegypti mosquitoes reduces the ability of mosquitoes to transmit human pathogenic viruses and is now being developed as a biocontrol tool. Successful introgression of Wolbachia-carrying Ae. aegypti into native mosquito populations at field sites in Australia, Indonesia and Malaysia has been associated with reduced disease prevalence in the treated community. In separate field programs, Wolbachia is also being used as a mosquito population suppression tool, where the release of male only Wolbachia-infected Ae. aegypti prevents the native mosquito population from producing viable eggs, subsequently suppressing the wild population. While these technologies show great promise, they require mass rearing of mosquitoes for implementation on a scale that has not previously been done. In addition, Wolbachia induces some negative fitness effects on Ae. aegypti. While these fitness effects differ depending on the Wolbachia strain present, one of the most consistent and significant impacts is the shortened longevity and viability of eggs. This review examines the body of evidence behind Wolbachia’s negative effect on eggs, assesses nutritional parasitism as a key cause and considers how these impacts could be overcome to achieve efficient large-scale rearing of these mosquitoes.