No abstract

Dengue, chikungunya and zika viruses are pathogens with an increasing global impact. In the absence of an approved vaccine or therapy, their management relies on controlling the mosquito vectors. But traditional controls are inadequate, and the range of invasive species such as Aedes albopictus (Asian Tiger Mosquito) is expanding. Genetically modified mosquitoes are being tested, but their use has encountered regulatory barriers and public opposition in some countries. Wolbachia bacteria can cause a form of conditional sterility, which can provide an alternative to genetic modification or irradiation. It is unknown however, whether openly released, artificially infected male Ae. albopictus can competitively mate and sterilize females at a level adequate to suppress a field population. Also, the unintended establishment of Wolbachia at the introduction site could result from horizontal transmission or inadvertent female release. In 2014, an Experimental Use Permit from the United States Environmental Protection Agency approved a pilot field trial in Lexington, Kentucky, USA. Here, we present data showing localized reduction of both egg hatch and adult female numbers. The artificial Wolbachia type was not observed to establish in the field. The results are discussed in relation to the applied use of Wolbachia-infected males as a biopesticide to suppress field populations of Ae. albopictus.

As part of the response to autochthonous Zika transmission in the United States, the City of South Miami implemented a 6-mo period in which Wolbachia-infected WB1 Aedes aegypti (L.) males were released into an ~170-acre area. Intracellular Wolbachia bacteria infections in Ae. aegypti cause early embryonic arrest (known as cytoplasmic incompatibility [CI]) and egg hatch failure, and inundative introductions have been suggested as a potential control tool. Throughout the release period, the Ae. aegypti population was monitored within both the release area and an equivalent area that did not receive WB1 male releases. The results show a significant reduction in egg hatch at the area receiving WB1 males, which is consistent with expectations for CI. Similarly, the number of Ae. aegypti was significantly reduced at the area receiving WB1 males, relative to the untreated area. The observed population reduction and results encourage additional work and replication of the Wolbachia biopesticide approach against Ae. aegypti, as an additional tool to be integrated with existing control tools for the control of this medically important vector and nuisance pest.

BackgroundThe auto-dissemination approach has been shown effective at treating cryptic refugia that remain unaffected by existing mosquito control methods. This approach relies on adult mosquito behavior to spread larvicide to breeding sites at levels that are lethal to immature mosquitoes. Prior studies demonstrate that ‘dissemination stations,’ deployed in mosquito-infested areas, can contaminate adult mosquitoes, which subsequently deliver the larvicide to breeding sites. In some situations, however, preventative measures are needed, e.g., to mitigate seasonal population increases. Here we examine a novel approach that combines elements of autocidal and auto-dissemination strategies by releasing artificially reared, male mosquitoes that are contaminated with an insecticide.MethodologyLaboratory and field experiments examine for model-predicted impacts of pyriproxyfen (PPF) directly applied to adult male Aedes albopictus, including (1) the ability of PPF-treated males to cross-contaminate females and to (2) deliver PPF to breeding sites.Principal FindingsSimilar survivorship was observed in comparisons of PPF-treated and untreated males. Males contaminated both female adults and oviposition containers in field cage tests, at levels that eliminated immature survivorship. Field trials demonstrate an ability of PPF-treated males to transmit lethal doses to introduced oviposition containers, both in the presence and absence of indigenous females. A decline in the Ae. albopictus population was observed following the introduction of PPF-treated males, which was not observed in two untreated field sites.Conclusions/SignificanceThe results demonstrate that, in cage and open field trials, adult male Ae. albopictus can tolerate PPF and contaminate, either directly or indirectly, adult females and immature breeding sites. The results support additional development of the proposed approach, in which male mosquitoes act as vehicles for insecticide delivery, including exploration of the approach with additional medically important mosquito species. The novelty and importance of this approach is an ability to safely achieve auto-dissemination at levels of intensity that may not be possible with an auto-dissemination approach that is based on indigenous females. Specifically, artificially-reared males can be released and sustained at any density required, so that the potential for impact is limited only by the practical logistics of mosquito rearing and release, rather than natural population densities and the self-limiting impact of an intervention upon them.

Wolbachia pipientis Hertig and Wolbach (Rickettsiales: Rickettsiaceae) are intracellular α-proteobacteria that occur naturally in Aedes albopictus (Skuse) (Diptera: Culicidae) and numerous other invertebrates. These endosymbionts can invade host populations by manipulating host reproduction. Wolbachia infections have been shown to impart both costs and benefits to hosts in terms of development, survival, and fecundity. Here, we monitor intraspecific competition among independent cohorts of infected or uninfected larvae. Levels of competition are manipulated by varying initial larval densities and food levels. Although larval density is observed to have major impacts on immature survivorship, sex ratio of eclosing adults, and developmental rates, the Wolbachia infection status had minimal impact on male immatures and no effect on immature females under these experimental conditions. Female and male immatures were observed to respond differently to competitive pressure, with the functional relationships of females and males consistent with scramble and contest competition, respectively. The results are discussed in relation to the evolution of naturally occurring Wolbachia infections in Ae. albopictus (i.e., natural population replacement events) and public health strategies that propose the manipulation of Wolbachia infections in Ae. albopictus populations.