Both mosquito species associated with Zika virus, dengue, yellow fever and chikungunya exhibit a behavior called skip-oviposition. Unfortunately, this does not mean that Aedes mosquitoes skip the oviposition portion of their gonotrophic cycle. The term actually describes a behavior where Aedes aegypti and Aedes albopictus skip from oviposition site to oviposition site to distribute their eggs. This behavior limits the efficacy of mosquito control measures that are focused on eliminating larval habitats. The World Health Organization recommends habitat elimination as one facet of Integrated Vector Management.
A 2013 study found that Aedes aegypti would deposit the majority of its eggs in one site and distribute the rest in groups of 1-30 among other oviposition sites. This behavior did not change significantly from one gonotrophic cycle to another, although the total number of eggs deposited was lower for older females. This study was carried out entirely under laboratory conditions, so the results may not be fully representative of mosquito behavior in the wild. However, field studies using ovitraps tend to find that a majority of the traps contain less than 30 eggs.
While skip-oviposition behavior make it more difficult to control mosquitoes’ potential breeding sites, it does provide an upside. By taking the time to seek out multiple oviposition sites, the mosquitoes extend the duration of each gonotrophic cycle. Considering that Aedes mosquitoes can produce a new batch of eggs within 24 hours of finishing the last batch, any delay to their reproduction is welcome. Even a small delay of a few hours may represent a 10% loss, but the study mentioned above found that the mosquitoes would continue skip-oviposition for up to 72 hours after first being introduced to potential breeding sites. When propagated through the many generations of mosquitoes that can develop in the course of a season, skip-oviposition behavior represents a large reduction in potential vectors for Zika and other infectious diseases.
Taking Advantage of Skip-Oviposition Behavior for Mosquito Population Control
We know that Aedes aegypti and Aedes albopictus mosquitoes prefer not to share their oviposition sites with each other. However, they are excited to share amongst individuals of their own species. We will touch more on this phenomenon in a future post. It is also reasonable to assume that we cannot control all potential oviposition sites in a real world scenario. Can these pieces be used in a solution?
A 2003 study at North Carolina State University found that the insect growth regulator pyriproxyfen was effective at inhibiting the development from larva to adult Aedes mosquito. At only 0.2 parts per billion (approximately 0.2 micrograms per liter water), pyriproxyfen kept 50% of Aedes albopictus larvae from reaching adulthood. This is such a small amount of pyriproxyfen that it could be transferred to an untreated or uncontrolled oviposition site on the legs of ovipositing mosquitoes. A single female visiting a clean container reduced adult emergence by 4-30%, but it could reduce adult emergence by as much as 73% with five treated females visiting the clean oviposition site. The NC State study also found that females who had been exposed to pyriproxyfen in their first gonotrophic cycle had a 30% lower egg hatch rate in later batches.
Brazil began introducing pyriproxyfen in their drinking water in 2014 to reduce the number of mosquito larvae growing around residences. This was a response to growing insecticide resistance among mosquitoes in the country, and was met with some criticism. Pyriproxyfen’s toxicity in mammals does not begin to show until you reach a dosage of 5 grams per kg of body weight. This means that the average person would need to ingest more than 9 million liters of water to reach a lethal dose. The World Health Organization advises the use of 0.01 mg/l pyriproxyfen in drinking water in areas where dengue vectors are prolific.
Pyriproxyfen’s efficacy is significantly reduced after 10 days of use. As mosquito populations are reduced from a variety of measures, the probability that any given uncontrolled oviposition site will continually be treated with pyriproxyfen falls proportionally. Without complete control of mosquito oviposition sites pyriproxyfen alone will not resolve the Zika crisis. However, it is effective enough to be worth consideration as one of multiple facets in a vector control program.
Program Manager for Biomedical Engineering
Eagle Medical Services, LLC