Selection of oviposition sites by Aedes aegypti: Behavior of gravid mosquitoes and mechanisms of attraction
Gravids preferred oviposition sites that contain larvae over those that contain only water. The degree of preference increased with increasing larval instar, and also increased with increasing larval density up to a density of 2 larvae per ml. At higher densities the response became negative. Larvae act as indicators of site-suitability, and may indicate imminent competition at high densities, regardless of the presence of contaminating microfauna. Gravids rely on olfactory and chemo-tactile cues to detect the presence of larvae at oviposition sites, with a greater reliance on olfactory cues. Visual cues are unimportant in the detection of larvae.
Gravids avoided ovipositing in a site already harboring eggs. This negative response increased with increasing egg density. Eggs are indicators of direct competition in the larval habitat. This avoidance reaction is mitigated by both olfactory and chemo-tactile cues. That egg distribution did not affect subsequent oviposition suggests the chemo-tactile response is more likely due to local concentration of chemicals than due to direct contact with eggs.
Females visit more smaller sites when all available oviposition sites are identical, and lay fewer eggs in smaller treatment sites, than they do compared to larger sites. The gravid response is independent of substrate surface area and water volume. Eggs were not distributed differently in oviposition sites of different size but equal diameter, suggesting females may select different size-sites based on water surface area. Size is probably an indirect measure of available resources.
The oviposition response does not vary with either chronological age of the gravid, nor with her gonotrophic cycle. Females separated in age by 2 weeks in post-emergence age or by 2 gonotrophic cycles responded to oviposition sites in the same manner. Oviposition behavior is flexible but does not take into account a female's relative fitness.
The response to oviposition sites containing an attractive density of larvae was further investigated. Females could not discriminate between control and treatment sites in a wind tunnel, even when released but 22 cm from the attractive treatment. The olfactory cues associated with larval rearing habitat act as close range oviposition attractants.
0573: Public health