Behavioural and antennal responses of Aedes aegypti (l.) (Diptera: Culicidae) gravid females to chemical cues from conspecific larvae

Abstract

Mass trapping of gravid females represents one promising strategy for the development of sustainable tools against Aedes aegypti. However, this technique requires the development of effective odorant lures that can compete with natural breeding sites. The presence of conspecific larvae has been shown to stimulate oviposition. Hence, we evaluated the role of four major molecules previously identified from Ae. aegypti larvae (isovaleric, myristoleic, myristic [i.e. tetradecanoic], and pentadecanoic acids) on the oviposition of conspecific females, as well as their olfactory perception to evaluate their range of detection. Using flight cage assays, the preference of gravid females to oviposit in water that previously contained larvae (LHW) or containing the four larval compounds was evaluated. Then, compounds and doses inducing the highest stimulation were challenged for their efficacy against LHW. Only isovaleric acid elicited antennal response, suggesting that the other compounds may act as taste cues. Pentadecanoic acid induced significant oviposition stimulation, especially when dosed at 10 ppm. Myristoleic acid and isovaleric acid deterred oviposition at 10 and 100 ppm, while no effect on oviposition was observed with myristic acid irrespectively of the dose tested. When the four compounds were pooled to mimic larvae's chemical signature, they favored oviposition at 1 ppm but negatively affected egg-laying at higher concentrations. When properly dosed, pentadecanoic acid and the blend of compounds may be promising lures for ovitraps as they could compete with LHW. Due to their low volatility, their effect should be further evaluated under field conditions, in addition with long-range attractants for developing effective tools against gravid females.

Fig 1. Oviposition activity index (OAI) of Ae. aegypti gravid females towards larval holding water (LHW) at three different densities: 5, 20 and 100 larvae / mL (mean ± S.E.M., n = 5 replicates per density).

OAI value of 0 indicates no difference in oviposition between LWH and UPW bowls. Different letters indicate significant differences between densities (Tukey’s post-hoc test: P < 0.05).

Fig 2. Oviposition activity index (OAI) of Ae. aegypti gravid females towards the different synthetic solutions at four different doses: 0.1; 1; 10 and 100 ppm (mean ± S.E.M., n = 5 replicates per condition).

Asterisks show significant effect of the condition over water (paired Student’s t.test: * P < 0.05, ** P < 0.01, *** P < 0.001).

Fig 3

A: Normalized antennal responses of engorged Aedes aegypti females towards synthetic compounds at 10⁻⁴ g (mean ± S.E.M., n = 10 individuals per group). Normalized EAG values correspond to the response to the compound divided by the response to the solvent (i.e. hexane). Light blue bars correspond to females before oviposition (9–12 days old); dark blue bars correspond to females after oviposition (12–15 days old). Asterisks indicate significant differences in the antennal perception compared to the solvent (Tukey’s post-hoc test: *** P < 0.001). B: Exemplar traces obtained from Autospike® software for the two groups of mosquitoes toward the different test solutions.

Fig 4. Antennal dose-responses of engorged Aedes aegypti females towards the four carboxylic acids and the positive control at 5 different doses, from 10⁻⁸ to 10⁻⁴ g following a log₁₀ increase (mean ± S.E.M.).

A: Isovaleric acid (n = 6). B: Myristoleic acid (n = 5). C: Myristic (= tetradecanoic) acid (n = 5). D: Pentadecanoic acid (n = 5). E: 1-octen-3-ol (positive control, n = 7). Different lowercase letters indicate significant differences in perception between doses within a compound (Tukey’s post-hoc: P < 0.05).