Oviposition attractants and oviposition stimulants for Culex mosquitoes

J. G. Millar

Dept. of Entomology, Univ. of California, Riverside CA 92521, USA

Culex mosquito oviposition is mediated by chemical and physical cues that are associated with typical oviposition sites such as ponds and other bodies of stagnant water that contain decomposing plant debris. We have identified a number of volatile chemical cues that influence mosquito oviposition from aqueous infusions of decaying plant materials mimicking natural oviposition waters. Compounds were identified both by traditional bioassay-driven fractionation of solvent extracts of infusions, and by collection of headspace volatiles, followed by analysis by coupled gas chromatography-electroantennogram detection. Behaviorally and physiologically active compounds included indole and 3-methylindole, phenols, aldehydes, and sulfur compounds. We also investigated the development of bioassays to determine which steps in the oviposition sequence (attraction or oviposition stimulation) were influenced by various chemical cues. Several general conclusions could be drawn from experiments with Culex quinquefasciatus and Cx. tarsalis. First, concentration was critically important; compounds which were attractive or stimulatory at parts per billion levels in water became repellent at higher concentrations, particularly to Cx. tarsalis. Second, oviposition stimulation (treatment vs. control frequently > 10:1) appeared to be more strongly mediated by chemical cues than attraction (treatment vs. control, ~3:1). Third, blends of compounds were more biologically active than individual compounds, with the increase in activity being approximately additive rather than synergistic. Fourth, correct formulation of blends was important because the release rates of compounds from aqueous solutions was dependent on their hydrophobicity and hydrogen bonding abilities. Thus, if the relative proportions of compounds in the headspace volatiles from infusions were used to formulate synthetic blends, hydrophobic compounds were overrepresented, and hydrophilic compounds were underrepresented. Once blends had been adjusted to compensate for these effects, the corrected blends were more biologically active than the unadjusted blends. Overall, considerable progress has been made in identification of compounds that mediate Culex mosquito oviposition, but even the best synthetic blends are not yet as active as crude infusions of decaying plant material, indicating that further compounds remain to be discovered.

Index terms: Culex tarsalis, Culex quinquesfasciatus, oviposition stimulant, oviposition attractant

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