Innate and learned responses of foraging predatory mites to polar and non-polar fractions of thrips’ chemical cues

Abstract

Plant-inhabiting predatory mites (Acari, Phytoseiidae) are important natural enemies of herbivorous mites and insects and globally used in biological control in diverse agro-ecosystems. Foraging predatory mites predominantly use chemical cues to find plants harboring prey, to locate prey patches within plants and to detect prey individuals within patches. While a great deal of research has been dedicated to the response of predatory mites to herbivore-induced plant volatiles, comparatively little is known about close range chemosensory orientation such as the response to kairomones emanating from prey bodies. We addressed this issue in the interaction between the predatory mite Amblyseius swirskii Athias-Henriot and its prey Western flower thrips Frankliniella occidentalis Pergande. First, we assessed the predators’ innate response to polar and non-polar fractions of whole-body extracts of thrips, which were generated by four solvents, i.e. hexane (strongly non-polar), ethyl acetate (moderately non-polar), ethanol (moderately polar) and distilled water (strongly polar). The predators avoided the non-polar hexane extract, did not show a clear response to the ethyl acetate and ethanol extracts, and were strongly attracted by the polar water extract. Second, we evaluated the effects of early life experience with the aversive hexane and attractive water extracts on the foraging behavior of adult predators presented live thrips. The predators memorized which type of cue they had experienced early in life. Experience of the aversive hexane extract reduced, whereas experience of the attractive water extract enhanced, the attack likelihood and predation rate on thrips, relative to thrips extract-naïve predators. GC–MS analysis of the hexane extract revealed six major compounds, i.e. decyl acetate, dodecyl acetate, linolenic acid, pentacosan, heptacosane and an unidentified unsaturated acetate. Decyl acetate and dodecyl acetate are well known compounds released by thrips upon predation threat; whether the other four compounds convey any information to the predators is unknown. Our study provides evidence that polar and non-polar fractions of thrips’ chemical cues elicit opposite responses in foraging predatory mites. The finding that early learning of thrips chemical cues profoundly changes the predators’ foraging success later in life may be exploited to optimize biological control of thrips.