Abstract
Host location and acceptance by egg parasitoids can be mediated by close-range host stimuli. In this study, we tested the response of Trichogramma japonicum Ashmead to cuticular extracts of adult and larval rice yellow stem borer (YSB), Scripophaga incertulas Walker. We also studied the wasps' response to extracts from YSB larval frass. Laboratory bioassays revealed that hexane extracts of the adult host body stimulate ovipositor probing of T. japonicum. Extracts of larval frass also stimulated parasitization. In contrast, host larval cuticular extracts had no effect on parasitization rates. Fractionation of the crude extracts of adult YSB cuticular extracts was performed using silica gel chromatography, followed by bioassays of the individual fractions to test their effects on wasp behavior. Analyses of the most active fractions by gas chromatography-mass spectrometry revealed that the extract contained saturated long chain alkanes and alkenes, with carbon numbers ranging from C20 to C32. Hydrocarbons were applied onto host eggs to test their effects on parasitization rates. Treatments of eggs with docosane, tetracosane, pentacosane, and eicosane enhanced host egg parasitization, while pentadecane, hexadecane, and nonadecane deterred oviposition.
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