Oviposition deterrents in larval frass of the cotton boll worm, Helicoverpa armigera (Lepidoptera: Noctuidae): chemical identification and electroantennography analysis.

Oviposition deterrents in the frass of cotton bollworm (CBW), Helicoverpa armigera larvae fed on an artificial diet (FA) and on cotton Gossypium hirsutum leaves (FC) were investigated by behavioral bioassays and electroantennography analyses in the laboratory. It was found that a water suspension or a hexane extract of the frass FA or FC, in contrast to the corresponding foods, significantly deterred oviposition of conspecifics. When hexane extracts of the frass FA and FC were further partitioned into polar and neutral lipid fractions, two polar fractions significantly reduced oviposition. The neutral fraction from frass FC also exhibited significant deterrence, although the activity was much lower than that of the corresponding polar fraction. The polar lipid fractions contained several fatty acids, mainly palmitic and oleic acid at the ratio nearly 1:1. A blend of authentic fatty acids of the same composition found in frass FA or FC mimicked the deterring effect. Moreover, these fatty acids and their blend at the ratio found in frass FA or FC elicited significant electroantennogram responses and typical dose-response curves. Thus, it is suggested that CBW larvae may deploy two types of oviposition deterrents: a non-specific and a specific one. The former is a blend of fatty acids, independent of food and plays an important role in oviposition deterrence, whereas the latter may be produced only when the larvae feed on cotton leaves. The possible explanations of this deployment have also been discussed.