Cucurbitacins as feeding and oviposition deterrents to insects

Cucurbitacins, the bitter triterpenes common to all Cucurbitaceae, are thought to be potent feeding deterrents for all insects not adapted to exploiting cucurbits. Yet appropriate choice tests not confounded by possible effects from phytochemical induction have only been conducted on a few chrysomelid leaf beetle spp. To examine the extent to which cucurbitacins deter feeding across several taxa of mandibulate and haustellate insects, we conducted feeding preference tests, pitting host tissues exogenously coated with a methanol dilution of cucurbitacin B against tissues coated with only methanol. Four mandibulate herbivores [Popillia japonica Newman, Cerotoma trifurcata (Forster) Leptinotarsa decemlineata (Say) and Trichoplusia ni (Hubner)], 2 mandibulate detritivores [Tenebrio molitor L. and Nauphoeta cinerea (Olivier)] and 3 haustellate herbivores [Gargaphia solani Heidemann, Corythucha ciliata (Say), Peregrinus maidis (Ashmead), and Acyrthosiphon pisum (Harris)] were examined. We also investigated the ability of cucurbitacin B to deter Ostrinia nubilalis (Hubner) and Spodoptera exiqua (Hubner) females from ovipositing on heated substrates. Two patterns emerged from our feeding assays: nonadapted insects with mandibulate mouthparts were deterred from feeding on food tainted with cucurbitacins, whereas insects with haustellate mouthparts preferred cucurbitacin-treated food. Oviposition assays with O. nubilalis and S. exiqua revealed significant deterrence at doses of 15 mu g cucurbitacin B per square centimeter. The data suggest that cucurbitacins are generalized antifeedants for non-adapted mandibulate insects but may actually be phagostimulants for phytophagous insects with haustellate mouthparts. We report on the oviposition deterrence for this class of compounds.