FEEDING DETERRENCE AND TOXICITY OF NEEM TRITERPENOIDS

The neem tree, Azadirachta indica, produces a plethora of triter penoi ds. One of these is azadirachtin, a complex compound with powerful ins ecticidal properties. The biosynthesis of azadirachtin starts with a s teroid precursor and proceeds via two levels of structural complexity, i.e., furan ring formation and C-ring opening, after which further mo difications yield azadirachtin. Antifeedant and toxic properties of az adirachtin against insects were compared with those of various compoun ds from the two lower levels of structural complexity. To larvae of th e moth Spodoptera littoralis, several compounds from the lower structu ral levels showed strong antifeedant activity, comparable to that of a zadirachtin. Nymphs of the locust Schistocerca gregaria in antifeedanc y tests were more sensitive to azadirachtin than to the other compound s rested. To Spodoptera littoralis, Schistocerca gregaria, and the bug Oncopeltus fasciatus, azadirachtin was very toxic, exerting severe gr owth and molt disrupting effects. In contrast, compounds from the lowe r structural levels than azadirachtin showed virtually no toxicity to these three insects. Thus, although several simpler triterpenoid struc tures other than aza dirachtin have insect antifeedant activity, the c ombination of strong anti feedant and toxic properties of azadirachtin may provide the more important chemical protection of the neem tree. Our data reveal no explicit link between antifeedant activity and toxi city of individual neem triterpenoids.