Feeding deterrence and toxicity of neem triterpenoids

The neem tree, Azadirachta indica, produces a plethora of triterpenoids. One of these is azadirachtin, a complex compound with powerful insecticidal properties. The biosynthesis of azadirachtin starts with a steroid precursor and proceeds via two levels of structural complexity, i.e., furan ring formation and C-ring opening, after which further modifications yield azadirachtin. Antifeedant and toxic properties of azadirachtin against insects were compared with those of various compounds from the two lower levels of structural complexity. To larvae of the moth Spodoptera littoralis, several compounds from the lower structural levels showed strong antifeedant activity, comparable to that of azadirachtin. Nymphs of the locust Schistocerca gregaria in antifeedancy tests were more sensitive to azadirachtin than to the other compounds tested. To Spodoptera littoralis, Schistocerca gregaria, and the bug Oncopeltus fasciatus, azadirachtin was very toxic, exerting severe growth and molt disrupting effects. In contrast, compounds from the lower structural levels than azadirachtin showed virtually no toxicity to these three insects. Thus, although several simpler triterpenoid structures other than azadirachtin have insect antifeedant activity, the combination of strong antifeedant 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 toxicity of individual neem triterpenoids