AZADIRACHTIN - AN UPDATE

Research into the insecticidal effects of azadirachtin, a limonoid fro m the Indian Neem tree, Azadirachta indica, has been ongoing for some 30 years. Its strong antifeedant, insect growth regulatory and reprodu ctive effects are now well understood and documented although its bioc hemical effects at the cellular level are still unknown. Antifeedancy varies markedly between species with Lepidoptera being particularly se nsitive to azadirachtin. The physiological effects on growth, moulting and reproduction are more consistent between species although cuticle or gut may provide barriers to bioavailability in some species. The m ode of action of azadirachtin lies in (i) effects on deterrent and oth er chemoreceptors resulting in antifeedancy (ii) effects on ecdysteroi d and juvenile hormone titres through a blockage of morphogenetic pept ide hormone release (e.g. PTTH; allatotropins) and (iii) direct effect s on most other tissues studied resulting in an overall loss of fitnes s of the insect. The complexity of the molecular structure of azadirac htin has precluded its synthesis for pesticide use although novel synt hesis of the parent molecule is now almost complete and research into simpler mimetic substances is ongoing. Applied research has concentrat ed on a variety of natural formulations from neem seed kernels which c ontain azadirachtin together with several structurally related molecul es. This review brings the reader up to date with both pure and applie d research in the field and provides a detailed overview of present th inking into the mode of action of azadirachtin. Wherever possible comp arative approaches have been made between species of the effects of pu re azadirachtin and areas for future research are indicated.