AZADIRACHTIN - AN UPDATE

Research into the insecticidal effects of azadirachtin, a limonoid from the Indian Neem tree, Azadirachta indica, has been ongoing for some 30 years. Its strong antifeedant, insect growth regulatory and reproductive effects are now well understood and documented although its biochemical effects at the cellular level are still unknown. Antifeedancy varies markedly between species with Lepidoptera being particularly sensitive 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 mode of action of azadirachtin lies in (i) effects on deterrent and other chemoreceptors resulting in antifeedancy (ii) effects on ecdysteroid and juvenile hormone titres through a blockage of morphogenetic peptide hormone release (e.g. PTTH; allatotropins) and (iii) direct effects on most other tissues studied resulting in an overall loss of fitness of the insect. The complexity of the molecular structure of azadirachtin has precluded its synthesis for pesticide use although novel synthesis of the parent molecule is now almost complete and research into simpler mimetic substances is ongoing. Applied research has concentrated on a variety of natural formulations from neem seed kernels which contain azadirachtin together with several structurally related molecules. This review brings the reader up to date with both pure and applied research in the field and provides a detailed overview of present thinking into the mode of action of azadirachtin. Wherever possible comparative approaches have been made between species of the effects of pure azadirachtin and areas for future research are indicated.