EntomologiaExperimentalisetApplicata 77: 69-80, 1995. 69 @ 1995 KluwerAcademic Publishers. Printed in Belgium. Behavioural and neurophysiological responses of Spodoptera littoralis to azadirachtin and a range of synthetic analogues M. S. J. Simmonds 1, W. M. Blaney 2, S. V. Ley 3, J. C. Anderson 4, R. Banteli 3, A. A. Denholm 3, P. C. W. Green 1, R. B. Grossman 3, C. Gutteridge 3, L. Jennens, S. C. Smith 4, P. L. Toogood 4 & A. Wood 4 1Jodrell Laboratory, Royal Botanic Gardens, Kew, Surrey, TW9 3AB, UK 2Department of Biology, Birkbeck College, Malet Street, London, WC1E 7HX, UK 3Department of Chemistry Cambridge University, Lensfield Road, Cambridge, CB2 1EW, UK 4Department of Chemistry, Imperial College of Science, Technology and Medicine, London, SW7 2AY, UK Accepted: February2, 1995 Key words: azadirachtin, azadirachtin analogues, Spodoptera littoralis, antifeedant, behaviour, electrophysiology Abstract The antifeedant activity of azadirachtin and 56 azadirachtin analogues, including 22,23-dihydroazadirachtin, against larvae of Spodoptera littoralis was investigated using behavioural and electrophysiological bioassays. None of the analogues was as active as azadirachtin, although many showed significant antifeedant activity at high concentra- tions. The majority of the analogues stimulated a dose-dependent response from a neurone in the medial styloconic maxillary sensilla which correlated with the behavioural activity. Methylation of the hydroxy substitutions on the azadirachtin molecule usually resulted in a decrease in antifeedant activity, as did the addition of bulky groups to the dihydrofuran ring. Introduction Azadirachtin, an oxidised limonoid isolated from the neem tree Azadirachtin indica A. Juss (Butterworth & Morgan, 1968), is a potent antifeedant compound for numerous species of insects (Mordue (Luntz) & Blackwell, 1993). In general, lepidopteran larvae are deterred from feeding by lower concentrations of azadirachtin (Heliothis virescens, 0.07 ppm, Yamasaki & Klocke, 1987; Peridroma plorans, 0.4 ppm, Cham- pagne et al., 1989) than either species of Hemiptera (Myzus persicae, 100 ppm, Nisbet et al., 1992) or Coleoptera (Diabrotica undecimpunctata howardi, 100 ppm, Reed et al., 1982). Slight modification to the structure of azadirachtin can result in a decrease in its antifeedant activity and this decrease appears to be consistent across the dif- ferent species tested (Blaney et al., 1990; Nasiruddin & Mordue (Luntz), 1994). Spodoptera littoralis larvae have been used in glass-fibre disc bioassays to show that the dihydrofuran part of the azadirachtin molecule is involved in its potent antifeedant activity (Blaney et al., 1990, 1994). However, there is very little infor- mation about the role of the various functional groups on the decalin portion of the molecules in determining the antifeedant effect of the molecule as a whole. In this paper we investigate how a range of syn- thetic analogues derived from azadirachtin differ from azadirachtin in their antifeedant effect and in their abil- ity to stimulate responses from neurones in the medial maxillary styloconia sensilla of final stadium larvae of S. littoralis. We thereby investigate how functional groups on the azadirachtin molecule relate to its potent antifeedant activity. Materials and methods Compounds. Figure 1 shows the structure of azadirachtin and indicates the differences between it and the analogues used in these experiments. Azadirachtin was purified from crude neem samples