Biological activity of neem seed kernel extracts and synthetic azadirachtin against larvae of Plutella xylostella L

The activity of two neem extracts, AZT and NEEM‐AZAL (containing 30 and 3 mg azadirachtin ml −1 respectively) and synthetic azadirachtin (AZ) against second‐instar larvae (L2) of Plutella xylostella L. was examined using leafdip bioassays. On Chinese cabbage, AZ was significantly ( P <0.05) less toxic (3 to 4‐fold; LC 50 0.54 μg AZ ml −1 ) than either neem extract against a laboratory strain of P. xylostella (FS). The LC 50 values for AZT against the FS and another laboratory strain (Wellcome) were not significantly different on Chinese cabbage. The activity of AZT against the FS and Wellcome strains was similar on Chinese cabbage and Brussels sprout. AZT was significantly less toxic (3‐fold) on Brussels sprout against an acylurea‐resistant field strain (Sawi) when compared with the FS strain on Chinese cabbage. Larval mortality (at day 13) was found to increase with increasing exposure time of P. xylostella (FS) larvae to AZT‐treated Chinese cabbage, although there was little difference in mortality between 48 and 120 h exposure. When AZT, NEEM‐AZAL and AZ were applied at a dose (1 μg AZ ml −1 ) which gave end‐point mortalities between 50 and 90% (at day 13), all treatments delayed the development of a proportion of surviving larvae but no morphogenetic abnormalities were observed in larvae which reached pupation. Evidence for antifeedant (reduced weight gain) and repellant effects (choicechamber) for AZT were observed with L2 P. xylostella (Wellcome) on Chinese cabbage. AZT was also shown to have ovicidal activity against P. xylostella (Wellcome) at relatively high dose ranges (10‐1000 μg AZ ml −1 ) as well as some contact activity (FS strain) in topical bioassays. In residual bioassays on glass with adults of the hymenopteran endo‐larval parasitoid of P. xylostella, Diadegma semiclausum (Ichneumonidae), AZT showed little or no activity at rates up to 1000 μg AZ ml −1 . In medium‐volume (MV, 200 litre ha −1 ) and ultra‐low‐volume ( c. 1 litre ha −1 ) spray bioassays on Brussels sprout, AZT gave 16‐92% and 88‐100% mortality respectively (Wellcome strain) at rates approximating to 1‐20 g AZ ha −1 . The residual activity of AZT and NEEM‐AZAL against P. xylostella (FS) on Brussels sprout (MV spray) was observed to decrease appreciably after three days, the decline in activity being particularly marked for NEEM‐AZAL.