The use of Bacillus thuringiensis and Neem alternation on Plutella xylostella (Lepidoptera: Plutellidae) and its effects on natural enemies in cabbage production

The diamondback moth (DBM), Plutel/a xylostella (L.) is a major pest of cabbage in Senegal. Chemical control is the most commonly used control method despite its environmental and health issues. Bacillus thuringiensis (Bt) and Neembased products are considered as relevant alternatives to synthetic chemical insecticides. The aim of this study was to assess the effect of the alternation of Bt and Neem (Azadirachta indica) on P. xylostella and its effect on parasitoids compared to sole applications of Bt, Neem and Dimethoate. Plants treated with Dimethoate recorded three times more P. xylostella compared to applications of Bt, Bt/Neem and Neem. Results showed that although there were no significant differences between Bt, Bt/Neem and Neem, populations of P. xylostella were considerably reduced in tbese treatments as compared to Dimethoate and control. Four parasitoid species were recorded of which two species Oomyzus sokolowskii and Apanteles litae were important both in terms of abundance and parasitism. The parasitism rate was higher in the Neem treatment. The correlatioo between abundance of P. xyloste/la and parasitism rate was observed in all the treatments except that of Dimethoate and was much stronger in Bt/Neem and Neem. This study suggests that in the absence of chemical insecticides, parasitoids contribute significantly in DBM population control. Also the use of only four alternated applications of Bt and neem is as effective as sole treatmeots in the control of P. xylostella. This approach which îs more environmeotally friendly seems to be also more cost effective to farmers