Integration of biological control and botanical pesticides - evaluation in a tritrophic context.

The plant kingdom is by far the most efficient producer of chemical compounds, synthesising many products that are used in defence against herbivores. Extracts made from some plants, particularly extracts from plants within the Meliaceae (mahogany) family, have been shown to have insecticidal properties. We investigated the potential of these extracts and the possibility of integrating botanical pesticides with biological control of the diamondback moth, Plutella xylostella. Sub-lethal doses of botanical extracts were prepared from leaves of the syringa tree (Melia azedarach) and commercial preparations (Neemix 4.5®) from the neem tree (Azadirachta indica). In "no-choice" tests, bioassay trays were used to test the impact of three different doses on first- instar larvae. In "choice" tests, half a leaf was treated with extract and the other half left untreated. The impact that these extracts had on natural enemies was investigated using two parasitoid species, Cotesia plutellae and Diadromus collaris. Results indicated that these extracts had a significantly negative impact on first-instar larvae of P. xylostella. However, the extracts had no direct negative impact on their parasitoids. Therefore, it appears that biological control and botanical pesticides can be combined to control P. xylostella.