Impact of botanical extracts derived from Melia azedarach and Azadirachta indica on populations of Plutella xylostella and its natural enemies: A field test of laboratory findings

Abstract

Differences between results from ecological laboratory studies and what actually happens in the field can be large. Therefore, field experiments are essential to validate laboratory findings. In previous laboratory trials we investigated the impact of aqueous leaf extracts from the syringa tree, Melia azedarach L. (Meliaceae) and commercial formulations from the neem tree, Azadirachta indica Juss. (Meliaceae), Neemix 4.5®, on the biology and behaviour of the diamondback moth Plutella xylostella and two of its most abundant parasitoids, Cotesia plutellae and Diadromus collaris. In the laboratory we had demonstrated that these botanical extracts had adverse effects on survival, fecundity, development, oviposition and feeding of P. xylostella, but no direct negative effects on the survival and foraging of the parasitoids. In the current study, we verified the importance of these previous laboratory findings through field experiments. We treated cabbage plants in the field with the neem product and syringa extract and assessed the infestation levels of P. xylostella and the parasitism rates by natural enemies. Infestation levels of P. xylostella were similar in the plots treated with the botanical extracts and the control plots. However, the damage in the treated plots was significantly lower than in the control plots, indicating that reduced feeding by P. xylostella was a more important factor in the reduction of damage than the actual population density. The proportion of marketable cabbages was significantly higher in the treatments than in the control. The proportion of parasitoids found emerging from P. xylostella was also significantly higher in the treated plots than in the control plots and direct observations indicated that parasitoids still visited cabbage plants that had been treated with the botanical extracts.