Evaluation of larvicidal activity of selected plant extracts against Plutella xylostella (Lepidoptera: Plutellidae) larvae on cabbage

Abstract

Plutella xylostella (Lepidoptera: Plutellidae) popularly known as the moth-of-crucifers is a major pest of brassica worldwide which can cause damage to brassica crops up to 100% loss of total production. The use of synthetic pesticides for its control has harmful effects on public health and the environment, besides the development of resistance to these synthetic insecticides. One of the alternative control methods developed in recent years has been the use of plant extracts. In the present study, was evaluated the insecticidal potential of the leaves, root bark and stem bark extracts of three Mozambican medicinal plants: Trichilia emetica, Anacardium occidentale and Cymbopogon citratus to control the development of diamondback moth (L.) on cabbage. The plant extracts were obtained by maceration in organic solvents (n-hexane, ethyl acetate and methanol). The preliminary phytochemical tests were conducted to identify classes of chemical constituents present in the extracts. To assess insecticidal activity, the extracts were tested on the second instar larvae of diamondback moth using the leaf disc immersion methodology, where the larvae were fed with cabbage leaves dipped in each extract, being observed larval mortality in function of extract concentration. The evaluation of the insecticidal activity showed that the methanol crude extract from the root bark of Trichilia emetica caused the highest larval mortality (LC50 = 0.94 mg.ml^-1). The LC₅₀ values obtained for all the extracts tested indicate that the crude methanol extracts have a higher larvicidal potential than those obtained in the sequential extraction. The results suggest that the larvicidal activity of the extracts under study can be related to the presence of the identified metabolites that act synergistically or individually in producing larval mortality.

Keywords: Plutella xylostella, larvicidal, Trichilia emetica, Anacardium occidentale, Cymbopogon citratus.