EFFECTS OF INTERACTIONS BETWEEN HOST PLANTS AND SELECTIVE INSECTICIDES ON LARVAE OF PLUTELLA-XYLOSTELLA L (LEPIDOPTERA, YPONOMEUTIDAE) IN THE LABORATORY

The residual toxicity of two selective insecticides, teflubenzuron (ac ylurea) and Bacillus thuringiensis Berliner sap. aizawai (microbial), to laboratory and field strains of Plutella xylostella L. was shown. i n the laboratory to be significantly affected by leaf nutritional stat us, other host-plant resistance factors, cultivation method and plant age. With plants offering some degree of host-plant resistance, the to xicity of the insecticides was either increased or decreased compared with highly susceptible plants, depending on the specific nature of th e plant-herbivore interaction. Differences in residual toxicity of the insecticides varied up to nine-fold on different host plants (=host-p lant- + insecticide-induced mortality) despite less than four-fold dif ferences in P. xylostella mortality in controls (=host-plant-induced m ortality alone). Host-plant nutritional status also had a substantial effect on the damage potential of larvae. Different response times by P. xylostella to the two insecticides tested on host plants of varying nutritional status were related to the contrasting modes of action of the respective active ingredients. The present studies suggest that i nsecticides applied to Brassica oleracea L. var. capitata with partial plant resistance may contribute to improved control of P. xylostella. A conceptual model is used to describe likely mortality responses by macrophagous larvae to insecticides applied to plants of varying resis tance status. The implications of the findings in relation to the inte grated management of P. xylostella are considered.