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 (acylurea) and Bacillus thuringiensis Berliner ssp. aizawai (microbial), to laboratory and field strains of Plutella xylostella L. was shown in the laboratory to be significantly affected by leaf nutritional status, other host-plant resistance factors, cultivation method and plant age. With plants offering some degree of host-plant resistance, the toxicity of the insecticides was either increased or decreased compared with highly susceptible plants, depending on the specific nature of the plant-herbivore interaction. Differences in residual toxicity of the insecticides varied up to nine-fold on different host plants (=host-plant- + insecticide-induced mortality) despite less than four-fold differences in P. xylostella mortality in controls (=host-plant-induced mortality 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 insecticides 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 resistance status. The implications of the findings in relation to the integrated management of P. xylostella are considered.     

Evidence for Resistance to Bacillus thuringiensis (Bt) subsp. kurstaki HD-1, Bt subsp. aizawai and Abamectin in Field Populations of Plutella xylostella from Malaysia

The efficacy of Bacillus thuringiensis (Bt) subsp. kurstaki HD-1 (‘Dipel’™; Btk; CryIA & CryII) and Bt. subsp. aizawai (‘Florbac’™; Bta; CryIA & CryIC) was assessed against larvae from various field populations of Plutella xylostella (F2 generation) collected in the Cameron Highlands, Malaysia in April 1994 and a lowland population (SERD 2; F10 generation) collected in December 1993. Evidence of resistance to Btk and to a lesser extent Bta is reported in these populations (LC₅₀ Toxicity Ratios [TR]=3–14 and 2–8 respectively), most notably in SERD 2. The first recorded evidence of resistance to abamectin (TR=17–195-fold) in field populations of P. xylostella is also reported. In an unselected sub-population of SERD 2, the TR values for Btk, Bta and abamectin declined 2- to 3-fold (P<0·01) over six generations in the laboratory (F10–F16) while in sub-populations of SERD 2 selected with these products (F11–F15) there was a significant (P<0·01) increase in the TR (15-, 3- and 2·5-fold respectively) when compared with the F10 generation. This suggests the presence of marked resistance to Btk and some resistance to Bta and abamectin. There is also evidence of slight cross-resistance to Btk in the Bta-selected sub-population but no evidence for the reverse selection of resistance or for cross-resistance between Btk and abamectin. Concurrent selection studies (F11–F15) with another sub-population of SERD 2 demonstrated resistance to the acylurea insect growth regulator, teflubenzuron (‘Nomolt’™) (29-fold increase in TR). Based on the selection experiments with SERD 2, estimates of realised heritability (h²) of resistance gave very high values for teflubenzuron and Btk (c.0·7) and moderate values for abamectin and Bta (c.0·3). The results are discussed in relation to integrated pest management (IPM) and insecticide resistance management (IRM) strategies for P. xylostella.