Bioassays (at generation G2) with a newly collected field population (designated CH4) of Plutella xylostella (L.) (Lepidoptera: Plutellidae) from farms in the Cameron Highlands, Malaysia, indicated resistance to acetamiprid, deltamethrin, lambda-cyhalothrin, and esfenvalerate. At G3, the field-derived population was divided into two subpopulations, one subpopuplation was selected (G3 to Go10) with acetamiprid (aceta-SEL), whereas the second subpopulation was left unselected (UNSEL). A significant reduction in the resistance ratio for each compound was observed in UNSEL at G,1, indicating that the observed resistance to each insecticide was unstable. For aceta-SEL, bioassays at G,, found that selection with acetamiprid gave a resistance ratio of 409 compared with UNSEL. The LC50 values for deltamethrin, lambda-cyhalothrin, and esfenvalerate to aceta-SEL were similar at both G11 and G2. This suggests that acetamiprid selection maintained the otherwise unstable resistance to these compounds in the aceta-SEL population. Logit regression analysis of F1 reciprocal crosses between aceta-SEL and the susceptible Lab-UK indicated that resistance to acetamiprid was inherited as an autosomal, incompletely recessive (DLC = 0.26) trait. Tests of monogenic inheritance and weight distribution suggested that resistance to acetamiprid was controlled by a single locus
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