INHERITANCE AND FITNESS CONSEQUENCES OF RESISTANCE TO FENVALERATE IN SPODOPTERA-EXIGUA (LEPIDOPTERA, NOCTUIDAE)

Inheritance and fitness consequences of resistance to fenvalerate in Spodoptera exigua (Hubner), the beet armyworm, were evaluated with inbred lines of fenvalerate-resistant and susceptible populations. Ancestors of the fenvalerate-resistant population were exposed to a selection regime of multiple insecticides in the wild and were subsequently selected intensely with fenvalerate in the laboratory. Tolerance distributions of six generations of known relationship, derived from crosses of the inbred lines and their offspring, were estimated using susceptibility data. Because the tolerance distributions of the parental and F1 generations overlapped, discrimination between a polygenic and monogenic model of fenvalerate resistance inheritance was difficult. By fitting mean tolerance of these and the F2 and two backcross generations to additive-dominance models, variation of mean tolerance among generations was adequately explained by a reduced polygenic model of five parameters that included epistatic effects of additive-by-additive and additive-by-dominance digenic interactions. Therefore, a polygenic model of inheritance seemed most plausible to explain fenvalerate resistance expression. Measurements of fecundity and development time differed among generations. Although an additive fitness cost explained a significant amount of the observed variation, resistance may be acting on these traits in a complex manner. Polygenic inheritance of resistance and fluctuating fitness effects because of changing patterns of fenvalerate use may contribute to the spatial and temporal variation in resistance seen in wild populations.