| Pesticide resistant and susceptible strains of spider mites, Tetranychus urticae, were studied throughout development to compare stage structure and fitness components of pesticide resistant and susceptible mites exposed and not exposed to sublethal doses of pesticide. Large numbers of virgin female mites were isolated with males on leaf disks of the red kidney bean (Phaseolus vugaris) and daily counts of the life cycle stages made throughout the life of the parent female, and until all of the offspring became adults. These experiments were repeated under exposure to a low, sub-lethal dose of dicofol, an organochlorine pesticide. Results showed that pesticide resistant strains had a higher fitness as measured by the intrinsic rate of increase, r, when not exposed to pesticide, and about equal fitness under pesticide exposure compared with susceptible strains. The sex ratio, closer to fifty percent female in the resistant strains not exposed to pesticide, was even more male-biased under pesticide exposure, increasing the genetic contribution of females producing low offspring sex ratios in a population with a female biased sex ratio, in subsequent generations. Systems models of the development process are presented and analyzed. |
