Rapid adaptation of squash bug, Anasa tristis, populations to a resistant cucurbit cultivar

We conducted laboratory experiments on two field-collected populations of the squash bug, Anasa tristis (De Geer) (Hemiptera: Coreidae)], to compare their responses to five generations of selection on a resistant squash cultivar Waltham Butternut, a susceptible cultivar Early Prolific Straightneck, or alternating generations on the resistant and susceptible cultivars. Before testing, one population was maintained for 1 generation on susceptible squash plants, while the other was maintained on the same cultivar for 5 generations. In both populations, nymphal survival decreased markedly in the first generation on resistant squash, but then approximately doubled in the second generation reared on the resistant cultivar. By the end of the experiments, survival on the rearing host was the same for A. tristis maintained continuously on a single cultivar, regardless of which cultivar. Immediately following one or more generations on resistant plants, A. tristis nymphs had lower survival on the susceptible cultivar than did nymphs that had been maintained for consecutive generations on the susceptible cultivar. Results of reciprocal crosses between lines of A. tristis reared on either resistant or susceptible squash for five generations indicated a genetic basis for differences in nymphal survival on the two cultivars.In contrast with the changes in nymphal survival, the preoviposition period became progressively longer when A. tristis was reared for several generations on the resistant cultivar compared to the susceptible one. Prior rearing on resistant plants increased the preoviposition period of adults placed on susceptible plants. However, adult feeding on resistant plants following nymphal development on susceptible plants did not result in an increase in the preoviposition period on resistant plants. Our findings indicate that squash bugs can adapt to resistant plants within a relatively short period in the laboratory. However, ecological and agricultural factors should prevent squash bugs from overcoming resistance in the field.