COMPARATIVE FITNESS OF 3 STRAINS OF COLORADO POTATO BEETLE (COLEOPTERA, CHRYSOMELIDAE) IN THE FIELD - SPATIAL AND TEMPORAL VARIATION IN INSECTICIDE SELECTION

Fitness components were measured for permethrin-resistant, susceptible, and hybrid strains of Colorado potato beetle, Leptinotarsa decemlineata (Say), on insecticide-treated and untreated potato, Solanum tuberosum L., under field conditions. The hybrid strain was created by crossing resistant females and susceptible males. In separate experiments, survivorship was determined for adults and larvae of various ages (1, 2, 4, and 6 d) that were exposed directly to a permethrin (Ambush 2EC) spray. Results indicated that there is significant selection against susceptible genotypes during aIl larval stages and the adult stage. One- and 2-d-old larvae, as well as adults, of the hybrid strain and the resistant strain had similar survival. Survival in 4- and 6-d-old larvae of the hybrid strain was intermediate between the susceptible and resistant strains. Weights of 4- and 6-d-old larvae of the resistant and hybrid strains that survived treatment with insecticide were significantly lower than weights of untreated larvae. In another set of experiments, viability of 1-d-old larvae and viability and fecundity of adults were estimated when beetles were placed on potato foliage at various intervals (2, 5, 8 and 13 d) after permethrin application. Selection against susceptible 1-d-old larvae was strongest when placement was 2 d after application and selection diminished on subsequent placement dates but remained significant for the duration of the experiment. Selection against susceptible larvae was more intense on old foliage than new foliage. Larvae of the hybrid and resistant strains had similar survival on treated foliage. When hybrid 1-d-old larvae were placed on foliage 2 and 5 d after application, the sex ratios of survivors were 91 and 75% male, respectively. These values were significantly higher than that of the resistant strain survivors on treated plants and hybrid survivors on untreated plants. This is consistent with the reported sex-linkage of this trait in Colorado potato beetle, but surprisingly, survivorship of hybrid males was significantly higher than survivorship in the resistant strain males for day 2 and 5 placements. Differences were not detected in survivorship and fecundity of adults ofthe three strains placed on plants at various intervals after insecticide application. Implications of these fitness studies for resistance management of Colorado potato beetle are discussed.