Resistance to insecticides is often linked with fitness costs. Although costs affecting lifetime reproduction in females will have the greatest effect on resistance evolution, fertilization costs in males, through less vigorous mating behavior or less effective sperm in resistant individuals, will slow the spread of resistance. We used a sterile male technique to measure the relative mating competitiveness of Colorado potato beetles, Leptinotarsa decendineata (Say) (Coleoptera: Chrysomelidae), resistant or susceptible to imidacloprid. We showed that 5,000 Krad from a cesium-137 source was sufficient to sterilize male beetles. We tested mating competitiveness by placing a single resistant and a single susceptible male beetle with a virgin susceptible female in circular acetate enclosures surrounding potted potato plants, and we observed frequency of mating or mounting with each male and subsequent fertilization success. Resistant males mate just as often as susceptible males during 24-h competitive interactions. Hatch rate was correlated with the proportion of matings or mounts observed with the nonirradiated male, but no effect of the first or last male, nor of resistance on fertilization success was observed, and resistance does not seem to incur a behavioral or sperm-competitive fertilization cost.
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