Fitness Costs Limit the Development of Resistance to Indoxacarb and Deltamethrin in Heliothis virescens (Lepidoptera: Noctuidae)

Insecticide resistance in Heliothis virescens (F.) (Lepidoptera: Noctuidae) has been documented from all over the world and is often associated with reduced fitness. Fitness costs could delay the development of resistance depending upon the prevailing conditions. We were interested in establishing whether a field-collected population from Washington County, MS, was resistant to spinosad, indoxacarb, and deltamethrin and whether any such resistance was associated with fitness costs. Bioassays results showed that the insecticides were equally toxic to the field population. Upon laboratory selection (generations [G]3 to G8), the resistance ratio increased only 2-, 3-, and 1-fold for spinosad, deltamethrin, and indoxacarb, respectively, compared with the field population. In contrast, the resistance ratios increased 213-, 65-, and 55-fold compared with an unselected population at G9. The estimated realized heritability (h2) after six generations of selection was 0.17, 0.03, and 0.12, respectively, and the number of generations required for 10-fold increase in LC50 of Spino-SEL, Indoxa-SEL, and Delta-SEL was estimated to be 14.3, 50, and 14.3. Comparison of life traits between the selected and unselected populations revealed that the selected populations laid a significantly lower number of eggs and that a lower percentage of eggs hatched. This also was reflected in both the net replacement rate and the intrinsic rate of population increase, which were both lower for the selected populations. It also was observed that the mean relative growth rate of the larvae was lower for the selected populations; not only did the larvae take longer to pupate but the mean weight of the prepupae from the selected populations was lower. Our data suggest that due to fitness costs the development of resistance to the insecticides was limited such that after six generations of selection the larvae were no less susceptible to the insecticides than the field population although were considerable more resistant than the unselected population