Characterization of fitness costs associated with insecticide resistance in the diamondback moth, Plutella xylostella, from Hawaii

Insecticide resistance has been well documented for the diamondback moth, Plutella xylostella, for many different insecticides from the vegetable-growing regions of Hawaii. Insects from this region were the first to develop resistance to Bacillus thuringiensis (Bt) and more recently to two novel synthetic insecticides: spinosad and indoxacarb. A colony of diamondback moths from Waipio, Hawaii, was established in the laboratory in Newark, Delaware, in an attempt to characterize resistance to nine insecticides, each representing a different insecticide class, and to assess potential fitness costs associated with these insecticide-resistant insects. A susceptible colony from Geneva, New York, was obtained from the field and established in the laboratory to be used in comparison. The resistance of each population to a variety of insecticides was characterized through bioassays, and the two populations were compared using resistance ratios. The population from Waipio was found to be resistant to every insecticide tested in comparison to the population from Geneva. Resistance ratios ranged from 4.9 for methoxyfenozide to 2,222 for esfenvalerate.; Following the characterization of resistance, laboratory life history studies were completed on both populations to determine if a fitness cost due to insecticide resistance was present in the Waipio population. Diamondback moths from both populations were followed from egg to adult, then set up in mating pairs. Factors examined included survival, development time, and reproduction. The Geneva, NY, population had a one day shorter development time from larva to pupa than the Waipio population. This difference could have a substantial impact on fitness over many generations. In addition, the susceptible population hatched 23.9 more eggs per female than the resistant population. A difference this large could have a substantial impact on fitness very quickly, especially when combined with a shorter development time.