BACKGROUND: A strain of Helicoverpa armigera with 171-fold resistance to indoxacarb was introgressed with a susceptible strain by serial backcrossing and reselection with indoxacarb resulting in the creation of the near-isogenic GY7-39BC4 strain. Fitness was compared on artificial diet under diapause and non-diapause conditions in resistant, susceptible and F 1 progeny from a reciprocal backcross of the two strains using life history trait analyses. Selection experiments were used to determine stability of resistance. RESULTS: There were no significant differences between strains in survival, female fertility or realized fecundity. A comparison of the intrinsic rate of population increase showed similar relative fitness between strains. Lower male fertility and male longevity in the resistant strain and one of the F1 strains compared with the susceptible strain suggests small non-recessive costs may be associated with male reproductive capacity in individuals with indoxacarb resistance alleles. However, there was no significant decline in resistance in the GY7-39 strain when reared in the absence of insecticide for five generations. Following an artificially induced diapause, survival was reduced by 52% and pupal weights were significantly lower in the resistant strain compared with the susceptible strain. CONCLUSIONS: These results suggest indoxacarb resistance does not confer a major fitness cost under standard laboratory conditions. However, a survival cost associated with overwintering highlights the imperative for adoption of management strategies in northern regions of Australia where a winter diapause does not occur.

The ability to effectively detect changes in susceptibility to insecticides is an integral component of resistance management strategies and is highly dependent upon precision of methods deployed. Between 2013 and 2016, F2 screens were performed for detection of resistance alleles in Helicoverpa armigera (Hübner) to emamectin benzoate, chlorantraniliprole, and indoxacarb in major cropping regions of eastern Australia. Resistance to emamectin benzoate was not detected. There were low but detectable levels of survival at discriminating concentrations of chlorantraniliprole and indoxacarb. Alleles conferring an advantage to chlorantraniliprole were present at a frequency of 0.0027 (95% CI 0.0012-0.0064; n = 1,817). Alleles conferring an advantage to indoxacarb were present at a frequency of 0.027 (95% CI 0.020-0.035; n = 1,863). Complementation tests for allelism in six of seven positive indoxacarb tests indicated that resistance was due to alleles present at the same locus. The majority (88%) of lines that tested positive for indoxacarb resistance deviated from a model of recessive inheritance. Pheromone-caught male moths contributed significantly greater numbers of F2 lines compared with moths derived from field-collected eggs or larvae. There was no difference in the detectability of indoxacarb resistance in F2 lines from pheromone-caught moths compared with moths derived from immature stages collected from the field and reared to adult under laboratory conditions. Therefore, we recommend the use of pheromone traps for sourcing insects for F2 screening as a more cost- and time-efficient alternative to traditional methods of sampling.

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