Fipronil Resistance in the Diamondback Moth (Lepidoptera: Plutellidae): Inheritance and Number of Genes Involved

Abstract

Bioassays (at generation 1, G 1) using fipronil, spinosad, indoxacarb, and Bacillus thuringiensis toxins Cry1Ac and Cry1Ca with a newly collected field population of Plutella xylostella (L.) from farmers fields in the Cameron Highlands, Malaysia, indicated a resistance ratio of ≈400-, 1,170-, 330-, 2,840-, and 1,410-fold, respectively, compared with a laboratory-susceptible population of P. xylostella (ROTH). At G 3, the field-derived population was divided into two subpopulations, one was selected (G 3 to G 7) with fipronil (fip-SEL), whereas the second was left unselected (UNSEL). Bioassays at G 8 found that selection with fipronil gave a resistance ratio of ≈490 compared with UNSEL and ≈770 compared with ROTH. The resistance ratio for fipronil, spinosad, indoxacarb, Cry1Ac, and Cry1Ca in the UNSEL population declined significantly by G 8. Logit regression analysis of F 1 reciprocal crosses between fip-SEL (at G 8) and UNSEL indicated that resistance to fipronil in the fip-SEL population was inherited as an autosomal, incompletely recessive ( D LC = 0.37) trait. At the highest dose of fipronil tested, resistance was completely recessive, whereas at the lowest dose it was incompletely recessive. A direct test of monogenic inheritance based on a backcross of F 1 progeny with fip-SEL suggested that resistance to fipronil was controlled by a single locus. The fip-SEL population at G 8 showed little change in its response to spinosad and indoxacarb compared with G 1, whereas its susceptibility to Cry1Ac and Cry1Ca increased markedly over the selection period. This suggests that there may be some low level of cross-resistance between fipronil, spinosad, and indoxacarb.