Diamondback moth, Plutella xylostella (L.) is one of the most destructive pest of Brassica vegetable and oilseed crops throughout the world and it is now estimated to cost the world economy US $ 4-5 billion annually. It is one of pesticide resistance recidivist pest species and has ability to develop resistance quickly to any group of pesticide used for its control. Indoxacarb is a relatively new molecule and has been used for management of P. xylostella in India. To ensure the long term use of this new insecticide it is very crucial to study the genetic basis of insecticide resistance in P. xylostella.The present investigation was undertaken to study the rate of resistance development, mode of inheritance and cross resistance relationships in indoxacarb resistance strain. The homozygous resistant strain of P. xylostella was developed through laboratory selection with increasing doses of indoxacarb using leaf dip bioassay technique. The mode of inheritance was elucidated through bioassay analysis of the response of resistant (Px-R) susceptible (Px-S), F1 hybrid progeny of P. xylostella using leaf dip bioassay. Cross resistance relationships was also studied for different group of pesticides on Px-R strain. The 238.41-fold indoxacarb resistance strain (Px-R) was generated after the 14 generation of selection when compared with the susceptible strain (Px-S). The LC50 of Px-S, first generation of selection (G1) and Px-R strains were 1.90ppm (0.28µg/ml), 18.17ppm (2.63µg/ml) and 453.22ppm (65.72µg/ml), respectively. Theresistance to indoxacarb was appeared to be stable, though it was slightly dropped down when selection pressure was released continuously for two generation. The dose mortality relationship among LC50 values of the homozygous susceptible, resistant and heterozygous individuals indicated that resistance to indoxacarb was autosomal and inherited as a semi-dominant trait. The degree of dominance (D) and heritability (h) of F1 hybrid progeny ranges from -0.001 to 0.0012 and 0.499 to 0.506, respectively. Cross resistance analysis revealed that, positive cross resistance relationships was observed between indoxacarb and synthetic pyrethroids (cypemethrin) in P. xylostella. Given the semi-dominant inheritance of indoxacarb resistance, the sub lethal doses and frequent use of indoxacarb should be avoided for management of P. xylostella. Moreover, indoxacarb resistance strain of P. xylostella shown positive cross resistance to synthetic pyrethroids (cypemethrin), therefore indoxacarb and synthetic pyrethroid should not be recommended together for management of P. xylostella.
Key words: P. xylostella, indoxacarb resistance, inheritance, cross resistance