Could resistance to insecticides in Plutella xylostella (Lepidoptera:Plutellidae) be overcome by insecticide mixtures?

To investigate if synergism occurs between pyrethroids, organophosphates and new insecticides, we tested representatives of these compounds (bifenthrin, chlorpyrifos, spinosad, indoxacarb and emamectin) against the diamondback moth (Plutella xylostella). Larvicidal activity of these insecticides was assessed separately and together on a susceptible strain (Lab-UK) of P. xylostella as well as a field population collected from Multan. The field population showed significant resistance to chlorpyrifos (331 100-fold), bifenthrin (45 200-fold), emamectin (1800-fold), spinosad (11-fold) and indoxacarb (5600-fold) when compared with the Lab-UK population. When insecticides were mixed based on LC50 and tested at serial concentrations against Lab-UK, significant synergy (CI < 1) occurred between bifenthrin, spinosad and emamectin. In contrast, the interaction between bifenthrin and indoxacarb was additive (CI B< 1). The toxicity of bifenthrin against the field population increased significantly (P < 0.01) when combined with spinosad, emamectin and indoxacarb. Synergistic effects could be attributed to the complementary modes of action by these insecticide classes acting on different components of nerve impulse transmission (which are not identical forbifenthrin and indoxacarb either). However, chlorpyrifos/bifenthrin mixture was not significantly different either from bifenthrin or chlorpyrifos alone, indicating an additive affect. In combination with spinosad and emamectin, tested against the resistant field population, the toxicity of chlorpyrifos increased significantly and even more so with indoxacarb. Mixtures could also give rise to multiple resistance that may extend across other chemical classes and thus become difficult to manage. Therefore, alternative strategies such as mosaics or rotations should be considered. That is, though synergistic effects have been found, this should not be followed up as a strategy to manage resistant field populations.