Mechanisms and management of insecticide resistance in Australian diamondback moth
Diamondback moth (DBM) (Plutella xylostella L.) is the main pest of Brassica vegetable crops in Australia, and has international notoriety for rapidly acquiring insecticide tolerance, which then leads to field control failures. DBM resistance to older insecticide classes is widespread in Australia; hence the choices for DBM control are increasingly limited to several newer synthetic pesticides and Bacillus thuringiensis products. In this project we have demonstrated that some field populations of diamondback moth have now developed significant levels of resistance to two important groups of these newer insecticides (IRAC Groups 6 (emamectin benzoate) and 28 (chlorantraniliprole and flubendiamide), and that these resistance levels are already capable of reducing the field control achieved with these Group 6 and 28 insecticides. We have also demonstrated that high levels of resistance can result in the laboratory from repeated exposure to low concentrations of synthetic (eg. Group 6 insecticide emamectin benzoate) or microbial (eg. Bacillus thuringiensis var. kurstaki) insecticide. In this selection study we found that in the early stages of tolerance acquisition a novel inducible tolerance mechanism was involved. As the selection process with the synthetic insecticide progressed, a combination of induced metabolic tolerance and genetic mutation(s) were found to contribute to the resistance. Studies are continuing to identify the molecular basis of this resistance, and to determine the full implications of the inducible tolerance process to resistance management. Further, the DBM lab strain which has acquired high resistance to emamectin benzoate is exhibiting cross-resistance to Group 28 diamides insecticides. Studies are underway to determine if the resistance mechanism(s) in this lab-selected strain is/are the same as those evolving in DBM field populations in response to growers’ use of emamectin benzoate. If this is shown to be the case, then cross resistance between Group 6 and 28 insecticides in the field is likely, and the ‘two-window’ resistance management strategy will need to be revised in the light of such findings. In summary, in the near future the Australian Brassica vegetable industry is likely to experience declining efficacy with a number of important synthetic insecticides (most notably Group 6 and 28 insecticides). More extensive studies of field strains from all major production areas are required to ascertain the immediacy and magnitude of this threat, and new non-insecticidal technologies for DBM management in Australian Brassica vegetables must be developed to provide sustainable crop production practice.