Monitoring of chlorantraniliprole resistance in populations of Plutella xylostella

Plutella xylostella (L.), is the main pest of Brassicas in Brazil and its control is very difficult mainly because of its genetic plasticity, which has lead it to evolve resistance to most insecticides.  Since November 2009, chlorantraniliprole, a diamide class insecticide has been marketed with great activity towards resistant strains of P. xylostella to other insecticide classes. Diagnostic concentration was used to monitor the resistance to chlorantraniliprole in 10 field Brazilian populations of P. xylostella, collected in the northeast areas in 2011. Concentration-response bioassays were done to determine the resistance levels among the populations. Early 2nd–instar larvae (F1) were exposed to 0.3 mg chlorantraniliprole/L of water by immersion of Brassica oleracea var. acephala leaves. Concentration-response curves were performed with at least 7 concentrations and three replicates for each treatment. Bioassays were kept inside a growth chamber at 27±0.5°C and 65±5% RH and repeated at least once.  Percentage mortality was corrected by the control mortality and the data subjected to Probit analysis. All the populations assessed showed 100% survival at the diagnostic concentration. Even when tested with the label rate, all the populations showed less than 10% mortality. Clearly, resistance evolved very fast (< two years) to this new molecule in the field. Resistance ratios were very high for most populations (< 16800-fold). Further studies are underway to better characterize the resistance to diamides in these strains. This panorama reveals how important is to devise an early program of resistance monitoring and pest management in areas of Brassicas cultivation.