High resistance levels in brazilian Plutella xylostella populations: needs for adjustments in field concentration

ABSTRACT Plutella xylostella cause severe damage on cruciferous plants all over the world. Farmers in several regions of Brazil report increasing inefficiency of chemical control, even when using high insecticide concentrations. We evaluate here the susceptibility of brazilian diamondback moth populations to the Premio® (Chlorantraniliprole), Dipel® (Bacillus thuringiensis var. kurstaki) and Lannate® BR (Oxime Methylcarbamate) insecticides, frequently used in Brazil. Susceptibility bioassays with five field-collected and two laboratory diamondback moth populations were conducted with increasing concentrations of insecticides up to ten times above the recommended concentration. Extremely high and region-dependent resistances were found in field populations against Chlorantraniliprole and B. thuringiensis, with Resistance Ratios up to 370.0 times for the Camocim de São Felix population. For Oxime Methylcarbamate, we were not able to do Probit analyses for the field populations because of very low mortality rates. Laboratory populations showed resistance to the three tested insecticides with all LC50 concentrations exceeding the recommended doses by at least 3 times. Our results show strong and variable resistance to the three tested insecticides according to the region of origin. To maintain efficient pest control in a large country like Brazil, local levels of resistance need therefore to be monitored by the authorities and reassessment and adjustments of regional doses of insecticides should be implemented as a public policy, to prevent massive spread of insecticides in the field, as well as increases in cases of resistance.