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.摘要：小菜蛾(Plutella xylostella)在全球范围内对十字花科植物造成严重为害。巴西多个地区的种植户反映，即便使用高浓度杀虫剂，化学防治的效果也持续下降。本研究针对巴西境内常用的Premio®（氯虫苯甲酰胺(Chlorantraniliprole)）、Dipel®（苏云金杆菌库斯塔克亚种(Bacillus thuringiensis var. kurstaki)）以及Lannate® BR（肟基氨基甲酸酯(Oxime Methylcarbamate)）三种杀虫剂，评估了巴西小菜蛾种群的敏感性。研究采用5个野外采集种群与2个实验室饲养的小菜蛾种群，设置梯度杀虫剂浓度（最高至推荐剂量的10倍）开展敏感性生物测定。结果显示，野外种群对氯虫苯甲酰胺与苏云金杆菌表现出极高且具有区域特异性的抗性，其中Camocim de São Felix种群的抗性倍数最高达370.0倍。针对肟基氨基甲酸酯，由于野外种群死亡率极低，无法完成概率单位分析。实验室饲养种群对三种供试杀虫剂均表现出抗性，所有半致死浓度(LC₅₀)均至少超出推荐剂量3倍。本研究结果表明，小菜蛾对三种供试杀虫剂的抗性水平因种群原产地不同而存在显著差异且程度较强。在巴西这样的广袤国家中，若要维持高效的害虫防治效果，相关部门需对各地的抗性水平开展监测，并将杀虫剂区域剂量的重新评估与调整作为公共政策予以落实，以避免田间杀虫剂的过度使用以及抗性案例的大规模扩散。