LAUSR

Abstract Dimethoate belongs to organophosphate (OP) group of insecticides, used worldwide against several economic insect pests. This study was done to evaluate the fitness costs and stability of dimethoate resistance in a previously selected strain of dusky cotton bug, Oxycarenus hyalinipennis Costa (Hemiptera: Lygaeidae), a serious sucking pest of cotton. Previously, dimethoate selected (Dimeth-SEL) strain was developed after selecting O. hyalinipennis for six generations in the laboratory. The selected strain expressed resistance ratios (RRs) of 157.76-, and 14.13-fold to dimethoate when compared with Lab Pop, and Unsel Pop, respectively. Data on fitness traits of selected strain reveal that the relative fitness (Rf) of Dimeth-SEL was 0.77 along with significant decrease in nymphal survival, hatchability, net reproductive rate (R0), intrinsic rate of natural increase (rm), and biotic potential (Bp). The Rf evaluated for Cross1 and Cross2 was 1.02 and 0.91, respectively. For resistance stability, Dimeth-SEL (G6) was left unexposed to dimethoate for five generations (G7-G11) and the bioassay results at G11 showed that removal of selected strain from selection pressure resulted in a significant decline in dimethoate resistance. It can be inferred from these results that resistance to dimethoate was developed in O. hyallinipennis at the expense of very high energetic costs and that these costs might also contributed in lowering the resistance levels towards susceptibility after stopping the selection for several generations. These results have practical implications in managing the dimethoate resistance against O. hyallinipennis by implementing the insecticide rotation strategy and also by delaying the dimethoate spray in the field for some generations.