Effect of Sublethal Doses of Bifenthrin and Chlorpyrifos Administered Alone and in Combinations on Esterases of Stored Grain Pest, Trogoderma granarium

ABSTRACT Toxic effect of sub-lethal concentration(LC20) of bifenthrin, chlorpyrifos and their combinations was studied on the specific activities of esterases in the 4th and 6th instar larvae of susceptible laboratory strain(Lab-S) and deltamethrin resistant population(GUW) of stored grain pest, Trogoderma granarium. After exposure to LC20 of bifenthrin, chlorpyrifos and their combinations of 3:1 and 1:3, the activities of total esterases, cholinesterase and carboxylesterase increased significantly in both the 4th and 6th instar larvae of Lab-S and GUW populations when compared with their respective controls. Acetylcholinesterase(AchE) activity was significantly decreased(28.94 and 42.42%) in the 4th instar larvae of Lab-S population after treatments with bifenthrin and bifenthrin: chlorpyrifos combination 3:1, respectively. After exposure to chlorpyrifos and bifenthrin: chlorpyrifos combination 1:3 the AchE activity was increased 45.24 and 64.28%, respectively. This increase was much more prominent in the 6th instar larvae viz., 93.95, 45.57, 182.52 and 132.61% following treatments with bifenthrin, chlorpyrifos, 3:1 and 1:3 combinations of two insecticides, respectively. In the resistant population this increase was 35.71, 48.10 and 75.61% in the 4th instar, respectively and 9.47, 28.17 and 60.29% in the 6th instar after chlorpyrifos, 3:1 combination and 1:3 combination treatments, respectively. This activity was decreased 63.99% in the 6th instar but increased in the 4th instar 13.36% after bifenthrin exposure. Arylesterase activity was declined after exposure to bifenthrin, chlorpyrifos and 3:1 combination in both the 4th instar(89.47, 64.33 and 36.65%, respectively) and 6th instar(84.49, 54.01, 4.28%, respectively) of Lab-S as well as in the 4th instar(77.73, 55.81 and 35.96%, respectively) of resistant population. This activity was upregulated(84.92, 141.86 and 192.88%, respectively) in the 6th instar of resistant population, following exposures to bifenthrin, chlorpyrifos and 3:1 combination. Whereas, this activity was increased in the 4th instar(53.22%) and 6th instar(124.60%) of Lab-S population as well as in the 4th instar(35.19%) and 6th instar(234.75%) of resistant population after treatment with 1:3 combination, respectively. The findings of current investigation suggests that insects upregulate the synthesis of esterases in general cholinesterase and carboxylesterase in response to exposure to insecticide to combat with the stressful condition