Chironomids are a globally-distributed family of insects that can serve as biological indicators of environmental pollution. Pollution due to pesticide usage is of particular importance due to the heavy application of these chemicals. Three routinely-applied insecticides, omethoate, chlorpyrifos, and cyhalothrin, were selected for our study to investigate their toxicity against Propsilocerus akamusi and to determine their effects on carboxylesterase activity of 4th-instar larvae. After 12 h of exposure, the LC₅₀ values of omethoate, chlorpyrifos and cyhalothrin were 12.508, 2.478, and 0.046 μg/L, respectively, indicating cyhalothrin has the highest level of toxicity to P. akamusi. The omethoate induced carboxylesterase activity at 3 and 12 h post-application when P. akamusi was challenged with 0.05 μg/L. When challenged with the higher rates, 0.05, 0.125, 0.25, 0.5, 1, 2 and 4 μg/L of omethoate, carboxylesterase activity was found to increase after 48 h. In addition, carboxylesterase activity was inhibited at 24 h and 48 h for the 0.05 μg/L of omethoate while enzyme inhibition was observed for the earlier time points of 3, 12 and 24 h for all doses tested. The inhibition carboxylesterase activity ranged from 5.149%-50.587% and 3.225%-36.403% under treatments with 0.125, 0.25, 0.5, 1, 2, 4, and 8 μg/L at 3 and 12 h, respectively. Overall, at 24 h, the inhibition of carboxylesterase activity by omethoate ranged from 20.441%-48.523%. In comparison, when chlorpyrifos was applied with the concentration of 0.125, 0.25, 1, 2 and 4 μg/L, the level of carboxylesterse was inhibited dramatically after 3 h treatment. This trend was also found at 12 h with the exception of the lower concentrations of 0.125 and 0.25 μg/L. Overall, the inhibition of carboxylesterase was found by the treatment of chlorpyrifos with the concentration of 0.125, 0.25, 1, 2 and 4 μg/L ranged from 14.145%-51.254% at 24 h and 9.772%-39.659% at 48 h. For the cyhalothrin test, carboxylesterase activity was inhibited by 0.00625, 0.0125, 0.05, 0.25, and 1μg/L of cyhalothrin treatments at 3, 12, 24, and 48 h. The magnitude of the inhibition caused by cyhalothrin was found to be decreased following the cyhalothrin concentrations and treatment time. The differences in the carboxylesterase activities caused by omethoate, chlorpyrifos, and cyhalothrin indicated different responses of P. akamusi to different dose and time of these insecticides. The poisoning symptoms also indicated differences between the three insecticides tested. The symptoms of P. akamusi to omethoate and chlorpyrifos poisons in P. akamusi were overall excitation followed by a loss of consciousness and ultimately death. The poisoning symptoms of P. akamusi to cyhalothrin, however, showed an initial increase in activity followed by a period of inactivity, apparent recovery and finally death. These results suggest that the carboxylesterase activity of chironomids can be used as a biochemical marker to monitor pesticide pollution.