Biological performance of Menochilus sexmaculatus Fabricius (Coleoptera Coccinellidae) upon exposure to sublethal concentration of Imidacloprid

The effects of sublethal exposure to imidacloprid (Kendor® 18.3SL) on the biological performance of the ladybird beetle, Menochilus sexmaculatus F., the most common coccinellid beetle found feeding on aphids, were studied under ambient laboratory conditions of 27-32˚C and 50-75% RH. The corn leaf aphid, Rhopalosiphum maidis, was offered as prey. The LC50 obtained from the contact bioassay at 24 h post-treatment for the regression slope, b=1.08, indicated that imidacloprid was likely to be selective. Sublethal exposure to imidacloprid caused reduction in survival with the females reaching 50% mortality by the 24th day, while that of the control was the 36th day. Meanwhile, demographic parameters including the net reproductive rate (Ro), generation time (T), innate capacity of increase (rm) and the finite rate of increase (λ) of the treated females were markedly inferior as compared to the untreated females. The Ro value indicated that the control female was capable of producing 17.59 female offspring but treated female could only produce 2.55 female offspring during their generation time of 25.17 and 23.04 days, respectively. The capacity or instantaneous rate of increase (rc) declined from 0.114 to 0.041, which was parallel with the decrease in the intrinsic rate of increased (rm) value from 0.125 to 0.041. The values of λ were 1.133 and 1.042 for the control and treated population, respectively. In the meantime, the doubling time (DT) increased sharply to 16.86 days for the treated population, whereas DT for the control was 6.09 days. The sex ratio was biased towards the female and generally the females survived slightly longer (48 days) than the males (46 days), as observed in the control population.