Stage and age influence on the susceptibility of Coccinella septempunctata (Coleoptera: Coccinellidae) after direct exposure to Neemix, a neem insecticide

The effects of Neemix 4.5 EC on predatory Coccinella septempunctata L. larvae were determined after direct spray exposure. First instars were treated by direct application with 0, 40, 100, 200, 400, 600, and 1,000 ppm and 4th instars were treated with 400, 600, 800, and 1,000 ppm azadirachtin, the active ingredient in Neemix. Survivorship and development of the larvae were affected in a dose-dependent manner. Symptoms of exposure included delay or prevention of pupation, blackening of the pupal case, formation of pupal-adult intermediates, and deformation of wings and elytra in adults. To account for the slow action of the pesticide, probit analysis of survivorship data was performed after all surviving larvae from each replicate had emerged as adults: day 16 for 1st instars and day 13 for 4th instars. The LC50 values and 95% FL for 1st and 4th instars were estimated to be 1,120 (719-3,677) ppm and 520 (405-600) ppm azadirachtin, respectively. These values were much higher than the recommended rates for control of aphids (3 weekly applications of 20 ppm), suggesting that Neemix might be used in integrated pest management programs because application of rates that control aphids should not result in appreciable mortality of predators. The slopes and intercepts of the probit regression lines were significantly different, indicating that the 2 instars responded to the pesticide differently. Fourth-instar C. septempunctata were innately more sensitive to the growth disrupting effects of acute exposure to Neemix than Ist instars. Disruption of morphogenetic hormone levels is more critical immediately before metamorphosis than during early instars, thus accounting for the differential susceptibility observed. It may be possible for early instars to sustain the effects of Neemix as long as the pesticide is detoxified before the onset of pupation. Our results suggest that it is extremely important to examine >1 life stage of a species to estimate the total effect of pesticides.