Microplitis demolitor (Hymenoptera: Braconidae) is a mid to late season parasitoid of Helicoverpa spp. larvae. Female M. demolitor prefer to lay eggs in second instar larvae. The host larva is killed before serious damage has been done to the crop and where any reasonable reduction in numbers will be important. Parasitised larvae effectively stop feeding and consume only about 10% of that consumed by healthy larvae (Murray and Rynne, 1992). Numbers of M.demolitor can build up very rapidly in the field. The cotton industry is committed to reducing the use of insecticides because of the economic and environmental costs. With the imminent release of Bt cotton, a very real reduction in insecticide use is expected. Due to reduced expression of Bt toxins in Bt cotton late season (Fitt, 1996), the action of natural enemies should be encouraged at this time. Use of insecticides that disrupt natural enemies should be reduced, however, data on the effects of many insecticides on beneficial insects are lacking under Australian conditions.