Microplitis demolitor (Wilkinson) (Hymenoptera: Braconidae) is a critical component of IPM in Australian cotton. High rates of parasitism (up to 65%) occur naturally in the field at a crucial stage in the development of the crop. Parasitised larvae cause negligible damage to the crop. M demolitor adults were demonstrated to tolerate certain insecticides (chlorfluazuron methoxy fenozide and pirimicarb) but were very susceptible to many others (endosulfan<dimethoate< cyhalothrin< profenofos< deltametlirin< bifenttirin< spinosad). M. demolitor is extremely tolerant of most insecticides during the pupal stage. Larval M. demolitor were not directly affected by insecticides but survival was reduced due to host mortality. Susceptibility of the host to stomach insecticides was reduced about 5 days after parasitisation, while insecticides with a contact action caused host mortality throughout the parasitoid's larval stage.