The dynamics of pest-parasitoid-insecticide interactions.
Population models of host-parasite interactions are extended to include insecticide application acting in a variety of ways on pest hosts and/or their parasites. These models are analysed to show the extent of 'depression' or 'resurgence' resulting from insecticide use. Insecticides acting on the pest alone always contribute to host depression, to a degree dependent on whether they precede or follow parasitism and on the stage of the pest life cycle where equilibrium population size is measured. When insecticide timing also leads to the death of parasites, pest depression is reduced and pest resurgence may or may not appear, depending again on the timing of application and the pest stage measured. The relative toxicity of insecticides to pests, immature and adult parasites is shown to be crucial to the level of resurgence obtained. The limitation and implications of this model are discussed, with emphasis on the effect of insecticide timing, parasite effectiveness and insecticide selectivity on pest depression and resurgence.