The risks of biocontrol: Transient impacts and minimum nontarget densities

The biocontrol of insect pests may pose a risk to native insects if the biocontrol agent attacks nontarget species. Potential biocontrol agents are screened before release to determine their acceptance of nontarget species and the suitability of nontarget species for their development. Here we show that, even though a biocontrol agent has very low acceptance of a nontarget species, it may nonetheless have a large impact on the nontarget population. This impact does not require the nontarget species to be a suitable prey capable of supporting the biocontrol agent population, but instead may be a transient impact that occurs soon after the agent is released. Because the population of biocontrol agents is likely to increase rapidly in response to the high density of its target pest, the resulting high density of the agent population may dominate its low acceptance of the nontarget species, causing a strong decline or even local extirpation of the nontarget. We demonstrate this possibility using models of host-parasitoid dynamics that incorporate a broad range of assumptions about the life histories of hosts and parasitoids, and that demonstrate how various common aspects of host-parasitoid biology are likely to reduce this risk considerably. The predictions of the models are reasonably approximated with a simple formula, which potentially provides a simple method for assessing the risk of transient impacts, but which should only be applied loosely (in a qualitative manner) and in the context of a fuller understanding of other factors affecting risk in the system in question.