Linking risk and efficacy in biological control host-parasitoid models

•Classical biological control entails risks as well as potential benefits.•We use a two-host one-parasitoid model to see how risk and efficacy may interact.•We focus on model determinants of host range and egg vs. time limitation.•Apparent competition can have important benefits for biological control.•However, under egg limitation, attack on low quality hosts can reduce control.

Exotic invasive arthropods present an increasing threat to native species, ecosystem function, crop production, forests, and other natural resources. Importation ('classical') biological control can be a cost-effective tactic for long-term pest management that reduces insecticide use. However, while importation of biological control agents has great potential benefits, it also entails risks to non-target native species. Therefore, candidate biological control agents are studied prior to release to predict safety. Little is known, however, about how traits affecting the safety of biological control agents may impact their efficacy in terms of reducing pest populations. We use a difference equation model to simulate a one-parasitoid two-host system and evaluate conditions under which biological control safety and efficacy interact. We vary the search efficiency and resistance to parasitism of both host species and interpret the results from the standpoint that one host is a target pest and the other a non-target species. We find that apparent competition can have important benefits for increasing biological control efficacy, even at low levels of non-target impact. However, under conditions of parasitoid egg limitation, high attack rates on resistant non-target hosts can dramatically decrease biological control efficacy while concurrently increasing non-target risk. These findings are discussed in the context of biological control agent pre-release risk-benefit assessment.

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