Parasitoids and ecological risk assessment: Can toxicity data developed for one species be used to protect an entire guild?

Parasitoid wasps are important control agents for a suite of economically important arthropod pests. However, it is well known that hymenoptera in general are highly vulnerable to the effects of chemical pesticides, and so represent an opportunity to better understand the compatibility of biological and chemical control of pests. Closely related species are often used in trials aimed at assessing the effects of different chemical toxicants on hymenoptera. Most of these tests focus on acute toxicity, with few exploring longer-term population outcomes. Here, using a simple mathematical framework, we quantify the relative vulnerability of a suite of parasitoid species to the sublethal effects of a toxicant acting on fecundity. Using an equation that describes a critical extinction threshold, we calculate levels of reduction in fecundity above which parasitoid species important for biological control are driven to local extinction. We apply this framework to four economically important braconid biological control agents (Diachasmimorpha longicaudata, Psyttalia fletcheri, Fopius arisanus, and Diaeretiella rapae), and find that threshold levels vary widely among them. In particular, we find that D. rapae is far more robust to pesticide exposure than the other three species, suggesting that these four species are not interchangeable in terms of risk assessment. Our findings imply that caution should be exercised in assuming species even within the same functional guild may be used as surrogate species for one another.

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► Braconid parasitoids are effective biocontrol agents but sensitive to pesticides.
► Risk assessment is often based on extrapolation from a few “surrogate” species.
► We used life history data to model the response of four braconids to toxicants.
► The four species exhibited different levels of susceptibility to toxicants.
► Use of surrogate species may be misleading in biocontrol risk assessment.