Deconstructing the IOBC tiered method: Are we overestimating the compatibility of pesticides and natural enemies?

• The International Organization for Biological Control (IOBC) uses a tiered approach to determine pesticide risk. • We modeled the effects of IOBC Tier 1 classes (harmless, 29 % mortality; slightly harmful, 79 % mortality) on natural enemies. 29 and 79 % mortality reduced natural enemy populations so much that they are ineffectual in protecting natural enemies. • Mortality levels that drove population growth rates < 1 were all lower than the threshold Class 1 mortality level of 30 %. 29 and 79 % mortality were shown to greatly reduce the effectiveness of a parasitoid to control the Oriental fruit fly. • We developed a regression equation that can be used to determine extirpation threshold mortality levels of natural enemies. Accurately estimating the risk of sensitive populations subjected to toxicant disturbances is central to our ability to protect ecosystem services. While the gold standard for assessing risk historically involves static measures such as the LD50 or LC50, more sophisticated approaches have been developed in an attempt to capture more nuanced outcomes. In the 1980s the International Organization for Biological Control (IOBC) developed a tiered approach to determine the compatibility of pesticides and natural enemies in the context of integrated pest management (IPM). We analyzed the IOBC approach using stage-based matrix models to project population outcomes for four parasitoid species, Diaeretiella rapae , McIntosh, Fopius arisanus , (Sonan), Diachasmimorpha longicaudata Ashmead, Psyttalia fletcheri (Silvestri) and the predator, Coccinella septempunctata L. By imposing mortality levels in matrix models equivalent to those outlined in the IOBC Tier 1 Class 1 (29 %) (harmless) and Class 2 (79 %) (slightly harmful) mortality classes, we explored discrepancies between the IOBC approach and population outcomes generated by these models. Our results highlight that the IOBC Class 1 and 2 levels of mortality are too high to protect many natural enemies from pesticides, setting the stage for unrealistically optimistic views of pesticide compatibility in many cases. Furthermore, a one size fits all approach to protect natural enemies from pesticides does not work because of differences in demographic rates among species which will be less negatively affected by Tier 1 levels of mortality and those that do not reproduce quickly and will thus be more vulnerable to pesticides. Therefore, the IOBC method should be used cautiously if at all, and results should be interpreted with the caveats and pitfalls highlighted here. Results of this study indicate that it is time to reevaluate how we estimate pesticide compatibility with natural enemies and adjust the methods and mortality thresholds based on more realistic measures of toxicity.