The effects of fungicides on non-target mites can be mediated by plant pathogens.

Field tests are used to evaluate the pesticides’ impact on predatory mites in realistic use situations. However, the distinction between the direct and indirect effects of a pesticide is not always possible in field tests. Generalist predatory mites belonging to the Phytoseiidae are considered as non-target organisms in pesticide side-effect evaluations. Plant pathogens of cultivated plants can be of importance as food resources for various phytoseiids. Pesticides with fungicidal activity may have a direct impact on phytoseiids, but they can also an indirect effect when reducing food availability for predatory mites. In field experiments on grapevine, we investigated whether the availability of the plant pathogen grape downy mildew has an effect on fungicides impact on the predatory mites Typhlodromus pyri and Amblyseius andersoni. We used fungicides having a differential selectivity towards predatory mites: copper compounds, folpet, and mancozeb. The abundance of predatory mites was associated with the plant pathogen foliar symptoms. The presence of predatory mites differed among treatments in response of the toxicological features of fungicides (direct effects), but also as consequences of plant pathogen availability induced by fungicide applications (indirect effects). Plant pathogen spread on untreated control determined contrasting results on pesticides effect. We segregated the direct effects from the indirect effects including a non-toxic reference in the experimental protocols.