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

The compatibility between the use of pesticides and beneficial arthropods is an important issue in IPM. The evaluations of pesticide side-effects on non-target organisms (NTO) are a part of regulatory procedures for the authorization of use and commercialization of pesticides. Field tests are deemed useful for the evaluation of the pesticides’ impact in realistic use situations where NTO naturally occurs. However, field evaluations of pesticide side-effects are not always able to account for the natural fluctuations of ecosystem parameters. These parameters maybe involved in the mechanisms that underlie the manifestation of direct and indirect effects of a pesticide on NTO. Predatory mites belonging to the Phytoseiidae family are widely considered as NTO in pesticide side-effect evaluations. Plant-pathogens of several cultivated plants can be of importance as food resources for various phytoseiids. Although their direct impact on phytoseiids, pesticides with fungicidal activity may reduce food availability for predatory mites leading to erroneous conclusions on their toxicity. Here, we present the results of field experiment series on the role of plant-pathogen availability in the evaluations of fungicides impact on predatory mites. Results indicated that the abundance of predatory mites was associated with the plant-pathogen foliar symptoms presence. The presence of predatory mites was different among treatments in response of the toxicological traits of a pesticide (direct effects), but also as consequences of differential plant-pathogen availability induced by fungicide applications (indirect effects). During the investigation, the variable plant-pathogen spreads on untreated control determined contrasting result on pesticides effect. We segregated the direct effects from the indirect food resource-mediated effects including a non-toxic reference in the experimental protocols. Based on these results we suggest that this point can constitute an ameliorative feature in the design of field evaluations on fungicides impact.