Do pesticides affect trends in microarthropod communities according to food preferences as indirect effect?

In agroecosystems the use of plant protection products is common practice. During the application a fraction of these products reaches the soil, making the soil community exposed to different pesticides. These products have been developed to kill the target organisms (fungi, plants or insects), but may give adverse effects also on non-target species. The effects can be direct on the species, or indirect, due to the effect of the pesticide on another organism (e.g. prey or predator). An example is the effect of a fungicide on fungi eaten by fungivorous animals. These possible indirect effects were studied in the field, by investigating a microarthropod community exposed to plant protection products in a vineyard in Northern Italy. Three sampling points were chosen, in a gradient of contamination: one inside the field, and two 4 m and 10 m away from the last plant row. The community was sampled in a sampling scheme drawn according to the insecticide applications. In particular, to evaluate indirect effects, three samples were examined: two after the application of herbicides and fungicides but before application of the insecticide chlorpyirifos, and the third after chlorpyrifos application. All the information about the active ingredients used was obtained and the exposure of the community was assessed with traditional ecotoxicological tools. The sampled organisms were identified at least to the order level, but for some taxa to the family level, or a division into major groups was made. Information on the diet was obtained from the literature to divide the different taxa into groups according to their food preferences. Results were related to the control 10 m away from the field. The hypothesis was that before the use of chlorpyrifos, there could be indirect effects on the abundance of organisms due to the elimination of food sources, while after chlorpyrifos application the direct effect of the insecticide should become visible. Different trends were observed in the community, according to the food preferences of the taxa: bacteriophages, for example, showed a decrease between the two dates before chlorpyrifos application, while the numbers of fungivorous, phytophagous and detritivorous organisms were almost constant. Coprophagous, necrophagous and secondary consumers showed a constant decrease during the entire productive season. The ratio between indirect and direct effects was highlighted, demonstrating that direct effects overshadow the indirect ones.