Field studies for the assessment of pesticides with soil mesofauna, in particular enchytraeids, mites and nematodes: Design and first results
Currently, earthworm field tests and litter-bag studies are regularly required as part of the environmental risk assessment of pesticides in soil. These tests give almost no indication if there is any structural impact, i.e. on the biodiversity of the soil organism community. Therefore, a design is presented here for such a structural study, which follows basically the design of litter bag studies. Our proposal is based on practical experience made in Southern Germany in 2007 / 2008. A new pesticide was tested, framed by a control (water) and a mix of two reference substances (the fungicide Benomyl and the insecticide Chlorpyrifos(ethyl)) of known effect, the latter in order to confirm the sensitivity of the studied soil organisms. For confidentiality reasons, the identity of the test substance cannot be given here. Six replicates were used per treatment (plot size: 7 m x 3 m). The study was performed on grassland and test chemical and reference substance were applied without soil incorporation. Grassland was chosen because diversity and abundance of enchytraeids, mites and nematodes (and soil mesofauna in general) is usually very low in agricultural soil. The duration of the study was one year and sampling was performed –2, 32, 89, 187 and 372 DAT (days after treatment). The taxonomic groups assessed were enchytraeids, soil-inhabiting mites, collembolans, and nematodes, but collembolans will not be covered in this contribution. All groups except nematodes were sampled using ISO standard methods. Specimens were sampled with soil corers (diameter and depth: 5 cm each). The enchytraeids were identified to the species level, but assessment is based on the genus level. Among mites only the oribatids were identified to species level, the rest to higher groups. The nematodes were only assessed quantitatively. The variance of the samples collected was small enough to detect a change in the taxonomic structure of the soil organism community. In this contribution results from the control, test substance and reference plots are presented. In comparison to the control, the number of enchytraeids on the reference plots was reduced by 60 % at individual sampling dates, thus validating the test design. Mites were not affected at all. The number of nematodes decreased by 48 % at most. The test substance itself showed no significant effect at all, except an unexplained reduction of enchytraeids at the 4th sampling date. Genera of enchytraeids behaved slightly differently, suggesting that in future studies the species level should be addressed. Referring to the experiences made in this study it is concluded that the use of soil mesofauna groups in field studies is a practical and promising tool in the environmental risk assessment of pesticides.
ecotoxicology, standardisation, grassland, Benomyl, Chlorpyrifos(ethyl)