Field assessment of toxic effects on reproduction in the earthworms Aporrectodea longa and Aporrectodea rosea

The aim of the present study was to estimate the in situ cocoon production in grassland of two earthworm species, Aporrectodea longa and A. rosea, in relation to application dose of Benomyl. The method used to estimate cocoon production was based on a detailed knowledge of the influence of temperatu re on development [R(T)] of earthworm cocoons, including the two species in this study. Soil temperature was measured throughout the study (May 5-June 6, 1997), and on June 6, the density of cocoons was estimated. The sampled cocoons were incubated in the laboratory and their hatching distribution w as recorded. Based on R(T), the average development time in the field of co coons produced on May 5 was predicted, and the hatched cocoons could thus b e scored as either produced during the study period or before. This made it possible to estimate the density of cocoons produced during the study. The density of adults was also estimated by soil sampling during the study, an d finally the reproductive rate could be calculated as density of cocoons d ivided by density of adults. In control plots, the reproduction of A. longa was estimated to be 2.5 +/- 0.7 (mean, SEM; n = 4) viable cocoons/adult du ring the 32-d study. In plots treated with 0.5 and 1.0 kg Benomyl (active i ngredient [AI])/ha, a 70% reduction of the reproductive rate was observed, but this was only statistically significant at the highest dose. In control plots, the reproductive rate of A. rosea was estimated to be 6.05 +/- 0.8 cocoons/adult during the study. Also in this species, there was a drastic d ecrease in reproduction, about 40% at both doses, due to Benomyl applicatio n. The results obtained in this field study were compared with results from laboratory reproduction tests with other earthworm species. There was good agreement between effects of Benomyl on reproduction in the laboratory and in the field. These results therefore suggest that standardized laboratory tests provide a reasonable prediction of the effect in the field.