Pesticide-induced surface migration by lumbricid earthworms in grassland: life-stage and species differences

Pesticide-induced changes in surface migration by earthworms in grassland were investigated using trapping and the fungicide benomyl. Traps were tended daily for 15 days after spraying, resulting in 2152 earthworms, five species, and juvenile predominance which reflected species/life-stage composition in the soil. Significant increases in migration (all worms) occurred already by day 2 due to spraying, final treatment level being 2.8× control. Life-stage composition indicated an increased juvenile proportion from 55% to 75% due to treatment. Spraying caused surfacing juveniles to increase significantly by day 2, reaching a final level 3.8× control, whereas for mature worms a significant increase did not occur until day 4. Species rank-order was Aporrectodea longa> A. rosea> Lumbricus terrestris> A. caliginosa in control areas, but A. longa> L. terrestris> A. rosea> A. caliginosa in sprayed areas; spraying altered the rank-order such that the anecic A. longa and L. terrestris dominated, jointly increasing from 59% to 78%. At species level, L. terrestris and A. longa exhibited significant increases of 4.6× and 3.6× in final migration levels in treated areas, the endogeic A. rosea and A. caliginosa having trends for increase. Species-specific differences for reaction time occurred, with significantly elevated migration already by day 1 for L. terrestris, and day 2 for A. longa and A. caliginosa. For each species, juveniles consistently showed greater increases than mature worms due to spraying, significantly so for juvenile L. terrestris, A. longa, and A. caliginosa, the two anecics reaching as high as 5.3× and 4.7×. The response of mature worms differed: A. longa and A. rosea increased surfacing due to treatment, L. terrestris showed a delayed reaction, whereas A. caliginosa exhibited suppressed migration. Results are discussed relative to behavior, ecological category, and risk of toxic exposure.