Do earthworms help to sustain the slug predator Pterostichus melanarius (Coleoptera : Carabidae) within crops? Investigations using monoclonal antibodies

Earthworms provide a major potential source of alternative food for polypha gous predators, such as carabid beetles, that are natural enemies of slugs, aphids and other agricultural pests. Non-pest prey may foster larger numbe rs of natural enemies, which then help to control pests, or alternatively m ay help to divert the predators away from pest control. An earthworm-specif ic monoclonal antibody was developed to study carabid-earthworm interaction s in the field and assess the role of earthworms as alternative prey. The a ntibody could identify as little at 7 ng of earthworm protein in an ELISA, and could detect earthworm remains in the foregut of the carabid beetle Pte rostichus melanarius for 64 h after consumption. Thirty-six per cent of fie ld-collected beetles contained earthworm remains. Quantities of earthworm p roteins in the beetle foreguts were negatively related to total foregut bio mass, suggesting that earthworm consumption increased as total prey availab ility declined. There was also a negative relationship between foregut biom ass and beetle numbers, but both quantities and concentrations of earthworm proteins in beetle foreguts were positively related to beetle numbers. Thi s suggests that as beetle activity-density increased, total prey availabili ty declined, or, as prey availability declined, beetles spent more time sea rching. In these circumstances, beetles fed to a greater extent on earthwor ms, an acceptable but nonpreferred food item. Earthworms may, therefore, pr ovide an ideal alternative prey for P. melanarius, helping to sustain it wh en pest numbers are low but allowing it to perform a 'lying-in-wait' strate gy, ready to switch back to feeding on pests when they become available.