SUBLETHAL NEUROTOXIC EFFECTS OF THE FUNGICIDE BENOMYL ON EARTHWORMS (EISENIA-FOETIDA)

Earthworms Eisenia fetida were treated by surface contact exposure for 4 days with the fungicide benomyl. Non-invasive electrophysiological recordings after treatment with sublethal concentrations of 0.2-25 mg benomyl litre-1 of water (= 0.003-0.435 .mu.g benomyl cm-2 of filter paper) indicating concentration-dependent decreases in conduction velocity for the medial and lateral giant nerve fibres. Threshold concentration for these effects was approximately two orders of magnitude less than the LC50. The effects on velocity were not accompanied by alterations in refractory period or impairment of locomotory reflexes, but persisted for at least several weeks after treatment. Morphometric analysis of treated animals revealed a disruption of the myelin-like sheath surrounding giant fibres but no change in fibre diameters. It is concluded that benomyl had subtle and sublethal neurotoxic effects on earthworm giant nerve fibres. The data indicate that giant fibre conduction velocity is a particularly sensitive parameter for detecting these effects in intact worms.