Effects of pesticides on nerve-conduction velocity in earthworms : A dissertation submitted in partial fulfilment for a Bachelor of Science with Honours at Lincoln University

Abstract

Earthworms (Aporrectodea caliginosa) were exposed to three insecticides (carbaryl, chlorpyrifos and DDT) at differing concentration ranges that elicited sub-lethal effects. The conduction velocities (CV) of the medial giant fibers (MGF) and also the body weight changes were measured at regular intervals. Three experiments were conducted with two ages of the earthworm (2 and 4 weeks). Experiment 1 involved immature earthworms (4 weeks old) in petri dishes and individual changes in CV were monitored over 4 days. This study followed the OECD guideline 207 methodology for earthworm toxicity testing, but used CV measurements of the MGF as the test tool rather than LC₅₀. Experiment 2 involved juvenile earthworms (2 weeks old) in soil and studied long-term changes in CV and body weight over time. In experiment 3 immature earthworms were used to study the short-term changes in CV and body weights of earthworms in soil.

The results, in general indicated that exposure to carbaryl, chlorpyrifos, and DDT decreased the CV of the MGF in earthworms in all three experiments. However, this was significant at high doses with the 3 insecticides tested only in experiment 3. Body weight and CV in earthworms exposed to soil treated with similar concentrations of the insecticide (experiments 2 and 3) showed changes over time, but with immature earthworms the CV returned to baseline levels within 14 days.

In experiment 3, body weight increases were evident with insecticide at the low doses (0.0001 and 0.00001 mg/kg bw), and this was significant (p=0.002) with carbaryl. The body weights of the earthworms exposed to these two doses of carbaryl doubled compared to the body weight of the controls. However CV of the same earthworms exposed to these two doses remained relatively constant and was comparable to the CV of the controls during the same times. Also, at the higher concentration (0.001 mg/kg bw), the insecticide inhibited the body weight and also the CV with carbaryl, this was significant against preceding lower dose concentrations (p<0.05), but not against the control (p>0.05).

Based on the results with carbaryl, chlorpyrifos and DDT, this study confirmed that the non-invasive electro-physiological recording of earthworm CV in MGF is a sensitive method for detecting potential sub-lethal neurotoxic effects of insecticides on earthworms.