Life history characteristics of Lumbricus terrestris and effects of the veterinary antiparasitic compounds ivermectin and fenbendazole

Earthworms in pastureland may be exposed to residues of veterinary medicines excreted in the faeces of treated cattle. Possible sub-lethal effects of these drugs on life history characteristics are critical to earthworm populations, but there are no internationally accepted guidelines for the laboratory testing that is essential as a supplement to field studies. In the laboratory, Lumbricus terrestris were kept in artificial soil and offered dung collected from cattle with sustained-release boluses of ivermectin or fenbendazole, or from untreated cattle (control). Earthworms were therefore exposed to drug residues in concentrations excreted naturally by treated cattle. Furthermore, worms were exposed to the drug in the natural way, i.e. by ingestion. A matrix model based on survival and reproductive rates as functions of life-stages was used to quantify the population dynamics of L. terrestris. There were no adverse effects of either ivermectin or fenbendazole on the individual survival and growth of L. terrestris. Mature worms produced on average 2-3 cocoons per worm per month irrespective of drug treatment, and about 50% of these cocoons hatched successfully. The median time to hatching (incubation time) depended on the age of the parent worms and was significantly lower for cocoons in the ivermectin group compared to controls, as the worms became older. The incubation time varied considerably in all groups; even within a batch of cocoons produced during 1 month by one pair of worms, the period sometimes varied by more than 1 year. The intrinsic rate of natural increase (r≈0.0044 d⁻¹) was not affected by either drug treatment. The methods and the model developed in this study are suitable for long-term studies on L. terrestris and give ecologically relevant information on population dynamics. There were no negative effects of ivermectin and fenbendazole on life history characteristics of L. terrestris and it is unlikely that earthworm populations will be affected in the field following normal use of sustained-release boluses in cattle. Finally, the matrix model points to adult survival rate and cocoon viability as the most important variables to be included in future ecotoxicological tests on L. terrestris.