Effect of Soil Moisture on Pesticide Toxicity to an Enchytraeid Worm, Enchytraeus sp.

The aim of the study was to find out whether soil moisture affects toxicity of organic pesticides to an enchytraeid worm. Laboratory experiments were carried out with dimethoate and benomyl, using a small Enchytraeus sp. as the test species. Substrate was natural agricultural field soil cultivated without pesticides for several years. Experimental design consisted of three soil moistures (40, 55, and 70% of water holding capacity) and five pesticide concentrations, plus controls. Measured parameters were survival, size of the parent worms and number and size of juveniles produced. Dimethoate was relatively non-toxic to this species. Dimethoate did not decrease survival, but sublethal effects on adult size and number of juveniles were observed. Adverse conditions in dry soil masked these effects; dimethoate appeared to be less toxic in dry soil than in moist soil. Benomyl caused significant mortality and the effects were very abrupt. Toxicity of benomyl decreased with increasing soil moisture content; in moist soil the worms survived at higher benomyl concentrations than in drier soils.