The ecotoxicity of five insecticides to the Pheretima group (Oligochaeta) occurring on golf courses in the Pretoria region (South Africa)

Earthworms have an important role to play in turfgrass. They aerify and enrich the soil, enhance water infiltration and break down thatch. Turfgrass managers are advised to select pesticides non-hazardous to earthworms to maintain the long-term stability of a healthy turf An artificial soil test was used to assess the toxicity of five insecticides, used for turf grass pest management, on the Pheretima group (Megascolecidae). The recommended application rates of carbaryl, chlorpyrifos, irnidacloprid, cyfluthrin and fipronil were tested. The LC50 values were also determined by using five dosages and a control (five replicates each) per insecticide. The dosages tested in the experiment were determined on a surface area basis. The effects of the tested insecticides on earthworm mortality, earthworm biomass and individual earthworm mass were assessed 24 hours, 48 hours and 7 days after application of the insecticides against the recommended application rate. Biomass and individual earthworm mass were also determined for dosages of the probit analysis where less than 30% mortality occurred. Individual mass was determined by dividing the biomass with the number of earthworm survivors per time interval per insecticide. Intoxication symptoms were noted. Carbaryl and chlorpyrifos (recommended application rates), had a significant greater effect on earthworm mortality than cyfluthrin, seven days after the application of the insecticides. No other significant earthworm mortality was found. None of the insecticides had a significant influence on earthworm biomass. Cyfluthrin initially reduced individual earthworm mass, but not biomass, more than the other insecticides. Carbaryl reduced biomass more than the other insecticides for all the assessments. Carbaryl, imidacloprid and chlorpyrifos had a larger negative effect than the control, fipronil and cyfluthrin on earthworm biomass and individual earthworm mass, for the 14 and 21 day assessments. For the 24 hour interval, carbaryl was the most toxic to earthworms (lowest LC50 of 77.2 mg kg-I), followed by imidacloprid (155.3 mg kg-I), cyfluthrin (350.7 mg kg-I), chlorpyrifos (389.9 mg kg-I) and fipronil (> 8550 mg kg-I) as the least toxic. For the 48 hour and 7 day intervals, imidacloprid was the most toxic to earthworms (LC50 values of 5.0 mg kg-I and 3.0 mg kg-I respectively), followed by carbaryl (15.7 mg kg-I; 9.0 mg kg-I), cyfluthrin (128.4 mg kg-1;110.2 mg kg-I), chlorpyrifos (330.0 mg kg-I; 180.2 mg kg-I) and the least toxic was fipronil (> 8550 mg kg-I both intervals). The biomass and individual earthworm mass were negatively influenced by chlorpyrifos, imidacloprid and cyfluthrin at higher dosages than the recommended application rate. Fipronil only had a significant effect on biomass and individual earthworm mass 24 hours after application at higher dosages than the recommended application rate. The intoxication symptoms noted were impaired activity, coiled and helical configurations, bleeding prostomiums and red sores.