Response of Some Microorganisms, Earthworms and Snails to Pesticides (Carbofuran and Paraquat) under Tropical Conditions

Aim: to determine the response of microorganisms, earthworms and snails to pesticides under tropical conditions.

Study Design: soil experiment with a controlled lighting system of 12 hour of light and 12 hour of darkness was adopted.

Place and Duration of Study: Department of Environmental Management and Toxicology, Federal University of Petroleum Resources, Effurun, Nigeria/three months.

Methodology: Acute effect of pesticides on earthworms and snails was assessed using a 14 day soil bioassay test as recommended by Organization for Economic Cooperation and Development (OECD) and International Organization for Standardization (ISO). Effect of pesticides application on microorganisms in soil was evaluated via standard microbial counts.

Results: Toxicity of Carbofuran to the earthworms and snails was higher relative to Paraquat with a median lethal concentration (LC50) of 10.7 mg/kg; 159571 mg/kg (earthworms) and 23.22 mg/kg; 759000mg/kg (snails), respectively. Microbial counts increased in pesticides treated soil after the initial decline in numbers. Total heterotrophic bacterial counts in Carbofuran and Paraquat treated soils increased from 1.59 × 106 CFU/g to 2.41 × 106 CFU/g and 1.46 × 106 CFU/g to 2.08 × 106 CFU/g, respectively for day 14 to 21. A reverse trend was observed in control soil. Fungal counts increased in Carbofuran (1.18 × 105 CFU/g to 1.42 × 105 CFU/g); Paraquat (1.53 × 105 CFU/g to 1.79 × 105 CFU/g) from day 14 to day 21. Actinomycetes counts were in magnitude of 104 CFU/g soil for both Carbofuran and Paraquat treated soils, while population of phosphate solubilizers and nitrifiers were in 105 CFU/g soil.

Conclusion: Toxicity test estimates the possibility that antagonistic environmental impacts/influence might take place or are taking place due to exposure to sole or additional pesticides. Pesticides had no adverse effects on the microorganisms at recommended field rates. Thus, their use must strictly be based on these recommended rates.