Toxicity of Insecticide-Contaminated Soil Used in the Treatment of Cotton Seeds to Bees

Insecticide-treated cotton seeds can pose risks to Apis mellifera Linnaeus, 1758 populations during crop establishment if chemical residues reach these insects near agricultural fields via dust drift produced during planting. However, the treatment of seeds with insecticides is essential to protect cotton plants from damage caused by pests, including thrips and aphids. Thus, the aim of this study was to evaluate the toxic effects (acute toxicity) of soil dust from cotton fields planted with insecticide-treated seeds on A. mellifera adults using a toxicity assay developed in the laboratory. Forager honeybees were maintained in 700-ml plastic cages with 7 g of surface soil where insecticide-treated cotton seeds were sown (270 g a.i. clothianidin, 270 g a.i. imidacloprid, 210 g a.i. thiamethoxam, and 75 g a.i. fipronil/100 kg of seeds, and an untreated group). Ten bees were placed in each cage. The experimental design was randomized, with five treatments and twelve replicates. The mortality rate was evaluated during the entire assay. Data were transformed to x + 0.5 and compared with a regression analysis and contrast test. The linear regression model revealed a significant relationship between bee mortality and exposure time. The mortality rate gradually increased as time progressed in all treatments and the control group. The contrast test did not reveal significant differences between the insecticide and control groups. Thus, residues of the products tested, thiamethoxam, clothianidin, imidacloprid, and fipronil, did not influence the mortality of A. mellifera adults as a result of exposure to soil contaminated with insecticide-treated seeds.