Linking Soil Sorption to Plant Uptake of the Systemic Insecticide Imidacloprid In Viticulture.

The systemic neonicotinoid insecticide imidacloprid is widely used in crop protection including for control of sucking insect pests in viticulture. Knowledge of the duration of systemic insecticide activity in plants at levels high enough to maintain insect control is critical for effective pest management. Plant uptake of a systemic insecticide is dependent upon the concentration of the compound in the soil solution that is taken up by the plant in response to evaporative demand. Thus, solution phase concentration and pesticide effectiveness will depend on the soil distribution coefficient (K_D) of the compound. A field study to determine the effects of K_D on the uptake of imidacloprid by grapes was carried out. Imidicloprid was applied to grapes in three different fields in the spring of 2010 and plant concentrations were measured bi-weekly throughout the growing season. The average concentration of imidacloprid was significantly higher in grapes grown in the soil with the highest K_D (1.43) compared to the other soils (K_D 0.71 and 0.76). Soil sorption was found to be correlated to soil clay content. Insect control of vine mealybug was also highly correlated to the concentration of imidacloprid in the grapes. Soil imidacloprid concentration following the growing season was also found to be 15.3 and 23 times higher for the soil with the higher K_D (1.84 mg kg^-1) than the other two soils (0.12 and 0.08 mg kg^-1). Soils with low imidacloprid sorptive capacity allowed the insecticide to rapidly leach from the soil profile before plant uptake could occur resulting in inadequate vine mealybug control. Soils with higher clay content provided some retardation of imidacloprid allowing for plant uptake and insect control.