Optimizing a Pesticide Fate Model Using Field Dissipation Studies on Alfalfa to Predict Honeybee Exposure and Risk

Pesticide fate modeling can be used to explore the expected effects of spray timing on pest and beneficial insects. To obtain accurate dissipation predictions, chemical-specific properties including the foliar photodegradation and foliar penetration rates are needed. In this work, photodegradation rates for three insecticides, as both active ingredient alone and in a commercial formulation, were measured. Chlorpyrifos degraded at a rate similar to the dark control so no photodegradation rate was reported. Reported λ-cyhalothrin and indoxacarb photodegradation rates were not significantly different when applied as the active ingredient or formulation. Measured photodegradation rates were incorporated into a pesticide dissipation model and it was evaluated using six field studies conducted on alfalfa fields using the insecticides chlorpyrifos and λ-cyhalothrin. A comparison of the predicted and measured insecticide concentrations in leaves showed good agreement after model optimization. Time-dependent honeybee risk quotients for the 7-day period following insecticide application were then calculated.