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.