Does persistence of toxic insecticide residues on field-weathered foliage correspond to the actual dissipation of toxic residues to honey bees (Apis mellifera L.) in the field?

Pesticide applicators rely on the Pollinating Insect Hazard Statement, which is found in the Environmental Hazards section of every pesticide label, to determine how toxic a product is to honey bees, as well as its residual toxicity, or the length of time after treatment until that toxicity dissipates. Residual toxicity information is key to current efforts to protect honey bees during crop pollination, by specifying which treatments, applied at dusk, will break down sufficiently before bees begin foraging the next morning.

Residual toxicity statements are informed by tests, whereby treated foliage is harvested at specified intervals to determine whether honey bee contact with this foliage results in mortality (EPA Ecological Guidance 850.3030 - Honey Bee Toxicity of Residues on Foliage).To determine whether this test adequately predicted the rate of dissipation of acute toxicity, we treated plots of white clover (Trifolium repens) and radish (Raphanus raphanistrum) that were preparing to flower or in full bloom, with ten different insecticides. Following treatment, we determine the 24h mortality of honey bees in contact with either weathered leaves, as specified by the EPA protocol, but also for flower heads. We also harvested pollen from flowers and determined the amount of residues using chemical analysis. We found that while there is relatively strong correspondence among residual toxicity values for insecticide residues in pollen, as well as for weathered residues in leaves and flowers, some insecticides had poorer fits among the three measures. Overall, our work suggests the general adequacy of EPA residual toxicity tests.