Toxicity of residues on foliage is a standard toxicity test that can be required by the U.S. Environmental Protection Agency (EPA) to assess the potential residual toxicity that pesticides may pose to honey bees (Apis mellifera). The Honey Bee Toxicity of Residues on Foliage (OPPTS Guideline 141-2 or OCSPP Guideline 850.3030) study is conditionally required if the honey bee acute contact (or oral) median lethal dose (LD50) value (obtained from a honey bee acute toxicity test such as OCSPP Guideline 850.3020) is less than 11 µg/bee. These studies are conducted using formulated pesticide products. The formulated products are applied to commodities under field conditions and allowed to weather; bees are then exposed to the weathered residues on the treated commodity. The study is used to determine the length of time that residues must be weathered to result in ≤25% mortality of bees crawling on the treated foliage. The residual time to 25% mortality (referred to as the RT25) data were compiled from registrant-submitted studies and were evaluated to determine whether the RT25 is related to the physical-chemical properties and/or abiotic/biotic degradation rates for each of the active ingredients. The analysis indicates that RT25 values were not correlated with the factors evaluated; given the variability in RT25 values across application rates and between crops and pesticide formulations RT25 values appear to be a function of a number of factors including application rate, crop, and pesticide formulation. This presentation includes the RT25 summary data and suggests that these data are formulation-specific and are not predictive. Although the RT25 value is frequently used to identify pesticides with extended residual toxicity, such use should be tempered with the understanding that the RT25 value is formulation specific.

What is the honey bee (Apis mellifera) RT25 and what does it mean?

254th National Meeting (2017)

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