2.2 Validation of the 22-day Honey Bee Larval Toxicity, Repeated (Chronic) Exposure Study Design

Abstract

Assessing the chronic toxicity of a compound to developing honey bees (Apis mellifera L.) has proven to be a challenge since the mid-2000s. Such data are requested by global regulatory authorities so they can evaluate the risk of compounds to larval honey bees when exposure is likely to occur in the environment. Poor test performance has led to frequent study failures and data uncertainty. Here we highlight a recent effort by the Pollinator Research Task Force (PRTF)¹ to validate the use of a method for evaluating the chronic toxicity of a compound (e.g., a pesticide) to an immature honey bee for use in a risk assessment. A ring test protocol was selected and based upon the current OECD guidance document No. 2392 with amendments developed at the University of Florida (Schmehl et al. 2016)³. Fifteen independent laboratories on three continents representing government, academia, and industry followed the same testing protocol to:
1) determine if test performance is robust across different geographic regions and different laboratory personnel and
2) identify limitations associated with the methodology.
The control performance criteria for a valid test according to OECD GD 239 is ≥ 85% survival at the end of the larval development and ≥ 70% survival through adult emergence. Thirteen trials (81.3%) satisfied the validity criteria and the test design’s performance was determined adequate for regulatory testing. The toxic reference chemical (dimethoate) had a consistent response with a 22-day EC ₅₀ range of 8-22 μg active substance (a.s.)/g diet. An acetone concentration at the maximum concentration allowed by the OECD GD 239 (2% acetone) was observed to be problematic to test performance. In conclusion, the ring test methods based upon the OECD GD 239 demonstrated that the repeat (chronic) exposure of a compound on developing bees can be successfully conducted. A copy of the full study report4 can be accessed here.