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In an acute contact test with bees the compound of interest is dissolved in a carrier solvent (frequently acetone) and then a droplet of the solution is placed on the dorsal thorax of the bee. The volume of the droplet is standardised to 1 µL for honeybees and to 2 µL for bumblebees. In practice the same droplet volume is used for bees with very different sizes. In this research the effect of the droplet volume was evaluated with acute contact tests with dimethoate for the alfalfa leafcutter bee, the red mason bee, the honeybee and the bumblebee. The results were analysed with a ToxicoKinetic ToxicoDynamic (TKTD) model to separate kinetic from dynamic effects. This allows to compare the sensitivity of the bee based on the effect threshold and not on the time, species and test dependent LD50s. The analysis of the test results indicates that the magnitude of the response of the bees increased with increasing droplet size. The results also showed that the manifestation of effects over time is slower for the red mason bee and the bumblebee compared to the honeybee and the alfalfa leafcutter bee. This implies that the result of a 2 day test with a fixed dosing volume results in different response for a bumblebee compared to the alfalfa leafcutter bee, not because of different sensitivities of the bees involved but due to the difference of relative dosed surface ratio. So comparing the sensitivity of bee species, based on standardised tests is biased and amplifies the sensitivity for the smaller bee species.

Risk assessment for bees is mainly based on data for honey bees; however, risk assessment is intended to protect all bee species. This raises the question of whether data for honey bees are a good proxy for other bee species. This issue is not new and has resulted in several publications in which the sensitivity of bee species is compared based on the values of the 48‐h median lethal dose (LD50) from acute test results. When this approach is used, observed differences in sensitivity may result both from differences in kinetics and from inherent differences in species sensitivity. In addition, the physiology of the bee, like its overall size, the size of the honey stomach (for acute oral tests), and the physical appearance (for acute contact tests) also influences the sensitivity of the bee. The recently introduced Toxicokinetic–Toxicodynamic (TKTD) model that was developed for the interpretation of honey bee tests (Bee General Uniform Threshold Model for Survival [BeeGUTS]) could integrate the results of acute oral tests, acute contact tests, and chronic tests within one consistent framework. We show that the BeeGUTS model can be calibrated and validated for other bee species and also that the honey bee is among the more sensitive bee species. In addition, we found that differences in sensitivity between species are smaller than previously published comparisons based on 48‐h LD50 values. The time‐dependency of the LD50 and the specifics of the bee physiology are the main causes of the wider variation found in the published literature. Environ Toxicol Chem 2024;43:1431–1441. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. [ABSTRACT FROM AUTHOR]

There are different tests for testing the impact of chemicals on bees: the acute oral test, the acute contact test and the chronic oral test. For honey bees, OECD guidelines are avialable stating how these tests need to be conducted. The endpoint of the tests is an LD50-value expressed in ug/bee, where the chronic test usually has the most conservative result. In current practise, the results of these tests are interpreted independently and the most conservative result is chosen for further evaluation. Unfortunately, in this approach it is not known how the different exposure regimes influence the result and what the time dependency of the LD50 values is. Extrapolation and interpretation issues between exposure regimes and time can be solved by using a mechanistic approach where time is explicitly considered and effects are interpreted with time-independent parameters. The already developed and published GUTS modelling framework was used as a starting point and was adapted to take into account the physiology of the bees and the details of the different existing tests for bees. It showed that the different bee tests (acute oral, acute contact and chronic) could be interpreted within this framework with one set of parameters describing the toxicity of a compound for bees. The framework was then applied to other be species to compare sensitivity leading to new insights in bee sensitivity and bee testing. [ABSTRACT FROM AUTHOR]

In an acute contact test with bees the compound of interest is dissolved in a carrier solvent (frequently acetone) and then a droplet of the solution is placed on the dorsal thorax of the bee. The volume of the droplet is standardised to 1 µL for honeybees and to 2 µL for bumblebees. In practice the same droplet volume is used for bees with very different sizes. In this research the effect of the droplet volume was evaluated with acute contact tests with dimethoate for the alfalfa leafcutter bee, the red mason bee, the honeybee and the bumblebee. The results were analysed with a ToxicoKinetic ToxicoDynamic (TKTD) model to separate kinetic from dynamic effects. This allows to compare the sensitivity of the bee based on the effect threshold and not on the time, species and test dependent LD 50 s. The analysis of the test results indicates that the magnitude of the response of the bees increased with increasing droplet size. The results also showed that the manifestation of effects over time is slower for the red mason bee and the bumblebee compared to the honeybee and the alfalfa leafcutter bee. This implies that the result of a 2 day test with a fixed dosing volume results in different response for a bumblebee compared to the alfalfa leafcutter bee, not because of different sensitivities of the bees involved but due to the difference of relative dosed surface ratio. So comparing the sensitivity of bee species, based on standardised tests is biased and amplifies the sensitivity for the smaller bee species. [Display omitted] • The sensitivity of bees to pesticides is mainly based on standardized tests with honeybees in mind. • Dedicated acute contact tests were carried out with honeybees, bumblebees, red mason bees and alfalfa leaf cutter bees. • The relative covered surface area for different bees was analysed for different droplet sizes in an acute contact test. • We show that the droplet volume affects both the speed at which effects develop and the sensitivity of a bee. • We show that the current approaches have a bias in amplifying the sensitivity of smaller bees. [ABSTRACT FROM AUTHOR]