Comparing Sensitivity of Different Bee Species to Pesticides: A TKTD modeling approach.

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;00:1-11. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Acute toxicity of pesticide mixtures to honey bees is generally additive, and well predicted by Concentration Addition.

Understanding the frequency of non-additive effects of pesticides (synergism and antagonism) is important in the context of risk assessment. The goal of this study was to investigate the prevalence of non-additive effects of pesticides to honey bees (Apis mellifera). We investigated a large set of mixtures including insecticides and fungicides of different chemical modes of action and classes. The mixtures included represent a relevant sample of pesticides that are currently used globally. We investigated whether the experimental toxicity of the mixtures could be predicted based on the Concentration Addition (CA) model for acute contact and oral adult bee toxicity tests. We measured the degree of deviation from the additivity predictions of the experimental toxicity based on the well-known Mixture Deviation Ratio (MDR). Further, we investigated the appropriate MDR thresholds that should be used for the identification of non-additive effects based on acceptable rates for false positive (alpha) and true positive (beta) findings. We found that a deviation factor of MDR = 5 is a sound reference for labeling potential non-additive effects in acute adult bee experimental designs when assuming a typical Coefficient of Variation (CV%) = 100 in the determination of the LD50 of a pesticide (a factor of 2× deviation in the LD 50 resulting from inter-experimental variability). We found that only 2.4 % and 9 % of the mixtures evaluated had an MDR > 5 and MDR < 0.2, respectively. The frequency and magnitude of deviation from additivity found for bees in this study are consistent with those of other terrestrial and aquatic taxa. Our findings suggest that additivity is a good baseline for predicting the toxicity of pesticide mixtures to bees, and that the rare cases of synergy of pesticide mixtures to bees are not random but have a mechanistic basis.

Chironomids: suitable test organisms for risk assessment investigations on the potential endocrine disrupting properties of pesticides.

Selecting an appropriate invertebrate assay has been a primary goal of national and international testing programs for endocrine disrupting chemicals. The available information on the endocrine system, its hormones and their modes of action in controlling physiological processes in invertebrates is limited and the selection of appropriate test species still presents a challenge. This paper outlines the development of a higher-tier full life cycle (FLC) test for pesticides with the non-biting midge Chironomus riparius (Insecta, Diptera, Chironomidae). As an insect, C. riparius represents the species' richest and ecologically one of the most important groups of invertebrates. In addition, the endocrine system of insects is one of the best studied among the invertebrates. Acute and chronic tests with Chironomus spp. are commonly used for testing and risk assessment of agrochemicals. A chironomid FLC test protocol has been developed and its suitability investigated in an inter-laboratory comparison. The protocol used is based on existing OECD and US-EPA test methods. To verify the suitability of the test to generate endpoints that encompass adverse effects on the arthropod endocrine system, a juvenile hormone analog was selected as positive control substance. Results have demonstrated that the proposed chironomid FLC can be performed in separate laboratories and that the selected arthropod juvenile hormone mimic causes effects. However, the observed toxicity is not proof of an endocrine disruptive mechanism and could equally be evoked by other compounds. Contrary to a screening assay, which aims at revealing a substance's mode-of-action, the FLC test generates robust, population-relevant endpoints that can be used in the risk assessment of agrochemicals. Since the initial results presented in this paper are encouraging we propose to complete the validation of this assay under OECD with high priority.