BEEHAVEecoGUTS - a new module for mechanistic predictions of lethal effects in honey bee colonies.

Honey bees are important pollinators of wild and cultivated plants. They are therefore a key species in the environmental risk assessment of plant protection products in the European Union. As empirical assessments of effects on honey bees on field and landscape level are challenging, mechanistic modelling offers a powerful complement. Such models can predict exposure and effects in the field and are therefore increasingly accepted within the regulatory framework. The BEEHAVEecotox model is a model which links exposure to colony effects in a mechanistic manner. It uses standard regulatory data to derive individual-level dose-response functions and includes larval mortality. However, it requires separate exposure pathways to be represented using pathway-specific dose-response parameters, making parameterization challenging. We developed an alternative based on the general unified threshold model of survival (GUTS), a generic toxicokinetic-toxicodynamic (TKTD) model. It combines exposure pathways into one effect module with one set of parameters. We present BEEHAVEecoGUTS, the integration of GUTS as an optional effect module in the modular design of BEEHAVEecotox. We found that the effect predictions of the original dose-response and the GUTS module are broadly comparable in semi-field tunnel scenarios, while exploratory repeated exposure scenarios revealed stronger divergences between the modules, particularly at higher application rates. GUTS is therefore a viable alternative with simpler and more robust compound specific parameterization and a broader scope for further expansion. It moves from single point estimates of toxicity and exposure to a holistic exposure-effect link.