Effects of brood termination rate on colony viability – A BEEHAVE modelling study how timing, magnitude and duration of effects determine colony strength

The brood termination rate (BTR) investigated in higher-tier bee brood studies for plant protection product risk assessment is the determinant of honey bee (Apis mellifera L.) mortality during development from egg to adult. It influences colony strength, and in turn pollination services, hive products and colony viability. According to the EFSA Bee GD (2013), a honey bee colony is regarded viable, if at least 5000 worker bees are recorded prior to hibernation. We investigate how magnitude and duration of effects on the BTR affect the strength of honey bee colonies before overwintering and therefore viability. For this purpose, we modified and applied BEEHAVE, a computer model to simulate honey bee colony dynamics. Our modifications allowed for in silico representations of higher-tier bee brood studies under semi-field conditions with the option to follow bee colony dynamics until the end of the season. We have found that bee colonies are rather resilient to an increased BTR, such that under common experimental conditions, the number of brood cells as well as the colony size can recover over time. Yet, if BTR was above ≥ 70% (approximately the effect size caused by the reference item fenoxycarb) for a long period of 20 days or the brood study was started late in the season (1st August), recovery was slow. Nevertheless, only if modelled experiments were started late in the season (1st August), there was a risk of colony sizes below 5000 worker bees before winter (31st October). This risk was found for treatments and control due to the seasonally reduced egg laying rate of the queen. Compared to the control the risk was only relevantly enhanced, if BTR was ≥ 70% for the entire brood cycle.