Bumblebee queen mortality along roads increase with traffic

Managing road verges to promote diverse and flower-rich plant communities has been proposed to mitigate the decline of pollinating insects caused by the loss of natural and semi-natural habitat. There is, however, a concern that flower-rich road verges can be ecological traps for pollinators as insects might be attracted to a habitat where there is a risk that they are killed by traffic. Therefore, we investigated the combined effects of traffic intensity and flowering plant diversity in road verges on the mortality and behaviour of bumblebee queens. The probability that an observed bumblebee queen was dead almost quadrupled when traffic increased from 100 to 6000 vehicles per day and tended to be lower when the flowering plant diversity in the road verge was high. Further, the number of nest-seeking bumblebee queens decreased dramatically with increasing traffic intensity in sites with low diversity of flowering plants, but not in sites with high diversity. Based on published data on bumblebee colony densities in different habitat types, we estimate that between 0.2 % and 32 % of all bumblebee queens present in our study landscapes were killed by traffic. We conclude that the flowering plant diversity in the road verge neither mitigates nor exacerbates the mortality from traffic. Road verges often provide pollinators with a high abundance of resources in florally poor landscapes. We therefore advise management to prioritise plant diversity along roads with low traffic intensity.