Peter Olsson

Road verges resemble natural bumblebee habitats in terms of flowers and nesting substrates. Hence, they may benefit bumblebee populations, especially in landscape where other floral and nesting resources are scarce. Such benefits may, however, be offset by the adjacency to traffic. As bumblebees move between habitat patches, effects on populations may manifest across landscapes rather than in the local road verge. Using a split-plot design, we studied how wild bumblebees respond to traffic intensity at local and landscape (within 1000 m) scales. We did this along a gradient of open, uncultivated land, including traditionally managed grasslands, which is an important habitat for bumblebees. In 20 study landscapes, we monitored bumblebees along each of three roads, one private road with negligible traffic and two public roads with contrasting traffic intensity (high and low). We assessed mortality, within-season variation in abundance (early queens, early and late workers, and new reproductives), and nest density (supported by scent-trained dogs). Densities of living early queens and workers increased with flower cover in the road verge but decreased with local traffic intensity, which was paralleled by increasing numbers of dead queens and workers. Nests were rare in road verges along the public roads compared to along the private road verges, where traffic was negligible. Despite that traffic clearly increased mortality and restricted the benefit of flowers in road verges, we observed no negative effect of traffic at the landscape-scale. This suggests that other factors are regulating bumblebee populations. We conclude that traffic limits the benefit for bumblebees of flower-enhancement strategies in road verges, but if implemented along roads with low traffic, such actions may be beneficial by promoting forage availability.