Close proximity to a major road does not impact the growth and reproductive success of Bombus terrestris colonies.

Background/Question/Methods

Road verges are often proposed as a key habitat for pollinator conservation; they can provide pollinators with food, nesting and overwintering sites, and improve connectivity within the landscape. However, there are growing concerns over whether the benefits provided by verges outweigh the costs of foraging and nesting in such close proximity to traffic. This study aimed to assess if there were any impacts, positive or negative, of roads and road verges on the growth and reproductive success of bumblebee colonies positioned adjacent to a major road. 16 colonies of Bombus terrestris were placed on the Elveden Estate in Norfolk, England between May and September 2021. All colonies were reared from wild-caught B. terrestris queens and were screened for pathogens prior to placement in the field. Eight were located within 5m of a major trunk road (the A11, average daily traffic flow 25,582 vehicles), which runs through the center of the estate, and the remaining eight were placed at least 1km from the road edge. Weekly surveys of colonies were carried out during dark hours, to record the following metrics: colony weight, number of workers, queen survivorship, and number of males and gynes.

Results/Conclusions

Previous field studies have experimentally demonstrated impacts of certain aspects of road pollution separately (collisions, noise, vibration, air/metal pollution). In reality, these factors will work together rather than in isolation, and this is the first study to assess how roads and verges impact bumblebee colony development in a real-world scenario. We found that colonies positioned on the road verge did not significantly differ from colonies located away from the verge with regards to overall size, reproductive success, queen survivorship, or the onset of male and gyne production. The results suggest an overall neutral effect of close proximity to the A11 on B. terrestris colonies. Although initially surprising, these results do align with previous work which revealed that larger-bodied insects with greater mobility, such as B. terrestris, are less likely to experience negative effects of roads. The lack of negative impacts on B. terrestris from a large and busy road suggests that road verge management should indeed take a significant role in bumblebee conservation.