Did the bee cross the road? If not, why not – and what does this mean for the flowers on the other side? In their latest research, Fitch & Vaidya investigate the influence of roads on pollinator movement and pollination by examining patterns of pigment transfer between focal plants of two species.

We know that large highways kill billions of insects each year, but whether roads also influence movement patterns, and how that impacts pollination, remains unclear.

To address this, our study focused on two plant species native to the midwestern USA: wild bergamot or monarda (Monarda fistulosa) and threadleaf coreopsis (Coreopsis verticillata). We expected these to be visited by distinct pollinator assemblages – with monarda visited primarily by large bees, and coreopsis by small bees.

We wanted plants that were visited by different pollinators, and specifically pollinators that differed in size, because we suspected that the movement of smaller insects would be more impacted by roads. To examine how pollen movement between plants was affected by the presence of an intervening road, we used powdered fluorescent pigment as a pollen analogue.

Individual potted plants were deployed in sets of three: one ‘pigment-added’ plant (which received a heavy dusting of fluorescent pigment to each open inflorescence and was set out at a roadside location), one ‘across’ plant (which we placed directly across the road from the pigment-added plant), and one ‘along’ plant (which we placed alongside the road on the same side as the ‘pigment-added’ plant, and at the same distance from the ‘pigment-added’ plant as the ‘across’ plant.

This setup allowed us to control for the fact that the habitat and pollinator communities might differ across sites, since our main focus was the difference between ‘across’ and ‘along’ plants in the amount of pigment they received.

The plants were left out for a day at each site. At the end of the day, we collected the plants and recorded the number of inflorescences on the ‘along’ and ‘across’ plants that now had fluorescent pigment. We did this at 32 roads and 15 car-free bike/pedestrian paths.

We chose roads that ranged in size from a single lane to five lanes of traffic, to determine if road size and traffic volume and speed mediated the effect of roads on pigment movement. To assess whether the two species were indeed visited by pollinators of different sizes, we conducted short observations of flower visitation at most sites.

So, did roads affect bee movement? In short, yes. Inflorescences on ‘across’ plants were, on average, 50% (for coreopsis) or 34% (for monarda) less likely to receive pigment than ‘along’ plants and as roads got wider and busier, fewer and fewer inflorescences received pigment – with the effect again being more dramatic for coreopsis.

As we suspected, coreopsis was mostly visited by very small bees (especially sweat bees), while monarda was visited primarily by the larger honey bees and bumble bees. Given that pigment receipt in coreopsis was more sensitive to roads, this suggests that roads are more of a deterrent to small than large bees.

Ecological field studies are famous for having very ‘noisy’ data, so we were surprised to find such consistent effects – and particularly to find that even narrow, quiet roads affected the movement of pigment.

Our results suggest that people concerned about bee populations, and about genetic connectivity of populations of the plants that depend on them, would do well to consider the effects of roads.

New road construction may serve to fragment plant populations, particularly if they rely on small insects for pollination, so the placement of such roads should be carefully considered. Our finding that wider and busier roads pose more of a barrier to the movement of bees is not shocking, but it does point to strategies for mitigating the effects of existing roads on bees and pollination

Intriguingly, many of the strategies that would be likely to reduce the impact of roads on bees – e.g. reducing road width, decreasing traffic speed – are also part of the Vision Zero campaign to reduce human traffic fatalities, which started in Sweden and is now being implemented across Europe and North America. This highlights the potential for win-win scenarios in designing the cityscapes and roadscapes of the future for both human safety and ecological health.

The full paper Roads pose a significant barrier to bee movement, mediated by road size, traffic and bee identity is free for a limited time in Journal of Applied Ecology.