Roadside rights-of-way as pollinator habitat: A literature review

Pollination of crops and naturally-occurring flowering plants is a critical ecosystem service provided by managed and unmanaged animal pollinators. Insects are the most studied pollinators, particularly managed honey bees, unmanaged wild bees, and butterflies. Bees and butterflies thrive in early-successional habitat featuring grasses, exposed soil, wildflowers, and shrubs, which is consistently found within transportation and utility rights-of-way (ROW). However, intensive management of ROW can reduce the amount of high-quality pollinator habitat; such practices include frequent mowing, broadcast herbicide use, and planting non- native cool season grasses. Here, we review peer-reviewed academic and non-peer reviewed gray literature describing ROW management practices and their effects on pollinator populations, focusing on applications of these practices in landscapes similar to those found in Maine and the northeast United States; that is, landscapes that are heavily forested and interspersed with agriculture, developed areas, and wetlands. The literature consistently recommends these management practices to provide pollinator habitat in ROW and promote plant and pollinator diversity and abundance:
1) Reduce mowing frequency and time mowing to pollinator activity.
2) Target herbicide applications to undesirable plant species using backpack sprayers.
3) Plant native seeds, seedlings, or shrubs, leaving some exposed soil for nesting.

We considered threats to plants and pollinators associated with ROW, including traffic volume and mortality, noise, light, and air pollution, and habitat fragmentation. The literature suggests that these threats vary widely across road sizes, types, and landscape context, and the overall negative impacts do not outweigh the potential benefits of promoting pollinator habitat in ROW. Landscape context also influences the composition of ROW plant and pollinator communities. In Maine, agriculture and grassland in the surrounding generally reduced bumble bee and butterfly abundance in Priority 1 ROW sites. Many state Departments of Transportation have incorporated integrative vegetation management (IVM)  principles into ROW management, and we summarize a number of case studies here. Restoration projects in high-visibility areas are common; further, these can lead to public support for additional pollinator habitat enhancement. Implementing new management practices can be difficult, therefore we discuss strategies to aid in successful adoption, including gathering public support, collaborations between public and private agencies, and innovative funding opportunities. While assessing vegetation management impacts on bee and butterfly communities in ROW is a rapidly expanding area of research, there are still many gaps in current knowledge. We conclude this report by addressing these gaps and provide suggestions for further study.