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. Both information
sources consistently recommend these management practices to provide
pollinator habitat in ROW and promote plant and pollinator diversity and
abundance: Reduce mowing frequency and time mowing to pollinator
activity.  Target herbicide applications to undesirable plant
species using backpack sprayers. 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
indicates that these threats vary widely across road sizes, types, and
landscape contexts and suggests that 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. 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; 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.