EVALUATING ROADSIDE INTEGRATED VEGETATION MANAGEMENT (IVM) TECHNIQUES TO IMPROVE POLLINATOR HABITAT

Abstract

Pollinator declines have spurred interest in managing road verges as early successional habitats that promote wildflowers. Reduced mowing is a common cost-effective pollinator habitat improvement strategy. Studies on verge management demonstrate the benefits of this method, but also highlight downsides including dispersal of weed seeds by mowing machinery that facilitates establishment and persistence of noxious weeds. Herbicides are commonly used on electric powerline rights-of-way (ROW) to manage invasive plants, as well as establish and maintain low-growing pollinator habitat. This method, however, has not been tested for its suitability for verge management.To explore this knowledge gap, we compare pollinator habitat quality and bee abundance in six Maryland, USA roadsides under three different vegetation management types: frequent mowing, reduced mowing, and spot-spray. For two growing seasons, we took standardized photographs of road verge transects to measure foliar cover using image analysis software (percent cover sampling), counted floral units in fixed quadrats (quadrat sampling), and recorded all insect-pollinated blooms throughout treatment areas and assigned perceived abundance categories to each species (scanning sampling). Management types were compared with respect to the density, native status and diversity of roadside flowers, the proportion of bare ground and leaf litter, and bee abundance. We found that road verges managed under spot spray and fall mow resulted in improved pollinator forage with higher vascular plant cover (4x), floral density (4 and 7x), proportion of native plant species (1.9 and 2.5x), and Hill number’s diversities (3 and 4x) compared to the control. The two treatment groups did not differ in measured metrics, with three exceptions: Spot spray sites had higher proportions of native floral units and potential nesting sites (bare and leaf covered ground); whereas fall mow increased plant species diversity. Surprisingly, we did not detect a significant difference in bee abundance among the three management regimes. However, season, site and year were major predictors of bee abundance. It is not unusual for bee populations to differ substantially throughout and between years Our study demonstrates that during the early transition stage from turf to meadow, spot spraying was similar to fall mowing at promoting roadside pollinator forage. Also, spot spraying resulted in higher proportions of bare ground and leaf litter that could potentially provide nesting and overwintering habitat. Thus there are alternatives to reduced mowing that may have an important role in future conservation efforts. Proper and selective use of herbicides may be especially relevant for areas where noxious weeds are pervasive, or species of concern are at risk from destructive biomass removal (i.e., nesting birds, turtles, and butterfly larvae).