PS 17-243 - The potential for elevated soil salinity to enhance the ecological trap effect of roadside pollinator habitat

Abstract: The management of roadside habitat may be an important component of the conservation of declining pollinators, but insect mortality along roadsides can be high due collisions with traffic, creating a potential “ecological trap.” Runoff from winter applications of deicing salt can increase soil salinity along roadsides, and if this results in higher sodium levels in floral nectar, it could increase the attractiveness of roadside plants to pollinators, possibly accentuating the ecological trap effect. Our study took place in the northern Shenandoah Valley (Clarke County, VA, USA) in June and July 2022. We compared the soil salinity in a highway median to soil collected from nearby fields and compared the Na⁺ concentrations in nectar from common milkweed (Ascelpias syriaca) from these two habitats. We also manipulated soil salinity in field populations of A. syriaca by adding brine solution to document changes in nectar Na⁺ and pollinator visitation. We found that soil salinity in the median of a highway was 2.3 times higher than the salinity of field soil, however nectar from A. syriaca in the two habitats did not differ significantly in Na⁺ concentration. Addition of brine to field soil increased soil salinity 17-fold and significantly increased the level of Na⁺ in A. syriaca nectar. Pollinators visited A. syriaca umbels that received the brine treatment twice as frequently as controls. The effect was most pronounced for honey bees (2.5 times more visits to brine treated plants) and sweat bees (4.5 time more). Bumble bees were the least affected pollinator. We conclude that elevated soil salinity is capable of increasing nectar Na⁺ and the attractiveness of A. syriaca flowers, but roadside soil salinity in our study was not high enough to enhance an ecological trap effect of roadside vegetation. In some parts of North American, soil salinity can reach levels similar to what we observed in our brine treatments. Searching for elevated Na⁺ floral nectar in these regions may be worthwhile.