Evaluating effects of neonicotinoid seed treatments and agroecosystem land management practices on solitary bees in Midwestern agroecosystems
Authors:   , Abney Robert Henry 
Published: December 2022

Bees are declining due to multiple factors including habitat loss and non-target exposure to pesticides resulting from agricultural intensification. Solitary bees are often overlooked as important pollinators, but they are valuable contributors to both the commercial success and ecological diversity of agroecosystems. Yet, solitary bee communities in agroecosystems may be adversely impacted by exposure to pesticides such as neonicotinoid insecticides, and it is unclear if bee habitat adjacent to agricultural fields can buffer against the potential negative impacts of neonicotinoids. Laboratory studies indicate lethal and sublethal effects of neonicotinoids on bees, and previous field studies have demonstrated reduced abundance, richness, and reproductive success of solitary bees in agroecosystems where neonicotinoids are present. Thus, we proposed two field studies of wild, solitary bee communities in agricultural field margins to determine the impacts of neonicotinoid insecticides and site-specific habitat variables (e.g., floral richness) on bees in Missouri agroecosystems. First, in 2019, we surveyed bee abundance and richness in field margins adjacent to 29 soybean fields located on public conservation areas (n = 18) and private land enrolled in the Conservation Reserve Program (n = 11). Public study fields were planted to one of three experimental treatments: untreated (no insecticide), treated (imidacloprid), or previously-treated (untreated, but neonicotinoid use prior to 2017). Privately-owned study fields were planted to one of two experimental treatments: treated (neonicotinoid) or previously-treated (untreated, but neonicotinoid use in 2018). Second, in 2018 and 2019, we placed solitary bee nest boxes in field margins of soybean fields (2018, n = 18; 2019, n = 24). In 2018, all fields were located on public conservation areas and, in 2019, privately-owned fields were added to the study. The experimental treatments for public and privately-owned study fields were the same as described for the first study. In the first study, we found wild, solitary bee abundance and richness was best explained by field treatment, with fewer bees and lower bee diversity observed in margins of previously-treated and treated fields compared to margins of untreated fields. We also found that bare soil and floral richness in field margins positively influenced bee abundance and richness, but it remains unclear if the positive influences of habitat can outweigh the negative impact of neonicotinoid seed treatments on bee abundance and richness. In the second study, we found field treatment had no negative impact on nesting effort of cavity-nesting bees. However, field treatment significantly reduced resin bee nesting success, with a lower percentage of resin bees emerging from nests collected from margins of previously-treated public (15 percent) and treated public (7 percent) fields compared to untreated fields (46 percent). We also found that the amount of woody vegetation within 800 m of nesting sites may impact nesting effort of resin bees, with more nest cells produced and cavities used in margins of fields with a greater amount of woody vegetation within 800 m. Reducing or eliminating neonicotinoid use in areas managed for wildlife may be beneficial to the conservation of wild bee populations.