Hedgerows and wildflower plantings are widely promoted in North America as strategies to augment pollinator abundance, diversity and pollination services within agricultural landscapes. These habitats have been well documented to provide important floral resources for wild and managed bees. They also are assumed provide, and even enhance, nesting substrates for pollinator populations; however, empirical tests of their ability to do so are very limited. We tested the ability of wildflower plantings to increase nesting of native soil-nesting bees in agricultural landscapes in Northern California. We also quantified whether differences in vegetation cover and edaphic characteristics between wildflower plantings and unplanted control sites, as well as microsite variation within sites, affected bee nesting. We monitored nesting over two seasons in replicated wildflower plantings and paired control sites using emergence trapping. At each trap location we also surveyed vegetation and edaphic characteristics. We tested differences in nest density and species richness of soil-nesting bees using GLMM. Bee communities, vegetation and soil characteristics were summarized using NMDS ordination and differences in each compared between wildflower plantings and control sites using permuational ANOVA.
In the first year of establishment nest density and species richness of bees were significantly greater in wildflower plantings than in control sites (P = 0.04, P < 0.005). Higher nest density and species richness were maintained in the second year, but they did not increase relative to the first year of establishment (enhancement x year interaction, P = 0.16, P= 0.53). These results suggest that wildflower plantings provide immediate nesting benefit for bees in such landscapes, but they may not continue to augment local nesting density over time. Wildflower plantings supported nesting of 22 species, 11 of which were not found at control sites. Unsurprisingly vegetation cover differed strikingly at wildflower plantings, but basic soil characteristics were similar between plantings and controls. These habitat differences coupled with differences in bee nesting, suggest potential strategies to encourage native soil-nesting bees and promote pollinator biodiversity within agricultural landscapes.