Investigating soils, host plants, and how their interaction mediates specialist bee abundance

Effective conservation of native bee diversity requires knowledge of both trophic and non-trophic habitat needs. For ground-nesting bees, non-trophic resources—such as nesting substrates—are often poorly understood. Nest site selection is complex, depending on multiple interacting factors, including soil properties that may also influence floral resource quality. We investigated how soil characteristics affect the occupancy and abundance of Andrena geranii, a specialist ground-nesting bee, at patches of its floral host, Hydrophyllum spp. Across 17 sites in Ohio that varied in soil silt and sand content, we recorded specialist bees visiting host plants during 15-min observations, quantified host flower abundance and quality, and measured soil variables relevant to nesting. We found 218 Andrena geranii occupying 16 of 17 sites, and higher abundance in sites with greater soil silt and lower soil sand content. High silt content may help maintain conditions favorable for larval development, such as higher moisture and stable temperatures. Fine particle size of silt-rich soils may also make it easier for females to excavate. We found that host plant quality tended to be higher in sites with higher silt content. Plants tended to be taller and have greater flower density in high-silt sites relative to high-sand sites. Specialist bee abundances tended to be greater at sites with higher plant heights, suggesting that resource quality also may influence specialist abundance. Further investigations will explore the mechanisms underlying specialist abundance, floral host quality, and soil properties.