The diversity of cavity nesting bees (Hymenoptera: Megachilidae) recolonizing a post-wildfire landscape and their pollen provisions, with reference to aerial seed mix applications

Abstract The assemblage of cavity nesting bees in a semi-arid post-wildfire pinyon–juniper woodland (2020 Pine Gulch Fire, Colorado, United States) was characterized and DNA metabarcoding was applied to identify composition of pollen provisions. Floral species composition of pollen provisions was compared to species composition of aerially applied seed mixes to evaluate which species were foraged upon by bees. Eight species of cavity nesting bees colonized nest traps in and around the fire perimeter; the mason bees Osmia montana Cresson, O. bruneri Cockerell, and O. californica Cresson were the most common. Bees overlapped in their use of floral resources and only 12 floral taxa were identified in pollen provisions, but the proportional composition of provisions diverged consistent with niche-partitioning. Bitterweed (Tetraneuris sp.) and 2 vetches (Hedysarum sp. and Astragalus sp.) were collected by most bee species. Although flowering forbs and shrubs constituted 16% of the aerial seed mix, only sunflower (Helianthus) was identified in both the applied seed mix and pollen provisions and represented only a small fraction (0.6%) of the seed mix. Vegetative cover was the most important factor for predicting the presence of Osmia nests; locations predominated by woody cover, especially Gambel oak (Quercus gambellii), were preferred nesting habitat. It is concluded that most forb and shrub species distributed in aerial seeding application were not used as forage by local cavity nesting bees. In this system, oak regeneration and key forage species (such as bitterweed and vetches) are relevant post-fire conservation targets that improve habitat value for certain pollinator taxa recolonizing burned sites.