Pollen provides bumblebees (Bombus spp.) with most of their lifetime protein, provisioned during larval development. Higher pollen protein in larval diets can increase larval weight and adult body size as well as adult survival. Because of the necessity of acquiring pollen and ample evidence for bee health correlating with pollen quality, it is important to further understand how workers forage and select for pollen. There is recent evidence that bumblebee workers detect differences in pollen quality and preferentially visit floral species with higher quality pollen. However, many of the implications of these findings are not yet known, such as how pollen foraging preferences may shift after disturbance which alters the floral community composition. We sought to investigate how floral visitation patterns for four bumblebee species would change with available pollen protein across a 25-year gradient of time-since-burn using a space-for-time substitution, while also comparing mixed and high severity burns. We hand netted bumblebees in four past burns, while also collecting their corbicular pollen loads as well as pollen from undehisced anthers of visited floral species. Total percent nitrogen, as a proxy for pollen protein, was assessed from pollen samples via combustion and gas chromatography–mass spectrometry.
Preliminary results suggest that corbicular pollen loads had significantly higher nitrogen content in mixed severity burns, and this pattern was consistent across time-since-burn. Corbicular pollen quality weakly but significantly increased linearly with the pollen quality of the visited floral species. Forb diversity between mixed and high severity burns was similar in the most recent burns, but mixed severity burns showed higher forb diversity with increasing time-since-burn. This could indicate the difference in corbicular pollen quality is not necessarily due to community-level effects of available floral resources, however it is not yet clear how available pollen quality at a site relates to forager selection. We believe these patterns may be driven primarily by a few species which dominate high severity, early time-since-burn sites and have relatively low pollen quality within our sampled flora. Therefore, our results suggest that although high severity burns typically have more abundant floral resources, the available pollen quality in high severity burns remains lower relative to mixed severity burns. Since we know from previous research bumblebees do not alter their provision sizes dependent on pollen quality, our work could imply that bumblebees are more likely to be nutritionally limited by pollen after high severity disturbances.