Extreme heat alters blueberry pollen nutrition and the health of a solitary bee pollinator, Osmia lignaria

Extreme heat events continue to increase in intensity and frequency with negative implications for flowering crops and their bee pollinators. During extreme heat events, flowering plants may experience losses in their reproduction due to heat-altered pollen quality, development, and functioning. Bee pollinators can also be indirectly affected by heat-stressed host plants, as these heat-altered pollen sources may not meet the dietary demands of foraging bees or their offspring. Despite this, few studies have quantified the effects of brief, field realistic heat stress on pollen nutrition in the context of plant functioning and bee health. To address this, we evaluated the protein, carbohydrate, and amino acid content of northern highbush blueberry pollen collected from flowers exposed to extreme heat (37.5°C for 4 h) or normal temperatures (25°C for 4 h) at various developmental stages (tight bud-anthesis). We found that blueberry floral buds vary in their sensitivity to heat, with bud swell being the most heat-sensitive developmental stage with significant reductions in pollen protein, total and several individual amino acids. When pollen from blueberry plants exposed to the same conditions was fed to Osmia lignaria larvae, we found that individuals fed pollen from heat-stressed plants were 7 times more likely to die compared to those fed pollen from non-stressed plants. Additionally, we observed a 39% reduction in fruit set following heat stress exposure at the bud swell developmental stage. These findings reveal how extreme heat can disrupt both plant pollination and bee survival through changes in pollen nutritional composition.