Characterizing wild bee communities and pollen limitation in spring- vs. summer-flowering specialty crops

Wild bees pollinate many specialty crops. However, differences in pollinator dependence between crops could result in crop-specific variation in floral visitation rate, pollen limitation, or pollinator communities. We characterized wild bee communities and pollen limitation in Indiana specialty crops with varying degrees of pollinator dependence: high tunnel tomato (low), field tomato (low), apple (moderate), blueberry (moderate), and watermelon (high). Timed floral observations were used to quantify visitation rates and species assemblages. Pollen deficits were evaluated by comparing fruit set in hand pollination, pollinator excluded, and open pollination treatments. Wild bee dominance over honey bees varied among crops, with apple and blueberry least reliant on wild bees (22% and 31% of observations, respectively) and watermelon and tomato most reliant (85% and 100% of observations, respectively). Community composition also differed among crops; Andrena and Bombus predominated in apples and blueberries, Halictidae in high tunnel tomatoes and watermelons, and Bombus griseocollis in field tomatoes. Evidence of pollen deficits, expressed as difference in fruit set between hand-pollinated and open-pollinated treatments, varied by crop: watermelon (5.0% higher), blueberry (5.8% higher), apple (7.2% higher), high tunnel tomato (7.2% higher), field tomato (8.1% higher).  While honey bee stocking may account for the majority of bee visitation in apples and blueberries, wild bees appear to be important pollinators of watermelon and tomatoes. These results indicate that wild bees variably contribute to pollination of diverse specialty crops; interestingly, the crop most reliant on wild bees– tomato– is one that was previously assumed to be primarily wind pollinated.