Pollination services of managed common eastern bumble bees (Bombus impatiens) in seedless watermelon (Citrullus lanatus)

Abstract

Seedless watermelon is a highly pollinator-dependent crop; in consequence, growers stock fields with managed pollinators to ensure high yields. Historically, only western honey bees (Apis mellifera) were used, but, more recently, growers have begun adding common eastern bumble bees (Bombus impatiens) as further insurance. While bumble bees are efficient watermelon pollinators on a per-visit basis, the impacts of field-scale colony supplementation are unknown. We conducted two field studies in 2017-2018 and 2025 across 36 commercial watermelon fields that were either stocked or unstocked with bumble bees. Focal flower observations were then used to investigate pollinator visitation frequency (individuals recorded per observation) and visitation rate (flowers visited per minute per individual) across and within (i.e., near vs. far from colonies) fields. Additionally, in 2025, we dusted bumble bee colonies with fluorescent powder to more directly quantify their flower visitation, and in 2024-225, we identified colony and foragers’ pollen loads to estimate watermelon’s relative contribution to foraging dynamics. Observations revealed that managed bumble bees did not increase visitation frequency to watermelon flowers at a whole field-scale, but only on plants adjacent to colonies within a field (3.6-fold increase within 10 m radius). Managed bumble bee presence also influenced other pollinators, with small solitary bees being, overall, most affected. Only 1.5% of flowers around bumble bee colonies contained fluorescent powder traces and 2.9% of pollen grains were identified as watermelon. Collectively, our data suggest that watermelon is not a preferred resource for managed bumble bees and thus stocking fields with colonies may not increase visits to crop flowers or contribute to enhancing yield.