Bee visit frequency and time of day effects on cumulative pollen deposition in watermelon

Measurements of pollinator effectiveness are essential to modeling pollination services delivered by animals, particularly for crops like melons that are entirely dependent on bees for fruit production. Pollinator effectiveness is often measured by allowing one visit from a pollinator to a clean stigma, then quantifying the pollen deposited. However, these measurements can be problematic when flowers are experimentally bagged and accumulate artificially high volumes of nectar, which influences pollen deposition differentially among bees. Another approach, used here, is to examine the effectiveness of the collective pollinator assemblage per unit time, beyond per-visit measurements. The rate at which pollen accumulates on female watermelon flowers (depleted of nectar) after multiple bee visits was documented per 20 minutes at seven suburban southern California farms. It was hypothesized that visit frequency would positively predict pollen accumulation rate up to a point, after which the rate would plateau. Results indicate that cumulative pollen deposition can be modeled linearly up to at least 20 visits and is best predicted by the visit frequency, not visit duration, of honey bees. Native bee visitation was very low and was not a significant predictor of deposition. Additionally, deposition decreased as the morning progressed at certain farms, despite no decreases in bee visit frequency or visit duration. This is likely due to variation in pollen removal and availability across melon cultivars and farms. These results support the use of per-visit measurements of pollinator effectiveness in watermelon, but highlight important variables that can influence these measures within and between study sites.