Effects of floral visitation frequency and time of day on cumulative pollen deposition by bee assemblages in southern California watermelon crops

Measurements of pollinator performance are vital to agriculture, especially for crops like watermelon that are fully dependent on bees to set fruit. Pollinator performance is often estimated by bagging a female flower before opening to ensure a “virgin” stigma, allowing a single visit from a pollinator, then quantifying the amount of pollen deposited. However, these measurements can be problematic if flowers accumulate artificially high volumes of nectar while bagged, which has been shown to induce overestimates of pollen deposition by honey bees. Another approach to modeling pollination services is to examine the performance of the collective pollinator assemblage per unit time, beyond single-visit estimates. This study aims to measure rates of cumulative pollen deposition on virgin female watermelon flowers (depleted of excess nectar) per 20-minute time period by the collective pollinator assemblage at various farms. It is hypothesized that visitation frequency and a higher abundance of native bee visitors will be positively associated with cumulative pollen deposition rates. An analysis of 34.7 hours of pollinator activity from 5 farms indicates that 20-minute cumulative stigmatic pollen deposition is positively associated with visitation frequency of honey bees but not with that of native bees, which accounted for only 3.7% of observed visits. Additionally, pollen deposition displayed a negative relationship with time of day at organic farms, despite no decreases in honey bee visit frequency over time. This suggests differences in pollen availability and deposition among farms or cultivars. Future research will focus on sampling farms with higher abundances of native bee pollinators.