Measuring the pollinator performance of honey bees on seedless watermelon

Background/Question/Methods

In light of managed honey bee (Apis mellifera) population declines, their performance as pollinators is of vital interest to agriculture, especially relative to potential alternative pollinators. A standarized method of estimating pollinator performance across species is therefore important. Pollinator performance is often estimated through pollen deposition, or the amount of pollen a bee deposits to a stigma in a single visit. In cucurbit crops like watermelon (Citrullus lanatus) where flowers are only open for one day, pollen deposition is measured using “virgin” female flowers which are bagged before opening. After opening within the bag, they are allowed one visit by a pollinator and the deposited pollen is quantified. However, previous research indicated that honey bees foraged differently on previously bagged virgin flowers than on non-manipulated, open-pollinated flowers. We tested the hypothesis that single visit pollen deposition and flower handling time is similar between initial and subsequent visits to experimental female watermelon flowers. We also examined temporal effects on pollen deposition and handling time. Previously bagged flowers were presented in the early morning (7:00 - 8:20 a.m.) to honey bees; others were left bagged until the late morning (10:45 - 12:30 p.m.) before being presented. In both time periods, half of the flowers were allowed one visit while the other half were allowed four visits by different bees. Any pollen deposited on the flowers was stained and quantified in the lab. Field work was conducted at Spadra Ranch in Pomona, California.

Results/Conclusions

Initial visits in the late morning were found to be about three times longer than initial visits in the early morning. Despite shorter initial visits in the early morning, twice as much pollen was deposited during an initial early visit than during an initial late visit. For both time periods, initial visits were longer in duration than subsequent visits (second, third, and fourth visits) to the same flower and each subsequent bee visit deposited on average half of the pollen deposited by the initial visit. Our results suggest that evaluating honey bee pollinator performance on the basis of initial visits to virgin watermelon flowers may yield an overestimate of per-visit pollen deposition by a factor of two. Similarly, time of day effects are likely to introduce significant variation in pollen deposition measures. The knowledge gained from this study will contribute to a better understanding of pollinator performance comparisons between honey bees and alternative native bee pollinators.