PLANT-INSECT INTERACTIONS Effects of Honey Bee (Hymenoptera: Apidae) Foraging on Seed Set in Self-fertile Sunflowers (Helianthus annuus L.)

Ten self-fertile commercial sunßowers cultivars were evaluated for seed set with and without exposure to bees. In the Þrst planting, the number of foraging honey bees was smaller than in the second, and seed set for most cultivars did not differ between those bagged to exclude bees and ones that were open pollinated. In the second planting, however, a majority of cultivars had significantly greater seed set when capitula were exposed to bees compared with when they were not. The weight of seeds fromopen-pollinated capitulawas greater than from thosewhere beeswere excluded. Environmental conditions also played a role in seed set as evidenced by differences between plantings in set on bagged capitula. In the Þrst planting, average maximum and minimum temperatures were signiÞcantly higher than in the second, and overall seed set was signiÞcantly lower in capitula where bees were excluded compared with the second planting. Under the high temperature conditions, however, some cultivars set four times more seed on open-pollinated capitula compared with those that were bagged. These results suggest that foraging activity and cross-pollination by bees might mitigate reductions in seed set caused by high temperatures.