She loves me / she loves me not: How petal and anthocyanin presence in cultivated sunflower affect native bee foraging preferences

Pollination is important for ensuring high yield in cultivated sunflower. In many areas of the United States, sunflowers are mainly pollinated by native bees. Bees (Hymenoptera: Anthophila) use multiple traits in flowers to inform foraging behavior, including color, nectar, pollen, and volatile organic compounds. Flower color may also affect floral temperatures. Temperature can act as a reward in and of itself, but it may also affect other floral traits in turn, such as nectar and pollen production. We examined wild bee preference for disc color, petal presence, and floret temperature in partially inbred sunflower lines that are apetalous and express high levels of anthocyanins, causing discs to appear red. We hypothesized that sunflower heads with darker discs and petals present would warm to a higher temperature earlier in the day and attract more bees than normal sunflowers. During sunflower bloom in 2024 in North Dakota, we conducted walking counts to determine the number of bees on blooming sunflowers in an experimental agricultural field. We surveyed bees and intra-floret temperatures throughout the morning while bees were active. We found that bees had no preference for disc color, but preferred flowering heads with petals over those with reduced or no petals. Intra-floret temperatures did not differ between sunflower heads with different disc colors or number of petals. These findings will be important for those selecting traits in insect-pollinated crops and as well as choosing cultivars used to promote wild pollinator populations.