More bees and fewer weevils: How petal number and disc color affect pollinator and pest presence in sunflower

In cultivated sunflower, pollination by bees (Clade Anthophila) can promote seed set, while red sunflower seed weevil (Smicronyx fulvus) is a major seed pest that lays eggs on blooming sunflower heads. To determine how petal presence and disc color affect bee visitation and seed pest damage in sunflower, we ran a full-factorial field experiment in 2024 and 2025 in Casselton, ND. We planted partially inbred lines where some were “apetalous” (without petals) and produced high levels of anthocyanin, causing the normally green disc to develop red pigmentation. To measure how bloom phenotype affects bee visitation, in July and August of both years we identified pollinating bees during walking counts. To assess pest damage, we collected heads in September 2024. After drying and X-raying seeds from heads, we examined X-ray images and counted the number of seeds damaged by red sunflower seed weevil and banded sunflower moth larvae (Cochylis hospes). We found that more bees visited sunflower lines with a normal petal number, regardless of disc color. Petal number had no effect on seed damage from larvae, but green disc lines exhibited more seed damage than red disc lines. Differences in seed damage levels were driven by red sunflower seed weevil larvae, not banded sunflower moth larvae. This may be because adult weevils lay eggs on blooming heads, but banded moths lay much earlier, before bloom phenotype is evident. Red disc color may be a future source of weevil resistance in cultivars that still maintain attractiveness to pollinators.