Indirect effects of field management on pollination service by honey bees (Apis mellifera) and seed set in hybrid onion seed production

Productivity in many agricultural crops depends on reliable pollination service; however, pollination may fail if crop management practices interfere with the attraction and retention of pollinators. Farmers must balance optimizing management decisions such as insecticide use and irrigation frequency for pest and disease management versus their potential to interfere with the pollination process. We investigated these issues in hybrid onion seed production, where previous research has shown that high insecticide use interferes with honey bee (Apis mellifera) attraction. We conducted field surveys of soil moisture, nectar production, honey bee visitation, pollen-stigma interactions and seed set at multiple commercial fields across two years. We then examined how management decisions, such as irrigation and insecticide use could affect the pollination process. We found that onion flowers produced maximum nectar at intermediate levels of soil moisture, and that high nectar production was positively related to honey bee attraction. Insecticide use was reduced compared to previous studies, and showed weaker effects on honey bee visitation. Similarly, there were weaker effects of insecticides on pollen-stigma interactions, but we did find that sites that were sprayed close to bloom had reduced pollen germination and pollen tube growth.  Ultimately, neither soil moisture nor insecticide use directly affected seed set– rather there was a high correlation between honey bee visitation and seed set, suggesting that crop management practices will ultimately affect yields via indirect effects on the pollination process.