Sublethal effects of imidacloprid-contaminated honey stores on colony performance, queens, and worker activities in fall and early winter colonies

Authors: Carroll, Mark; Brown, Nicholas; Reitz, Dylan

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/20/2023
Publication Date: 1/2/2024
Citation: Carroll, M.J., Brown, N.J., Reitz, D.C. 2024. Sublethal effects of imidacloprid-contaminated honey stores on colony performance, queens, and worker activities in fall and early winter colonies. PLOS ONE. 19(1). Article e0292376. https://doi.org/10.1371/journal.pone.0292376.
DOI: https://doi.org/10.1371/journal.pone.0292376

Interpretive Summary: Honey bees collect neonicotinoid insecticides in contaminated nectar which is then converted to honey stores where neonicotinoids persist. Bees exposed to very high levels of neonicotinoids may experience negative effects including reduced foraging, orientation, hygiene, mobility, queen egg-laying and stored sperm viability, brood rearing, and worker populations. Bees consume these contaminated sugar stores throughout the year, especially during dearth periods such as winter when outside floral nectar is very limited. We examined the effects of chronic neonicotinoid exposure on colony growth and performance, colony worker nutrition, and queen-worker interactions from early fall to mid-winter. Colonies were given the neonicotinoid imidacloprid in sugar syrup to simulate pesticide-contaminated nectars. Bee colonies received syrup containing 0 ppb (unexposed control), 20 ppb (moderately field relevant concentration), or 100 ppb (much higher than average field concentrations) imidacloprid over a from early September to mid-October to simulate collection of contaminated nectars from field flowers. Colonies were evaluated immediately before (pre-treatment), immediately after (post-treatment), and 10 weeks after this treatment period (mid-winter) to examine both immediate and delayed effects of dietary imidacloprid as colonies prepared for and entered winter. Significant negative effects were largely observed in colonies given 100 ppb imidacloprid syrup, which is a much higher concentration than observed in most agricultural crops. Post-treatment 0 ppb colonies had larger adult populations than either 20 ppb or 100 ppb colonies, while mid-winter 0 ppb and 20 ppb colonies had larger adult populations than 100 ppb colonies. Post-treatment 0 ppb colonies reared more brood than 20 ppb or 100 ppb colonies, while mid-winter 0 ppb colonies had more brood than 100 ppb colonies. Colonies experienced similar seasonal declines in stored pollen but no differences among treatment groups. Close monitoring of individual adult workers tagged with RFID chips revealed that neonicotinoid exposure affected outside worker activities rather than adult lifespans. Tagged workers from 100 ppb colonies spent less time outside the colony than workers from 0 ppb or 20 ppb colonies. Despite these treatment effects, adult workers appeared to be sufficiently well fed as indicated by well-developed head protein contents (associated with well-developed hypopharyngeal glands) across treatments and time points. Imidacloprid exposure partially affected queen reproductive physiology (ovary nutrient stores but not stored sperm), but not attraction of retinue workers (queen caretakers), or levels of queen pheromones that project her presence to the colony. Mid-winter 0 ppb queens had higher ovary proteins than 100 ppb queens and higher ovary lipids than 20 ppb queens. However, queen protein and lipid stores in a key non-reproductive tissue (fat bodies) did not vary across treatments. Queens from different treatment groups had similar levels of stored sperm required to produce viable worker offspring. Queens from different treatments were also tended by similar numbers of retinue worker attendants and had similar levels of four QMP (Queen Mandibular Pheromone) compounds in their mandibular glands. These results suggest that honey bees are susceptible to high concentrations of imidacloprid both in newly collected nectar and later during winter in contaminated honey stores. This study also suggests that high levels of imidacloprid-contaminated sugar stores can negatively impact fall colony preparation for overwintering and winter colony performance.

Technical Abstract: Honey bees collect neonicotinoid insecticides in contaminated nectar which is then converted to honey stores where neonicotinoids persist. Bees consume these contaminated sugar stores throughout the year, especially during dearth periods such as winter when outside floral nectar is very limited. We examined the sublethal effects of chronic neonicotinoid exposure on colony performance, colony worker nutrition, and queen-worker interactions from early fall to mid-winter. Colonies were given the neonicotinoid imidacloprid in sugar syrup to simulate a pesticide-contaminated early fall nectar flow. Bee colonies received syrup containing 0 ppb (unexposed control), 20 ppb (moderately field relevant concentration), or 100 ppb (much higher than average field concentrations) imidacloprid over a six week period (early September to mid-October). Colonies were evaluated immediately before (pre-treatment), immediately after (post-treatment), and 10 weeks after the treatment period (mid-winter) to assess both immediate and delayed effects of imidacloprid exposure as colonies prepared for overwintering. Significant sublethal effects were primarily observed in colonies given 100 ppb imidacloprid syrup, a much higher concentration than observed in most crop exposures. Post-treatment 0 ppb colonies had larger adult populations than either 20 ppb or 100 ppb colonies, while mid-winter 0 ppb and 20 ppb colonies had larger adult populations than 100 ppb colonies. Post-treatment 0 ppb colonies reared more brood than 20 ppb or 100 ppb colonies, while mid-winter 0 ppb colonies had more brood than 100 ppb colonies. Colonies experienced similar seasonal declines in stored pollen but no differences among treatment groups. Close monitoring of individual adult workers tagged with RFID chips revealed that neonicotinoid exposure affected outside worker activities rather than adult lifespans. Tagged workers from 100 ppb colonies spent less time outside the colony than workers from 0 ppb or 20 ppb colonies. Despite these treatment effects, adult workers appeared to be sufficiently well fed as indicated by well-developed head protein contents (associated with well-developed hypopharyngeal glands) across treatments and time points. Imidacloprid exposure partially affected queen reproductive physiology (ovary nutrient stores but not stored sperm), but not attraction of retinue workers (queen caretakers), or levels of queen pheromones that project her presence to the colony. Mid-winter 0 ppb queens had higher ovary proteins than 100 ppb queens and higher ovary lipids than 20 ppb queens. However, queen protein and lipid stores in a key non-reproductive tissue (fat bodies) did not vary across treatments. Queens from different treatment groups had similar levels of stored sperm required to produce viable worker offspring. Queens from different treatments were also tended by similar numbers of retinue worker attendants and had similar levels of four QMP (Queen Mandibular Pheromone) compounds in their mandibular glands. These results suggest that honey bees are susceptible to high concentrations of imidacloprid both in newly collected nectar and later during winter in contaminated honey stores. This study also suggests that high levels of imidacloprid-contaminated sugar stores can negatively impact fall colony preparation for overwintering and winter colony performance.