Impact of Temperature and Microbial Infection on the Honey Bee Hemocytes

Introduction Honey bees (Apis mellifera) rely on hemocytes for immunity, wound healing, and hemolymph homeostasis. While 32°C reflects optimal hive conditions, suboptimal temperatures (22°C) and stressors like pathogens (Varroa destructor, bacteria) or pesticides impair hemocyte function. Using clodronate liposomes (CLD) to deplete hemocytes, we assessed how immune suppression interacts with these stressors. Methods Newly emerged (5-day-old) and older (15-day-old) bees were assigned to: CLD, control liposomes (LP), PBS, injection wound, or no treatment. Cohorts were exposed to 32°C (optimal) or 22°C (stressful for non-immunosuppressed bees). Fifteen-day-old bees were challenged with Escherichia coli or Staphylococcus aureus. Survival, hemolymph volume, and hemocyte counts (total/differential) were tracked for 7 days. Results CLD-treated bees showed reduced survival and hemocyte counts, with declines exacerbated at 22°C (non-optimal) and after E. coli challenge. A critical threshold of ~220,000 hemocytes/mL was identified; below this, survival dropped sharply, particularly in CLD/LP groups. Granulocytes (phagocytosis) and plasmatocytes (encapsulation) were most affected. Notably, 22°C alone stressed healthy bees but was catastrophic for CLD-treated individuals. Discussion Hemocytes are essential for stress resilience. While 32°C supports immune function, 22°C suboptimal for healthy bees severely compromises immunosuppressed individuals. Bacterial challenges (especially E. coli) further deplete key hemocytes, revealing synergistic effects of environmental and pathogenic stressors. Beekeeping practices must prioritize hive temperature stability to mitigate immune risks. synergistic effects of environmental and pathogenic stressors. Beekeeping practices must prioritize hive temperature stability to mitigate immune risks.