Poor Air Quality Is Linked to Stress in Honeybees and Can Be Compounded by the Presence of Disease

SIMPLE SUMMARY: Climate change is associated with warmer and drier weather on average in central California. At the same time, honeybees are being transported from all over the United States to California for the completion of pollination services that ensure increased yields for a variety of crops, resulting in bees experiencing ambient abiotic stressors. Higher temperatures can reduce air quality, further exacerbating honeybee health challenges. We investigated the relationship between higher daily temperatures and poor air quality and demonstrated associations with incidences of pests (e.g., Varroa mites) and pathogens (e.g., Nosema ceranae). We also correlated the expression of genes linked to immune functioning, oxidative stress, and buffering against temperature and air quality stressors. High daily temperatures are associated with poorer air quality, and they are both associated with lowered immune system functions and increased oxidative stress protection against pests and diseases. Higher Varroa mite loads are correlated with the lower potential ability of honey bees to buffer against temperature stress. Our study provides insights into interactions between climate change-related abiotic stressors and their relation to biotic stressors, which underlie a decline in honeybee health. ABSTRACT: Climate change-related extreme weather events have manifested in the western United States as warmer and drier conditions with an increased risk of wildfires. Honeybees, essential for crop pollination in California, are at the center of these extreme weather events. We associated the maximum daily temperature and air quality index values with the performance of colonies placed in wildfire-prone areas and determined the impact of these abiotic stressors on gene expression and histopathology. Our results indicate that poor air quality was associated with higher maximum daily temperatures and a lower gene expression level of Prophenoloxidase (ProPO), which is tied to immune system strength; however, a higher gene expression level of Vitellogenin (Vg) is tied to oxidative stress. There was a positive relationship between Varroa mites and N. ceranae pathogen loads, and a negative correlation between Varroa mites and Heat Shock Protein 70 (HSP70) gene expression, suggesting the limited ability of mite-infested colonies to buffer against extreme temperatures. Histological analyses did not reveal overt signs of interaction between pathology and abiotic stressors, but N. ceranae infections were evident. Our study provides insights into interactions between abiotic stressors, their relation to common biotic stressors, and the expression of genes related to immunity and oxidative stress in bees.