A Note on the Honey Bee Parasitic Phorid Fly (Apocephalus borealis Brues) in an Urban Ecosystem

The honey bee is a crucial pollinator in urban ecosystems but faces multiple challenges, including habitat degradation, pollution, and parasitism by species such as the phorid fly, Apocephalus borealis Brues (Diptera: Phoridae). This study investigated honey bee abundance and the percentage of A. borealis parasitism in an urban environment in San Francisco, California. We monitored six sites weekly for six months using two sampling methods. Individual bees were weighed and observed for parasitoid emergence under controlled laboratory conditions. Our results indicate fluctuations in honey bee parasitism by A. borealis from September 2024 to May 2025, with four distinct peaks occurring in mid-September, February, late March, and early May. The highest parasitism rates exceeded 50% in early May, coinciding with increased temperatures and drops in relative humidity. These results suggest a potential link between abiotic conditions and parasitoid activity, highlighting the importance of long-term monitoring to understand the seasonal dynamics of host–parasite interactions in urban environments.