Abstract
<jats:p>Climatic extremes such as heat and drought pose environmental stressors for pollinators such as the western honey bee (Apis mellifera), threatening colony health and the ecosystem services these insects provide. Understanding how climatic variation impacts colony performance and parasitism under realistic field conditions is therefore of academic and applied interest. We conducted a 26-month field study in Southern California using commercially bred and locally adapted Californian honey bee genotypes exposed to natural hot and dry Mediterranean climates. We tested how climatic variation impacted colony resources, brood production, adult populations, Varroa destructor mite infestations, and colony and queen survival. Precipitation was correlated with increased pollen and nectar storage, brood production, and adult bee populations, whereas elevated temperatures, indicating heat stress, were associated with reduced colony performance. Despite continuous brood production throughout winter, Varroa mite intensities did not increase continuously, indicating that brood availability alone was insufficient to explain changes in mite infestations. Instead, mite levels correlated with colony performance and increased during a wetter, more productive first year but declined during a drought year, indicating that climatic conditions impacted parasite infestations through their influence on colony growth and resource availability. We also found differences in colony performance between the two genotypes. Locally adapted Californian colonies suppressed initially higher mite infestations during the first, more favorable year, when queen survival was higher than in commercial colonies. These differences largely disappeared during the second drought year, when colony performance and mite infestations were reduced in both genotypes. In summary, our findings demonstrate that climatic extremes in Southern California influence honey bee colony performance and parasite infestation. By linking environmental variation to key life-history traits of managed honey bee colonies, our study provides an ecological foundation for predicting pollinator performance in response to climate change and may better inform future management of domesticated pollinators.</jats:p>