Looking for a signal: how well do specialist and generalist bees track preferred host plants over time?

Understanding the factors that signal plant and pollinator phenologies is important for assessing the potential impacts of climate change. However, limited information is available on how well bees track preferred host plants over time and how traits like body size may govern differential responses among species, particularly in xeric areas where floral resources and climate are unpredictable. We studied the nesting phenology of six solitary, cavity-nesting bees that differ in host breadth and body size in the Monte Desert ecosystem, Argentina, over nine consecutive years. We used cross-correlation analysis to assess if the ability of bees to track the flowering phenology of their host plants and abiotic environment, as well as to detect potential differences between specialist and generalist bees. We found that nesting phenology is predicted by multiple flowering and climatic variables regardless of the bees’ level of specialization, and that there is a differential pattern in body size. The nesting phenology of smaller bees was predicted by the number of individuals in bloom, indicating some spatial pattern in resource availability. While the nesting phenology of some bees was predicted by flowering variables alone, that of other bees was explained by a combination of flowering and climatic variables. Our study also indicated that the inter-annual variability of nesting was greater in generalist bees than in specialist bees. These results suggest that if phenological decoupling occurs, bees might be able to restore it by detecting multiple environmental signals, and that generalist bees might be more vulnerable than previously expected.