Invasive mason bees are not limited by floral resources but may be limited by nest site availability
Osmia cornifrons and O. taurus are two non-native bees that were introduced to Maryland from Japan in the 1970s and 1980s. Both are rapidly expanding their ranges across the eastern US. Scientists have growing concerns about the impacts of these invasive bees on native bees and local plant communities, but little is known about their resource utilization or limitations. Invasive species are often habitat generalists making it easier for them to adapt to their new homes and reducing the likelihood of resource limitation. Osmia cornifrons and O. taurus require floral and nesting resources in order to persist. Both of these Osmia species are pollen generalists, utilizing a wide-range of flowering plants. They are also cavity-nesters, making their nests in pre-existing holes found aboveground. Appropriately sized cavities might be limited. We hypothesize that due to their generalist feeding behavior, neither Osmia species will be limited by floral resource availability, however both may be nest-site limited. To test our hypotheses, in March 2013 we placed artificial wooden nestboxes at 28 sites across Morristown National Historical Park in New Jersey. We manipulated nest availability by placing either one or two boxes at each site; each nestbox had twelve large and twelve small nest holes. The boxes were collected from the field in December 2013 and then surveyed in the lab to determine bee nest abundance. We compared these data to plant survey data collected at each site by the Northeast Temperate Network of the National Park Service.
Our nestboxes were exclusively colonized by Osmia taurus and O. cornifrons with O. taurus found in higher abundance than O. cornifrons. Total bee abundance was not related to plant species richness or abundance. Nor was it related to the presence of any specific plant species. In contrast, bees made more nests where more nest sites were provided, suggesting that nesting substrate does limit bee population size. However bees did not appear to have a preference for larger or smaller nest holes, possibly indicating some flexibility for both O. cornifrons and O. taurus in nest cavity selection. Our results suggest that these species are flexible in their floral and nesting resource use, which may be a reason they are able to expand their range within the United States. However, they may be limited by overall nesting resource availability.