Cavity-nesting bees and wasps in urban agroecology: Landscape determinants of diversity and foraging

Background/Question/Methods

A growing local food market is increasing the capacity and interest for agriculture in cities. Bees, by way of pollination, are integral components of fruit, vegetable, nut, bean, fiber, flower, and seed yield and thus their persistence is an essential component of any city planning targeted to support urban agriculture. Occupying a similar niche yet considerably less studied are solitary wasps, many of which collect pest or hyper-abundant invertebrate species to provision brood, thereby benefitting many of the same food and flower species dependent on pollination by bees. Unfortunately, what limits bees, wasps, and their functions in increasingly fragmented urban gardens are complex and generally not known. In this study, cavity-nesting bees and wasps, and their parasites from nest boxes set up at 200 community and private gardens, parks and green roofs in Toronto were evaluated to examine landscape and urban land use factors limiting diversity and foraging in urban food-producing gardens. Nest boxes (i.e., cardboard tubes of different widths encased in PVC pipe) were set out from April to October 2011. At the end of the year, bee, wasp and parasite larva were removed, reared and identified from each nest box. Using ArcGIS and Google Earth Pro, localities were buffered at 50m, 150m, and 250m to quantify landscape metrics (e.g. proportion of lawn, maintained garden, impervious surface) and population density at scales representing increasing foraging dimension and complexity for analysis with nestbox contents.

Results/Conclusions

Many cavity-nesting bees as well as wasps, including those that collect spiders, aphids, caterpillars, and leaf beetles, were found nesting in food-producing gardens and many other potential food-producing areas in the city. This first year of data demonstrates that cavity-nesting wasps are most abundant in urban parks (>1000m²) (45% of tubes, 96% of nest boxes colonized), while bees were more generally widespread. Species and functional diversity, particularly that of spider- and aphid-collecting wasps, as well as mason bees (Osmia spp.) in community gardens was significantly greater than on rooftops, suggesting pest-control and pollination by these species on roof gardens may be limited. The proportion of surrounding impervious surface was negatively correlated with colonization and diversity of bees and wasps, but not parasitism rate or parasite diversity, suggesting urban land use may contribute to an inverse density dependent parasite-host relationship. This research is part of a larger effort aimed at linking solitary bee and wasp diversity and stewardship directly to city planning and urban agriculture.