Natural enemy biodiversity in urban gardens

Urban consumers are increasingly interested in local food production. This has coincided with a conversion of underutilized vacant space to food production in Midwestern (USA) cities. As urban agriculture continues to thrive, there is a concern that commonly grown food crops will be susceptible to damage from insect pests. We investigated the pest and the natural enemy communities at three scales of urban agriculture - residential gardens, community gardens, and urban farms. Data collection was focused at 29 sites that planted brassica. We expected urban farms, with greater crop area, would have more cabbageworms as well as higher natural enemy abundance and richness. We analyzed the relationship between natural enemy and cabbageworm populations. Pests on brassica plants were primarily imported cabbageworm (Pieris rapae), and there was a trend (p=0.13) of fewer pests at residential gardens. Parasitoid wasps were the most common group of natural enemies. However, the community was similar across all scales of agriculture. Lady beetles, minute pirate bugs, and green lacewings were also observed in urban gardens. We found a significant negative relationship between parasitoid families that may parasitize cabbageworms and adult cabbageworms (p=0.02). Our results suggest a low risk for defoliation of brassica but a potential for greater pest populations exists at sites with more crop area. The similarity in natural enemy community composition across scales of agriculture and high numbers of specialist parasitoids indicates that biological control is a suitable option against cabbageworms.