Herbivores and natural enemies of brassica crops in urban agriculture

Abstract

Urban agriculture has been growing in popularity around the globe. As more land is allocated to urban agriculture, there is a greater need to understand arthropod herbivores and natural enemies in these sites. The area and production type of urban agriculture could influence the risk of pest outbreaks, since larger food production areas are hypothesized to host more abundant herbivore populations. To determine the potential for pest outbreaks, we sampled for pests and above-ground natural enemies at three types of sites with variable amounts of cultivated areas: residential gardens, community gardens, and urban farms. We focused on pests of brassica, a widely cultivated crop family attacked by cabbageworms and aphids. We investigated the relationships of garden characteristics and surrounding land cover on natural enemies and herbivores of brassica. More herbivores and plant damage were expected in larger community gardens and urban farms. However, herbivore populations were low across sites and not explained by garden characteristics or area of production. Herbivory on brassica averaged less than 15% at all sites. Limited plant damage and low herbivore numbers may be attributed to the positive effects of garden flowers on parasitoids and high numbers of parasitoids and generalist predators across sites. Garden area and floral resources had positive relationships with natural enemy abundance, although community composition was similar across sites. Since arthropod natural enemies were abundant across all types of food production, this suggests that urban agriculture is a beneficial habitat that could support regulating ecosystem services such as biological control.