Urban gardens provide refuges for biodiversity and benefit human communities by supplying fresh fruits and vegetables as well as recreational and educational benefits. For their long-term sustainability, urban gardens should be managed so as to optimize ecosystem services, particularly autonomous pest control. Studies have found strong insect herbivore control through predation and parasitism in urban settings, but less is known about the relative importance of direct and indirect top-down versus bottom-up forces and the drivers that affect these forces in highly managed urban agricultural systems. During the growing season (May-August) of 2016, we collected monthly data on cabbage aphid density and parasitism rates in 25 community gardens in three counties in the California central coast. We examined the effects of bottom-up and top-down direct factors (host plant characteristics and parasitization rates) and indirect factors (soil, garden, and landscape characteristics) on aphid density changes through the growing season. Our specific questions for this study were: 1) What is the relative importance of host plant variables, garden local and landscape characteristics, and aphid parasitism in determining aphid density? 2) How do host plant viariables, and garden local and landscape characteristics influence parasitism? and 3) Do patterns change during the growing season?
We found that aphid density varied widely across the three counties and the four sampling periods during growing season. On average, cabbage aphid density increased early in the season (average slope of density change from May to June: 0.58), did not change much in mid season (average slope of density change from June to July: -0.06) and decreased late season (average slope of density change from July to August: -1.36). Meanwhile, cabbage aphid parasitism increased during the growing season from 0.1% to 18%, 1.5% to 26%, and 1.4% to 17% in Monterrey, Santa Clara, and Santa Cruz counties respectively. In June, aphid density decreased with greater floral abundance but increased with host plant volume, while parasitism increased with habitat size but decreased with brassica density. In August, host plant volume increased with soil water holding capacity and these effects positively affected aphid density, whereas brassica density had a strong positive effect on parasitism. Our results suggest that urban gardeners can enhance autonomous pest control services through vegetation diversification and increasing floral resource abundance within their plots despite the landscape context in which their gardens are embedded.