Effects of garden land use, management practices, and landscape context on pest and beneficial insects in urban vegetable gardens

Background/Question/Methods

Compared to their rural counterparts, urban gardeners face reduced abundance and diversity of beneficial insects and increased invertebrate pest density. As a basis for developing ecologically-based pest management strategies for urban gardens, we asked: What are the temporal dynamics of pest and beneficial insects in New York City community gardens?  How do garden land use and management, and landscape context, affect pest and beneficial insect populations in urban vegetable gardens?

From June through September 2011, we collected biweekly scouting and sticky card data on pest and beneficial insects on key crops in 22 community gardens. For each garden, we also characterized local garden factors (land use, plant diversity, light availability, and management practices) and landscape context (green space and building units within a 500-m radius).  We then evaluated multiple regression models relating local garden and landscape factors to per-plant populations of pest and beneficial insects.

Results/Conclusions

Aphids on Brassica crops peaked in August and exhibited a positive association with canopy cover, indicating that garden trees may serve as secondary hosts.  Aphids on cucurbits and tomatoes were most problematic in June and declined to low levels by early July, suggesting a role for natural enemies in controlling these pests.  Both aphids and squash bug populations on cucurbits were positively correlated with the percentage of garden area in vegetable crops.  Flea beetle populations were negatively correlated with crop diversity and positively correlated with garden size and percentage of garden area in flowers (possibly a reflection of less intensive weed control in flower beds compared to vegetable beds, as flea beetles feed on Brassica weeds in spring).  Two-spotted spider mite damage on tomatoes decreased with the percentage of garden area in flowers.  On all crops, larger gardens supported greater per-plant populations of beneficial insects, suggesting that predator and parasitoid insects are more severely affected by habitat fragmentation than herbivorous insects.  On cucurbits, beneficial insects were also positively correlated with the percentage of garden area in flowers.

Our findings suggest several strategies for ecologically-based insect management in urban gardens, including: row cover to protect seedlings from early-season aphids; increasing floral, tree, and shrub plantings (both to decrease concentrations of host plants for pests, and to provide habitat for beneficials); and increasing the diversity of agricultural crops.  In the long term, advocating for larger garden spaces could create more favorable conditions for natural enemy populations and conservation biological control of insect pests in urban gardens.