Small scale additions of native plants fail to increase beneficial insect richness in urban gardens

1. Despite increasing interest in gardening for wildlife, experiments testing the conservation value of garden alterations are scarce, particularly in heavily urbanised landscapes. 2. In this paper, we assess the ability of discrete additions of native wildflowers (70 plants of seven plant species added and subsequently monitored for 2 years) to augment species richness of beneficial insects including bees, butterflies and predatory wasps in New York City community gardens. We also evaluate the overall use of native and introduced flowers by these insect groups in the urban garden study sites. 3. Bayesian analyses failed to find a strong influence of the native plant additions on beneficial insect richness, after correcting for prior floral abundance. 4. In addition, butterflies and megachilid (leaf-cutter) bees were found to heavily utilise introduced ornamental and crop flowers in these gardens, even when native flowers were present. 5. Our results suggest that exotic garden plants are utilised by, and important in maintaining richness of, beneficial insects in urbanised landscapes. Furthermore, garden manipulations need to be more substantial than often recommended, if they are to significantly influence the richness of beneficial insects in urban gardens.