Flower plantings offset the negative effects of urbanization on beneficial insects

Little is known about the effects of urbanization on insect natural enemy populations and whether landscape plantings in urban areas harbor natural enemy populations comparable to those in rural areas. This study looked at the effects of large and small scale land cover change on natural enemy assemblages. At a large scale, we hypothesized that as urbanization increased natural enemy abundance and diversity would decrease. At the scale of individual parcels, we expected ornamental landscaping features such as flower diversity would be positively correlated with natural enemy populations. To test these hypotheses, we monitored natural enemy abundance and diversity at 50 sites across Dane County, Wisconsin. Urbanization was measured as the percentage of impervious cover within a 500 meter radius around each study site. We selected 15 sites in the city of Madison that had greater than 60% surrounding impervious cover and 15 sites in rural Dane County that had less than 5% surrounding impervious cover. The remaining sites had intermediate proportions of impervious cover, 6-59%, surrounding the site. Within each parcel, we measured size and spatial arrangement of ornamental plantings and the proportion of perennial vs. annual flowers. Our results showed natural enemy diversity was 5% lower and abundance was 12% lower in urban sites compared to sub-urban and rural areas. However, urban sites with a greater proportion of flowering plants had up to 25% more beneficial insects. These results suggest the large scale impacts of urbanization may be partially mitigated by manipulating the design of landscape plantings.