Home sweet home: Evaluation of native versus exotic plants as resources for insects in urban green spaces

Insect decline and loss of biodiversity not only affect large-scale agricultural landscapes but are increasingly recognized in urban environments. It is undisputed that a greater supply of flowers in urban green spaces can provide insects with more food and habitat. However, controversy still surrounds the question of whether native wild plants or non-native ornamental plants and varieties are the right choice. The aim of our study was to investigate the number of insects interacting with different types of plants: twelve ornamental and six related wild perennials. In this context, not only the number of flower visitors was recorded, but also the feeding damage caused by insect herbivores on the leaves and stems of the study plants. We established 18 plant species in ten independent study plots in the city of Darmstadt, Germany. The plants were six native wild plant species, six ornamental plant species related to the wild plants from the same genus or family, and six exotic ornamental plant species from other genera and families than the wild plants. Native insects (wild bees, flies, beetles, wasps) that feed on pollen and nectar visited wild perennials significantly more often (67% of all visits) than related ornamental (24%) and unrelated exotic plants (9%). In contrast, honey bees (Apis mellifera) showed no preference for any of the three target plant groups and interacted with other plant species than most other insects. The assessment of leaf damage caused by insect herbivores on the individual plants confirmed a similar and significant difference in the insects’ choices. Leaves from wild plants showed the highest herbivory (mean 2.3% of the leaf area), followed by related ornamental plants (0.8%), whereas unrelated exotic plants were hardly consumed (0.1%) by herbivores. Our study shows that in urban green spaces, both flower-visiting and leaf-feeding insects are more likely to use native wild plants as a food source than closely related and exotic ornamental plants.

Methods
Flower visits:

Data were collected between 29 May and 19 September 2021 at the time of peak flowering of each plant species. All flower visitors were surveyed once during a continuous 10-minute survey period per plant species and plot between 9:15 am and 5:00 pm (1550 minutes in total). Insect individuals that could not be clearly identified in the field were collected and later identified in the laboratory. Identification was based on the specific literature for each insect order or family. For example, wild bees were identified using the identification tables of Scheuchl. It was not always possible to identify the species precisely so the taxa designated as species also contained individuals that could only be identified at the family or genus level. These flower visitors were then designated as morphospecies, for example, Andrena 1 and Andrena 2.

Feeding damage:

In addition to recording the flower visitors, the feeding damage was determined on those plants that were suitable for automatic damage measurement. The two Achillea species, whose leaves are pinnate, as well as the two Campanula species and L. corniculatus, whose very small leaves were not suitable for software-assisted evaluation, were excluded. For the other plant species, namely C. dealbata, C. jacea, K. macedonica, K. arvensis, S. verticillata, S. pratensis, A. tuberosa, K. uvaria, A. x cultorum, G. lindheimeri and C. plumbaginoides, five leaves per target plant species were taken blindly and randomly from different plant individuals in three-dimensional space.

The leaves were photographed directly on-site in a picture frame with anti-reflective glass using a Canon EOS 40D camera. The percentage of leaf damage was then evaluated in the laboratory using "BioLeaf", a mobile application for measuring leaf damage caused by insects. In the case of leaves whose leaf contours had been lost due to insect damage, auxiliary lines were drawn before the analysis in order to reconstruct the original leaf shape.