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.