Urbanization can affect herbivorous insect abundance and distribution both directly and indirectly. Increased heat from urban environments can speed insect development or can alter growing seasons for herbivores. Alternatively, urban environments may either increase or decrease levels of predation or parasitism on herbivores. We compared herbivory rates by leaf-mining flies (Phytomyza ilicicola, P. opacae, and Rhopobota dietziana) on American holly trees (Ilex opaca) planted around campus dormitories and in a natural setting at St. Andrews University in Laurinburg, NC. We collected preliminary temperature data around study trees, as well as phenotypic differences (numbers of leaf spines) between trees in different locations. We predicted that herbivory might increase in urban environments, either due to an “urban heat-island” effect (on a small scale), or due to reduced predation in more heavily managed urban systems.
Results/Conclusions
Our results suggest that two of the herbivores (P. ilicicola and P. opacae) were more abundant in urban environments. However, temperature data was more equivocal, and did not indicate measurably higher temperatures by trees planted in dormitory courtyards. Leaf spines were more abundant on I. opaca plants around dormitories, suggesting the possibility that spines may discourage foraging by predatory birds, which may increase abundance of herbivores in the more urban environment. These preliminary results suggest potential indirect effects of urban environments on herbivore environments, and suggest that indirect effects may be somewhat idiosyncratic.