Impacts of Native and Non-native plants on Urban Insect Communities: Are Native Plants Better than Non-natives?

Abstract

With continued suburban expansion in the southeastern United States, it is increasingly important to understand urbanization and its impacts on sustainability and natural ecosystems. Expansion of suburbia is often coupled with replacement of native plants by alien ornamental plants such as crepe myrtle, Bradford pear, and Japanese maple. Two projects were conducted for this thesis. The purpose of the first project (Chapter 2) was to conduct an analysis of existing larval Lepidoptera and Symphyta hostplant records in the southeastern United States, comparing their species richness on common native and alien woody plants. We found that, in most cases, native plants support more species of eruciform larvae compared to aliens. Alien congener plant species (those in the same genus as native species) supported more species of larvae than alien, non-congeners. Most of the larvae that feed on alien plants are generalist species. However, most of the specialist species feeding on alien plants use congeners of native plants, providing evidence of a spillover, or false spillover, effect. Results are concordant with those predicted by the Enemy Release Hypothesis, which states that alien plants are more successful in non-native areas due to reduced herbivore attack. With a reduction in primary consumer diversity, secondary consumers such as migratory birds and parasitoid wasps may also be impacted. These results highlight the need for further research into specific interactions between native and non-native plants. Suburban landscapes have contrived associations of native and non-native plants that may interact with insect communities across multiple trophic levels. The purpose of the second project (Chapter 3) was to investigate associational interactions (associational resistance or associational susceptibility) between native and non-native plants in urban environments, and how they impact insect communities. In a 2 year field study, abundance and diversity of eruciform larvae and natural enemies, as well as plant damage, were measured in 5 x 5 m plots in which a native red maple (Acer rubrum) was interplanted with either other native red maples, non-congeneric non-native crepe myrtles (Lagerstroemia indica), non-native congeneric Norway maples (Acer platanoides), or placed by themselves in a plot. Tree damage percentage, caterpillar abundance, and caterpillar species richness were all collected on focal red maples and one neighboring plant. Natural enemies were also measured using modified yellow pan traps. Damage rate and caterpillar abundance were similar between all treatment groups during 2014. In 2015, however, caterpillar abundance was greater for red maple surrounded by crepe myrtle than it was for red maples with no neighbors, red maples surrounded by other red maples, and red maples surrounded by Norway maples. Greenstriped mapleworm (Dryocampa rubicunda), a native, multivoltine specialist caterpillar, was the most abundant caterpillar found in the study, and its abundance was correlated with damage and total caterpillar abundance. We propose that D. rubicunda caterpillars feeding on red maples surrounded by crepe myrtles have an increase in larval and adult survival through a decrease in predation. There were no significant differences in insect natural enemy abundance or family richness due to treatments. We believe that our analysis of natural enemy community composition at the family level is insufficient and that results may differ if identified to a lower taxonomic level. The results indicate that associational interactions between native and non-native plants can greatly influence urban insect herbivore communities and that maximizing use of native plants in urban environments will decrease opportunities for plant pest outbreaks.