Pavement, pests, & parasitoids, oh my! Elm herbivores and their natural enemies in the urban forest

Abstract

Urban areas are the fastest growing habitat type in the world, and an increasing proportion of the United States and global population lives in urban areas. Urban forests provide essential ecosystem services to rapidly expanding urban populations, but their health is threatened by damaging herbivory from non-native, invasive insects. To address this problem, my masters research investigated two critical questions that limit our ability to sustainably manage invasive insects in urban forest ecosystems: (1) How do urban environments impact the density of invasive herbivores in the urban forest? and (2) Are predator and parasitoid natural enemies present, and, if so, what is the level of parasitism? I investigated these questions in the context of the elm-herbivore study system, analyzing a globally distributed host tree and its complex of invasive insect pests in Colorado, USA. In my first chapter, I address the first question and explore how a variety of environmental factors that vary across urban habitats influence the density of several invasive insects. Specifically, I evaluate how vegetational complexity, distance to buildings, impervious surface, canopy temperature, host availability, and density of co-occurring herbivores impact three invasive pests of elm trees: the elm leaf beetle Xanthogaleruca luteola Müller (Coleoptera: Chrysomelidae), the European elm flea weevil Orchestes steppensis Korotyaev (Coleoptera: Curculionidae), and the elm leafminer Fenusa ulmi Sundevall (Hymenoptera: Tenthredinidae). I found that insect responses to these factors were species-specific, and all environmental factors were associated with density of at least one pest species except for distance to buildings. Elm leafminer density decreased with higher temperatures and was influenced by an interaction between vegetational complexity and impervious surface. Elm flea weevil density increased with greater host availability, and elm leaf beetle density increased with higher temperatures. Both elm leaf beetle and elm flea weevil density decreased with greater leafminer density, suggesting that insect density is mediated by species interactions. Results of this study can be used to inform future tree planting efforts through the selection of "safe sites", or locations where trees will be less likely to experience damaging outbreaks of insect pests. Additionally, these results can be used to strategize preventative management on trees that are located in outbreak "hotspots", or locations where environmental conditions make trees predisposed to insect outbreaks. Finally, results of this study contribute to our knowledge of the dynamic ways in which multiple invasive insects interact in urban environments. This information will be especially valuable as non-native insect introductions continue to increase into the future. In my second chapter, I narrow my focus to two historically important and particularly damaging pests of elm, the elm leaf beetle and elm flea weevil, to address the second question listed above and explore the complex of natural enemies attacking these two pests. In many areas of these insects' invaded range, outbreaks severely damage elm hosts. Natural enemies are thought to be important in regulating elm leaf beetle and elm flea weevil populations in other regions, but whether natural enemies are present in Colorado is largely unknown. As such, the aim of chapter 2 was to identify which predators and parasitoids of these pests are present in Colorado, a state with frequent pest outbreaks and where the natural enemy community is almost entirely undescribed. In June – August 2021, I identified predators through field observations and laboratory feeding trials, finding seven species of predators from six arthropod orders that fed on elm leaf beetle or elm flea weevil. Additionally, I reared 58 elm leaf beetle egg clusters, 539 elm leaf beetle larvae, and 435 elm flea weevil mines to detect parasitoids. Two parasitoids of elm leaf beetle, the egg parasitoid Oomyzus gallerucae and the larval-adult parasitoid Erynniopsis antennata, are present in Colorado, representing novel records of these species in the state. However, combined parasitism of elm leaf beetle eggs and larvae was low at <3% across the season, with parasitoids nearly absent early in the season and peak parasitism occurring in late summer. I found five families of parasitoid wasps that emerged from leaves containing weevil mines: Chalcididae, Encyrtidae, Eulophidae, Euplemidae, and Pteromalidae. Parasitoids emerged from <20% of leaves containing weevil mines with almost no parasitism early in the season. Given the low parasitism rates and few predators observed in our study, it seems unlikely that predator and parasitoid natural enemies exert effective control over elm leaf beetle and elm flea weevil in Colorado. This finding challenges the assumption that natural enemies are a driving force of elm leaf beetle and elm flea weevil control in Colorado. Additional research is needed to confirm species identifications for parasitoids of the elm flea weevil, disentangle elm leaf beetle and elm flea weevil population dynamics, and establish effective and sustainable control methods amidst frequent pest outbreaks. Together, these two research projects enhance our knowledge of what triggers outbreaks of the elm leaf beetle Xanthogaleruca luteola, the elm flea weevil Orchestes steppensis, and the elm leafminer Fenusa ulmi in urban areas while also laying the groundwork for a renewed interest in biological control of elm leaf beetle and elm flea weevil. It is my hope that this work can be applied to other invasive insect pests in urban forest ecosystems and make urban forests more resilient in an era when they are increasingly vulnerable to insect attack.