How interactions with plants influence butterfly responses to environmental stressors: from pathogens to climate change
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Authors
Christensen, Tara
Issue Date
2025
Type
Dissertation
Language
en_US
Keywords
Alternative Title
Abstract
Global insect declines have been reported with increasing fervor from the scientific community as well as the public discourse. The factors contributing to declines are numerous and likely interactive, but the best-documented contributors are habitat loss, pesticide use, climate change, disease, invasive species, and pollution. For Lepidoptera, moths and butterflies, population trajectories are heterogeneous across taxa and geography. Butterflies, superfamily Papilionoidea, are among the most thoroughly studied groups in Lepidoptera due to their popularity and relative ease of identification. Several high-resolution (species-level) long-term monitoring efforts of butterflies have revealed widespread declines across diverse ecosystems. In this dissertation, I focus on a few of the main drivers of butterfly population declines – disease, invasive plants, and climate change, and ask whether butterflies’ relationships with their food plants can increase or decrease their susceptibility to these stressors. My four dissertation projects can be divided conceptually into two major questions: 1) do larval host plants, including exotic hosts, mediate herbivore-pathogen interactions in a specialist caterpillar? and 2) do nectar resources shape butterfly responses to anthropogenic change? Under the first conceptual framework, I used a laboratory rearing experiment, chemical analysis, and molecular viral detection methods to examine the effects of larval host plant species and secondary chemistry on viral transmission, infection intensity, and survival in caterpillars of a North American butterfly species, Euphydryas phaeton (Nymphalidae). Euphydryas phaeton are an appealing study system as they are chemical specialists on host plants containing iridoid glycosides, and have recently incorporated an exotic host plant into their diet. Therefore, they are ideal for investigating the evolution of diet breadth, and the effects of plant defensive chemistry on herbivore fitness and interactions with higher trophic levels.
In my first dissertation project we found that caterpillars utilizing their native host plant had reduced survival when experimentally infected with a viral pathogen, in comparison to those feeding on the exotic host plant, motivating the hypothesis of dietary range expansion driven by pathogen resistance on the novel host. Additionally, I utilized a field study to examine viral prevalence in the gregarious caterpillars of E. phaeton in their natural habitat. I found that the distribution of the virus at the landscape scale occurred with viral “hotspots” or areas with high infection prevalence, and that viral prevalence correlated with underlying host plant chemistry in a manner that was host plant dependent.
For the second major theme, I leveraged large datasets, including a long-term butterfly monitoring program from the Sierra Nevada mountains (Shapiro transect), natural history observations from GBIF, and a three-year intensive survey of nectar plants to better understand the role of floral composition and availability for diversity and population trends of montane butterflies. I found that there is variation in nectar breadth among adult butterflies of the Sierra Nevada, though the majority of species tended towards generalism. Surprisingly, nectar specialists had more positive population trajectories than generalists, indicating a possible fitness cost to generalism in these communities. Additionally, I gathered three years of floral data across six montane sites where long-term butterfly monitoring takes place. I found that butterfly diversity is strongly related to floral diversity, relative to floral abundance, indicating these butterflies are depending on a diverse assemblage of floral resources through the flying season. Additionally, it appears that an extreme heat event in California during the summer of 2024 may have driven advanced butterfly phenology relative to flower phenology, and accounted for reduced abundance and diversity later in the season.