Butterflies fly away: insect decline in California, USA
Authors
Halsch, Christopher
Issue Date
2023
Type
Dissertation
Language
Keywords
butterflies , climate change , insect decline , long-term monitoring
Alternative Title
Abstract
Since 2017, when Hallman et al. published their sensational study documenting reductions of insect biomass from protected areas in Germany, reports of insect decline have become common across the world. While some patterns have emerged, such as a 2% annual rate of decline in temperate terrestrial insects, many questions remain, including the importance of various drivers of decline, how these vary by region, and which species are the most vulnerable. This dissertation investigates factors associated with butterfly decline in the western United States. Specifically, I use a combination of observational and experimental approaches to assess the importance of climate change and pesticide use in causing the decline of butterflies over the past 50 years. Chapter 1 reviews the current literature on the impacts of climate change from long-term monitoring datasets of insects across the world. We find that climate change has been shown to be an important factor in influencing insect populations dynamics in natural areas throughout the world and that extreme events are more likely to have negative outcomes. We also recover a known bias in long term monitoring studies towards temperate regions and lepidoptera. Our results suggest that climate change impacts on insects have the potential to be considerable, even when compared with changes in land use. These results shed light on the complexity of insects responding to changing abiotic conditions. Chapter 2 investigates the direct and indirect effects of seasonal weather using long-term butterfly monitoring data, high resolution climate data, and a remotely sensed indicator of plant primary productivity (NDVI). We found that snowpack exerted a strong direct positive effect on butterfly occurrences and that low snowpack was the primary contributor to reductions during a drought. We then link these results to variation in overwintering strategies, finding that species that overwinter in more juvenile stages are more vulnerable. These results highlight mechanisms of weather driven declines in insect populations and the nuances of climate change effects in heterogeneous landscapes. Chapter 3 assesses the risk that pesticides pose to monarch butterflies in the Sacramento Valley (chapter 3A) and in milkweeds sold by retail stores nationwide (chapter 3B). In both studies, milkweed leaves were collected and sent to the Cornell core facility to be screened for pesticides. In chapter 3A, we detected 64 pesticides in milkweed leaf samples collected across land use types. On average, approximately 9 compounds were detected per plant across all sites, with a range of 1 to 25 compounds in any one sample. Chlorantraniliprole was identified in 91% of our samples and found to exceed a tested LD50 for monarchs in 58 out of 227 samples. Importantly, most compounds detected have not been tested on monarchs, thus the risk inferred from our results is necessarily an underestimate. In chapter 3B we detected 61 different pesticides in milkweed leaves collected in nurseries nationwide with an average of 12.2 (±5.0) compounds per plant. While only 9 of these compounds have been experimentally tested on monarch caterpillars, 38% of samples contained a pesticide above a concentration shown to have a sub-lethal effect for monarchs. We also found that plants with labels advertising their value for wildlife did not have fewer pesticides at concentrations known to have a negative effect on monarchs. Finally, chapter 4 takes an experimental approach to investigate the interaction between warming temperature, non-native host plant use, and pesticide exposure. We exposed caterpillars of the Melissa blue butterfly (Lycaeides melissa) to three concentrations of chlorantraniliprole (inspired by results from the previous work, described above), under three experimental climates, on a diet of a native or a non-native host plant throughout larval development in a fully factorial experiment. We found that high pesticide exposure and a non-native diet exhibit strong negative effects on caterpillars, resulting in 62% and 42% reduction in survival respectively, while interactive effects tend to be weaker, ranging from 15% to 22% reduction in survival. This study demonstrates that the cumulative effects of stressors acting in isolation (additively) are sufficiently strong to severely reduce survival and by extension population persistence in the wild. This dissertation advances our understanding of the realized impacts of Anthropogenic stressors on butterflies in the western US. It incorporates both observational and experimental designs, along with population specific life history information, to enhance our biological insight into large-scale patterns of persistence and decline in insects. Finally, this dissertation is a demonstration of the power and vital importance of long-term monitoring programs.
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Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 United States