Clark, S. 2025. The effects of global change on plant-insect interactions. Dissertation, Michigan State University, East Lansing, MI.

Globally, insect populations have declined for the past four decades. Insects are central to our food systems and provide ecosystem services including pollination and biological control of agricultural pests. Uncovering the mechanism and impacts of global changes on insect populations is paramount to forestalling or reversing insect decline. In three studies, I assess four of the primary global changes that are potential causes of insect declines, including climate warming, drought, insecticides, and habitat loss. To test the effects of warming, drought, and insecticides, I conducted two experiments: a field experiment and a lab experiment, using the cabbage white butterfly, Pieris rapae, as a model species. I expected that, within the butterfly’s thermal tolerance (10-35C), butterflies would increase performance until they reached their critical thermal maximum. When butterflies developed above or outside of their range of thermal tolerance, I expected their performance to drop swiftly.

In chapter 2, I conducted a field experiment manipulating three major global change factors: climate warming, drought, and insecticides. I used open-top chambers to simulate climate warming to an average range of 0.8-1.6C above average Michigan temperatures, rain-out shelters to exclude precipitation from any treatments underneath them, and a foliar spray of a neonicotinoid insecticide, Merit, in a fully crossed experiment. Control treatments were exposed to ambient temperature, precipitation, and no insecticides. Larval cabbage white butterflies were placed on plants before reaching second instar, and survival was documented weekly until pupation. I found no significant effect of climate warming, drought, insecticide exposure, or any interactions between them on caterpillar survival. While there were no significant differences between treatments, likely due to small sample size, there were trends that reflected patterns that have been noted previously. In chapter 3, I tested the effects of warming and drought treatments in a fully crossed lab experiment. I tested a range of temperatures to reflect cabbage white butterfly thermal tolerance by programming four growth chambers to 10C(CTmin), 20C, 30C, and 40C (beyond CTmax). I placed larval cabbage white butterflies on each plant, and watered them at either 30%VWC (standard soil moisture for broccoli) or drought-stressed to 12.5% VWC using a Hydrosense II soil probe. Results reflect a possible trade-off between size at pupation and time to pupation. Larvae reared at the 10C were larger (p < 0.0001) and took longer to reach pupation (p < 0.0001) than those reared at their thermal optimum, 30C. Larvae reared at 30C pupated more quickly (p < 0.001) but were smaller at this stage (p < 0.001). All larvae perished at 40C within 3 days. Higher temperatures may allow for more generations per year of smaller insects, but lower temperatures may result in the loss of a generation at the end of the growing season. Despite insects being larger under these conditions, long development times and temperatures below their thermal tolerance may result in population decline.

A different global change factor, habitat loss, was the focus of chapter 4. I tested how native prairie plants within farm fields impact insect biodiversity, and if there were detectable spillover effects into the surrounding cropland. I studied ants due to their unique roles as major predators of agricultural pests and contributors to plant and soil health. Working in a six-year study with 5m-wide prairie strips running through two cropland treatments that were 1ha in area, I tested whether row crop management, distance from the prairie strips, and establishment time of prairie strips impacted species richness. I found no significant difference in ant species richness between crop management (p = 0.171), and higher species richness in nearer prairie strips (p < 0.001). Species richness also increased over the five years that this experiment was conducted (p < 0.001). These results imply that the prairie strips are serving their intended purpose of increasing insect biodiversity over time, and there may be spillover effects into the surrounding cropland. Future farm management may benefit from the addition of native prairie plants to increase biodiversity adjacent and within fields.

My findings point to some interactions among global changes factors, especially temperature and precipitation, on insect performance. To reverse insect declines, we must first understand the vulnerabilities of insect performance to interacting global change, and second, integrate management strategies that increase biodiversity and mitigate species loss as insect decline continues.

Associated Treatment Areas:

• T3 Reduced Input Management
• T4 Biologically Based Management
• MCSE Main Cropping Systems Experiment

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