Stressed Plants and Herbivores: Exploring the Mechanisms of Drought's Impact on Cotton Physiology and Plant-Herbivore Interactions
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Drought is expected to become more prevalent in our future and influence plant-insect interactions in natural and agricultural systems. There is an established interest in predicting the effects of drought on plant-insect interactions, with over 500 published studies. Despite this intensive effort, researchers cannot accurately predict the effects of water deficit stress on insect performance. To address this, I tested hypotheses aimed to predict insect performance and abundance and developed a hypothesis that may better predict herbivore performance on stressed plants. I tested the Pulsed Stress Hypothesis which predicts that insect herbivores feeding on drought stressed plants will increase in abundance on plants that are pulsed stressed rather than continuously stressed. I conducted two, 10-week field studies to test the effects of drought on arthropods using 0.6 hectares of cotton. Stress was implemented by withholding water from continuously stressed plants and using pulsed watering for pulsed stressed plants. Piercing-sucking herbivores (i.e., thrips, stinkbugs, fleahoppers) were more abundant on pulsed stressed plants than continuously stressed plants. In contrast, chewing herbivores (e.g., grasshoppers, caterpillars) were similar in abundance on stressed plants. This suggests that the variation we see in herbivore response to stressed plants is dependent upon the severity and frequency of drought in addition to herbivore feeding guild. For my third field study, I tested the interactions of the timing of cotton aphid infestation, cotton development, and only pulsed stress. I had herbivore exclusion cages with only aphids inside and either on seedling or fruiting cotton. I largely found that cotton may compensate for early season damage from aphids and pulsed stress, but the combination of the two greatly impact cotton development. I conducted a meta-analysis on herbivore performance, macronutrients, and allelochemicals to determine the relationship between stress-induced changes in plants and herbivore performance. I used Metawin 2.0 to analyze the data from 42 published studies and found that macronutrients were the most important factor in determining herbivore performance on stressed plants. With this evidence, I devised the Nutrient Availability Hypothesis which predicted that the concentration of stress-induced changes in macronutrients in stressed plants will determine herbivore performance.
Sconiers, Warren Boyce (2014). Stressed Plants and Herbivores: Exploring the Mechanisms of Drought's Impact on Cotton Physiology and Plant-Herbivore Interactions. Doctoral dissertation, Texas A & M University. Available electronically from