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Document Type

Thesis - University Access Only

Award Date


Degree Name

Master of Science (MS)


Plant Science

First Advisor

Adrianna Szczepaniec


Neonicotinoid insecticides are widely used in row crop agriculture and provide effective control against a broad range of pests. Although their systemic nature reduces risk to the environment and non-target organisms, non-target impacts remain important to investigate. Spider mites (Acari: Tetranychidae), which are not susceptible to neonicotinoids, tend to increase after neonicotinoid use in other plant systems, which can result in additional applications of pesticides to suppress these secondary pests. Additionally, dry conditions often favor spider mite population growth. With the predicted increase in incidence and severity of drought events, it is important to consider how the combined effects of neonicotinoid insecticides and water deficit may affect spider mites. Thus, the objective of this research was twofold. First, I tested the effect of thiamethoxam seed treatments, imidacloprid foliar applications, and combinations of these treatments on arthropod abundance in soybean fields at two locations over two years in eastern South Dakota. Second, in greenhouse experiments I investigated the effect of thiamethoxam seed treatments and drought stress, alone and in combination, on spider mite abundance. I also measured the impact of these treatments on plant growth and fitness. I found that spider mites (Tetranychus urticae) were more abundant on plants exposed to the neonicotinoid insecticides in the field experiments, and that foliar applications of imidacloprid seemed to drive this increase in spider mite abundance. I did not note, however, consistent effects of the neonicotinoid applications on predators of spider mites. In the greenhouse experiments, I found that spider mites were consistently more abundant on plants exposed to the neonicotinoid insecticides and tended to be more abundant on plants that were exposed to drought stress or neonicotinoids and drought stress. There was a tendency for thiamethoxam to stimulate growth early in plant development. I also observed clear negative effects on plant fitness at the end of the experiment in plants exposed to drought stress. This research provides valuable insights into the relationship between neonicotinoid insecticides, spider mites, and plant physiology, and has important implications for sustainable management of spider mites and other herbivores in soybean fields.

Library of Congress Subject Headings

Neonicotinoids -- Physiological effect Spider mites -- Effect of insecticides on Soybean -- Effect of pesticides on Seed treatment


Includes bibliographical references (pages 51-55)



Number of Pages



South Dakota State University


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