Chapter Two:
Aphis glycines populations were greater in 2009 than in 2010. In both 2009 and 2010, the insecticide treatments reduced the exposure of soybean to A. glycines
populations when compared to the untreated control. Among the insecticides there were no significant differences for either year. In 2009, a yield reduction was observed in the untreated control, but not in the insecticide treatments. The yields among the insecticides were not significantly different. There was no yield reduction in 2010 due to low A.
glycines pressure.
Spirotetramat had the greatest residual activity in 2009, during the 10 to 19 DAT sampling period. The populations of A. glycines in the untreated control and imidacloprid treatments increased in 2009 during the 2009 10 to 19 DAT period. Only the untreated control had populations of A. glycines that increased in 2009 during the 18 to 27 DAT sampling period. In 2010, the data were inconclusive because all of the treatments demonstrated reductions in A. glycines populations. The reduction of A. glycines in the untreated control made it impossible to determine if populations were decreasing in insecticide treatments due to insecticide residual activity or some other environmental factor.
In 2009, biological control did not appear to be present during the 10 to 19 DAT or 18 to 27 DAT sampling periods. This conclusion was made because there were greater A. glycines populations on the uncaged plants than the caged plants that were used for the
study. This was the opposite of what was expected. In 2010, biological control was present in the untreated control and the imidacloprid treatment at the 10 to 19 DAT sampling period. Biological control did not appear to be occurring during the 18 to 24 DAT sampling period in individual treatments, but when we compared the pooled cage study data we were able to determine that biological control was occurring.
Based on the BSI values we determined that a bioresidual effect was occurring in 2010, during the 10 to 19 DAT sampling period in the imidacloprid treatment. A BSI did not appear to be occurring during the 18 to 24 DAT sampling period in 2010.
Chapter Three:
When all of the predaceous arthropods are combined between the two years we collected natural enemies from 22 taxa. These included both aphidophagous natural enemies as well as those natural enemies that do not consume A. glycines. The most abundant natural enemy in 2009 was H. axyridis, while in 2010 the most abundant natural enemy was O. insidiosus.
In 2009, all of the insecticides reduced the abundances of natural enemy populations.
In 2010, the broad-spectrum insecticide esfenvalerate reduced natural enemy abundances more than the other treatments. In 2009, H. axyridis abundances were greatest in the untreated control and the selective insecticide treatment spirotetramat. In 2010, O. insidiosus abundances were greatest in the selective insecticide treatment
imidacloprid. The untreated control had a greater abundance of natural enemies than the broad-spectrum insecticide treatment.
What does this all mean?
We now have an understanding of how selective insecticides work against the soybean pest A. glycines. They are able to reduce populations of A. glycines to levels that are comparable to control that is achieved with a broad-spectrum insecticide. Selective insecticides are also capable of preventing yield loss as well as the broad-spectrum insecticide that was used. The selective insecticide spirotetramat had greatest residual activity in 2009 between 10 and 19 DAT. The level of biological control varied between years, and between series, indicating that natural enemies were present in the field at different times.
We determined that a BSI was present in 2010 during the 10 to 19 DAT sampling period. It is possible that a bioresidual may not be important in soybean, since A. glycines populations are generally controlled around the R3 and R4 growth stages. After the insecticide residuals have worn off, the soybean plants have reached a growth stage where they are not negatively affected by A. glycines.
From our natural enemy data we can see that the most abundant natural enemy in 2009, H. axyridis, was not negatively affected by the selective insecticide spirotetramat.
In 2010 the most abundant natural enemy was O. insidiosus, and the selective insecticide imidacloprid had a reduced impact on its abundances. The natural enemy community varied between years, but this was most likely due to the presence of different pests between the two years. We saw that selective insecticides had a reduced impact on natural enemies when looking at the data from 2010. In 2009, all of the insecticides reduced the natural enemies when compared to the untreated control. We can determine
that selective insecticides are unlikely to cause a secondary pest outbreak, since it appeared that they did not reduce the natural enemies that do not feed on A. glycines.
ACKNOWLEDGEMENTS
First and foremost I thank God for the intellect that He has given me, as well as the perseverance to continue my education. Without His guiding grace I would never have made it as far as I have.
I would also like to thank my wonderful parents, Jerry and Donna Varenhorst, for all of their support and encouragement that I received when I reached points in my educational career that I didn’t think I could make it past. I also thank them for the initial financial support that helped me to get a start in college, which allowed me to find out that I have a love for learning. I thank my paternal, Norman and Lois Varenhorst, and maternal, Donald and Marie Held, grandparents, their pride in my academic successes has given me the determination to continue pushing forward to reach new goals, and I strive everyday to make them proud. I thank all of my extended family for their support, and encouragement. I also thank my girlfriend, Katie Bates, for her assistance in editing and helping me stay focused on achieving my goals.
I also thank my professor Matthew E. O’Neal. He offered me a research position in his lab, and over the course of my time at Iowa State University has helped me to become a better student, researcher, and overall person. His constructive criticism of my work has made it better than it was before. I thank him greatly for this opportunity to work on a research project, which has been an extremely fulfilling experience.
I thank my fellow soybean lab mates, especially Michael McCarville, Nicolas Schmidt, Kevin Johnson, and Kelly Seman. Without them I would have a much lower
probability of success. I must thank the numerous hourly workers who stood in the heat of the Iowa summers and helped me collect my data. Without them this experiment would have been impossible.