Degree Type


Date of Award


Degree Name

Master of Science




Sustainable Agriculture

First Advisor

Matthew E. O'Neal

Second Advisor

Alison E. Robertson


Soybean aphid, Aphis glycines Matsumura, is a phytophagous insect capable of causing yield reduction of 40-50%. The introduction and spread of this invasive pest has caused dramatic changes in commercial soybean production management. Due to various economic factors, prophylactic application of insecticides and fungicides to soybean has become increasingly common for yield protection. Impacts of prophylactic pesticide use on soybean aphid ecology are not well characterized, but frequent or poorly timed pesticide use in other systems is associated with aphid population increases due to decreased aphid mortality attributable to decreased fungal disease outbreaks. The first paper of this study examines the impact of growth stage-based applications of fungicides, insecticides, and tank mixes on soybean aphid populations to see if similar population increases are observed. This study also examines the effect of growth-stage based pesticide applications on soybean yield, as research-to-date focuses on an integrated pest management approach.

The second portion of this study employs Bayesian statistical methods to calculate the probability of management tactics from the above study providing cost-effective soybean aphid management. Prior studies have examined the effectiveness of insecticidal seed treatments and pesticide application at soybean growth stage R1; most disease and insect pressure appear later in the season, thus, prophylactic pesticide use at growth stage R3 is more likely. Cost-effectiveness estimates for these situations were nonexistent. To determine the effectiveness of treatments, cost estimates based on pesticide costs and scouting and application fees were used to calculate gain thresholds for each treatment under potential soybean market prices. The probabilities of each treatment reaching or exceeding estimated gain thresholds were calculated based upon collected yield data.

There is little literature available describing the community of entomopathogenic fungi utilizing soybean aphid as a host in North America. This may be due to the time-consuming bioassay and cultivation methods used to isolate and study these organisms. The third portion of this study endeavors to sequence the ITS region of a common entomopathogen, Conidiobolus thromboides, and to use this sequence information to develop a PCR-RFLP method to rapidly identify and distinguish C. thromboides in environmental samples.


Copyright Owner

Rebekah Marie Ritson



File Format


File Size

95 pages