Date of Award
5-2026
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Entomology, Soils and Plant Sciences
Committee Chair/Advisor
Jeremy Greene
Committee Member
Francis Reay-Jones
Committee Member
Michael Plumblee
Committee Member
Kendall Kirk
Committee Member
William Bridges
Abstract
Insect pests are a major limiting factor to profitable soybean, Glycine max (L.) Merr., production in South Carolina, yet current economic thresholds are largely based on historical data and may not reflect modern cultivars, production practices, or input costs. Key insect pests in South Carolina can be categorized into three major feeding groups: defoliators, stem feeders, and pod feeders. Defoliating insects include, but are not limited to, soybean looper, Chrysodeixis includens (Walker); velvetbean caterpillar, Anticarsia gemmatalis (Hubner); green cloverworm, Hypena scabra (Fabricius); grasshoppers; and numerous species of beetles. Stem feeders include, but are not limited to, kudzu bug, Megacopta cribraria (Fabricius); Dectes stem borer, Dectes texanus (LeConte); threecornered alfalfa hopper, Spissistilus festinus (Say); and grasshoppers. Pod feeders are predominantly podworm, Helicoverpa zea (Boddie) and stink bugs, including southern green stink bug, Nezara viridula (Linnaeus); brown stink bug, Euschistus servus (Say); green stink bug, Chinavia hilaris (Say); redbanded stink bug, Piezodorus guildinii (Westwood); and brown marmorated stink bug, Halyomorpha halys (Stål). Despite considerable changes in how soybeans are produced today compared with how they were managed decades ago, treatment thresholds for these pests have remained unchanged for years. We performed field trials to refine treatment thresholds of major insect pests and improve methodologies for evaluating insect injury by quantifying soybean yield responses to simulated defoliation and stem injury across multiple growth stages and years. To quantify these relationships, greenhouse and field studies were conducted from 2023 to 2025 using both mechanical and novel approaches to simulate insect injury. A 3D-printed pneumatic leaf puncher was developed by our team at Clemson University to deliver controlled and repeatable defoliation levels, providing a standard alternative to traditional methods of simulation such as hand plucking or clipping of trifoliates. Simulated defoliation treatments were applied at multiple growth stages (V8-R5), while stem injury was administered during mostly vegetative stages (V4-R1) using mechanical and chemical methods to represent feeding of major stem feeder pests. Additional field trials incorporating natural pest populations were conducted to evaluate a range of treatment thresholds under realistic production conditions. Across studies, increased levels of simulated injury consistently resulted in significant yield reductions, with injury later in the season causing significant yield impacts at relatively lower levels of injury when compared with early season injury. The pneumatic leaf puncher achieved high accuracy (≥90%) in delivering target defoliation levels, supporting its utility as a reliable research tool. Despite measurable injury-yield relationships, field trials across multiple years indicated that pest densities often remained below current economic thresholds, and no yield differences were observed among treatments under field conditions. In conclusion, these findings support maintaining current defoliation thresholds of 30% during vegetative growth stages and 15% during reproductive growth stages, while emphasizing the importance of integrated pest management to avoid unnecessary insecticide applications. The methodologies and results presented here provide a foundation for refining economic thresholds and standardizing techniques of simulated injury, with applicability across diverse soybean production systems regionally and beyond.
Recommended Citation
Whitfield, Adam, "Refining Treatment Thresholds of Insect Pests in South Carolina Soybeans Using Simulated Injury and Natural Populations" (2026). All Dissertations. 4244.
https://open.clemson.edu/all_dissertations/4244