Southern Soybean Research Program

Taynara Possebom, Dominic Duane Reisig

Stink bugs (Hemiptera: Pentatomidae) are major pests of soybean in the southeastern United States, causing $279 million in soybean costs and losses over the past 6 years. Current field-based direct scouting techniques could be supported by indirect monitoring systems. This study evaluated the effectiveness of seven pheromone trap types (delta, black pyramid, yellow pyramid, blue, clear, yellow, and white sticky cards) and two lure types/formulation to capture stink bugs across 68 soybean fields. From August to December 2023 and 2024, we checked the traps twice a month, counted stink bugs, identified stink bug species, and recorded trap captures at each soybean growth stage. Stink bug abundance varied by trap and lure combination. Pyramid traps captured the greatest species diversity. Trap captures also varied across soybean reproductive stages, with species-specific peaks occurring from flowering (R1) to maturity (R7). The dual-component lure (murgantiol <1%, methyl E, E, Z-2,4,6-decatrienoate <1%) consistently attracted more stink bugs than the single-component lure (methyl 2-E, 4Z-decadienoate <1%). These findings demonstrate that ground-deployed pyramid traps combined with dual-component lures may be effective tools for monitoring economically important stink bug species. Future research should explore the correlation between trap captures and in-field stink bug densities.

Delaney C. Foster, Thomas C. Mueller, Lawrence E. Steckel

Abstract Palmer amaranth, which is resistant to glyphosate and protoporphyrinogen oxidase inhibitors, remains a threat to cotton and soybean production in Tennessee. This is partly due to the recent evolution of dicamba-resistant Palmer amaranth in western Tennessee, which further complicates weed management. Experiments were conducted in 2021 and 2022 to determine the best timing between sequential applications and the order in which 2,4-D or dicamba should be used with glufosinate to control resistant Palmer amaranth. Palmer amaranth control increased when the interval between postemergence herbicide applications decreased from 21 to 7 d. At the 7-d interval in a dicamba-based system, the order of herbicides did not affect Palmer amaranth control. However, in a 2,4-D-based system, the greatest control was achieved when 2,4-D was applied first, followed by either 2,4-D or glufosinate. While weed height at the time of application had a significant effect on Palmer amaranth control with auxin herbicides, control was still unacceptable in the field at the labeled rates of dicamba or 2,4-D when applied to weeds that were &lt;10 cm tall (48% and 53%, respectively). Neither dicamba nor 2,4-D provided acceptable control of the Palmer amaranth populations evaluated in this study. Sequential applications separated by 7 d provided better weed control than those separated by 21 d. Given that the better 7-d sequential treatments provided less than 90% control and resulted in more than 64,000 surviving Palmer amaranth plants per hectare suggests that relying solely on these herbicides for Palmer amaranth control is not a sustainable weed management strategy.

Eric A. L. Jones, Ryan J. Andres, Jeffrey C. Dunne et al.

Abstract Complaints of control failures with acetolactate synthase (ALS)- and protoporphyrinogen oxidase (PPO)-inhibiting herbicides on redroot pigweed ( Amaranthus retroflexus L.) were reported in conventional soybean [ Glycine max (L.) Merr.] fields in North Carolina. Greenhouse dose–response assays confirmed that the Camden County and Pasquotank County populations were less sensitive to ALS- and PPO-inhibiting herbicides compared with susceptible A. retroflexus populations, suggesting the evolution of resistance to these herbicides. Sanger sequencing of target genes determined the Camden County population carried a Trp-574-Leu mutation in the ALS gene and an Arg-98-Gly mutation in the PPX2 gene, while the Pasquotank County population carried a His-197-Pro mutation in the ALS gene (first documentation of the mutation in the Amaranthus genus), but no mutation was detected in the PPX2 gene. Single-nucleotide polymorphism (SNP) genotyping assays were developed to enable efficient screening of future control failures in order to limit the spread of these herbicide-resistant populations. In addition, preliminary testing of these assays revealed the three mutations were ubiquitous in the respective populations. These two populations represent the first confirmed cases of PPO-inhibiting herbicide-resistant A. retroflexus in the United States, as well as the first confirmed cases of this particular herbicide-resistance profile in A. retroflexus inhabiting North America. While no mutation was found in the PPX2 gene of the Pasquotank County population, we suggest that this population has evolved resistance to PPO-inhibiting herbicides, but the mechanism of resistance is to be determined.

Eric A. L. Jones, Ramón G. León, Wesley J. Everman

Abstract Dicamba and glufosinate are among the few effective postemergence herbicides to control multiple herbicide-resistant weeds in southeastern U.S. cotton and soybean production. Field studies were conducted to determine the effect of weed size and the application of dicamba and glufosinate individually, mixed, or sequentially on common ragweed, goosegrass, large crabgrass, ivyleaf morningglory, Palmer amaranth, and sicklepod control. Sequential herbicide treatments were applied 7 d after the initial treatment. The tested weeds sizes predominantly did not affect weed control. Control of broadleaf weed species with sequential herbicide applications never increased compared to the initial herbicide application. Two applications of glufosinate and/or dicamba + glufosinate controlled grasses better than one application. The order of the herbicides in the sequential applications did not affect broadleaf species control, whereas herbicide order was important for the control of grass weeds. Grass weed control was higher when glufosinate was applied before dicamba. Dicamba + glufosinate additively controlled the weeds, except for goosegrass, for which control was less for dicamba + glufosinate compared to glufosinate alone. The results of the experiment provide evidence that dicamba and glufosinate applied individually, mixed, and sequentially are effective on common row crop weeds found in the southeastern United States, but the species present may dictate how the herbicides are applied together.

Thomas C. Mueller, Lawrence E. Steckel, Avat Shekoofa

Abstract Auxinic herbicides have been commonly used in production systems for broadleaf weed control for many years. One potential negative aspect to their use is their propensity to volatilize and move away from the treated area after application. This research examined three herbicide formulations and their relative amounts of vaporization following application under field conditions in Knoxville, TN, in 2017, 2018, and 2019. Herbicide treatments evaluated included 2,4-D choline, 2,4-D amine, and the diglycolamine (DGA) salt of dicamba. Ten field studies were conducted with major parameters including air sampler height (0.3 and 1.3 m) and applied surface condition (dry wheat stubble or green-plant vegetation). The relative volatility indicated by the study was that dicamba &gt; 2,4-D choline = 2,4-D amine. Detected herbicide concentrations were numerically higher at the 0.3-m sampling height and in the green-plant surface condition. These results confirm that dicamba is more volatile than 2,4-D and that there was no difference in vapor emissions between the amine and choline salts of 2,4-D under field conditions.