Research Highlights

Low Insect Pressure Trials Reinforce Value of Thresholds

By Laura Temple

Numerous insects feed on soybeans in South Carolina. To protect yield, farmers look for data on insecticide efficacy and application timing. In 2022, the South Carolina Soybean Board funded research to investigate those factors for stink bugs and other major insect pests at the Clemson University Edisto Research and Education Center near Blackville, South Carolina.

“Pressure from insect pests was relatively light in the 2022 trials, but some inference can be made from the results,” says Jeremy Greene, professor of entomology at Clemson University, who led the study. “Full-season protection and aggressive insect control with pesticides did not result in increased yield. Soybean producers should not spray insects based on crop growth stage or when insect populations are below treatment thresholds.”

The pest complexes Greene and his team studied included three categories:

• Stem feeders, like kudzu bug and threecornered alfalfa hopper.
• Defoliators, including grasshoppers, soybean looper and other caterpillars.
• Pod feeders, such as podworm and various species of stink bugs.

He considers the complex of stink bugs the most important insect-related limiting factor to soybean production, especially with the expansion of the invasive redbanded stink bug and brown marmorated stink bug into the region. 

“Both of those species of stink bugs have been observed in growing numbers in recent years, with notable reproduction in soybeans in 2021, prompting this study,” Greene says. 

He conducted trials comparing appropriate insecticides and timings for each category of pests. Despite the light insect pressure, Greene used a range of insecticide application dates to see if early or late sprays impacted yield with all pest categories.

He reports that terminating insecticide use at mid-vegetative (V) and different reproductive (R) growth stages did not impact yield. When the final insecticide treatment was applied at V8 or R2, the soybeans had a higher percentage of cumulative defoliation by the end of the season than those receiving a final insecticide treatment at later stages. Regardless, different application timings yielded no statistically significant yield differences under the light pest pressure.

“Throughout this study, with most insect populations below threshold levels, we consistently found that different timings for insecticide treatment resulted in no statistical differences in yield,” he says. “Soybeans can compensate for low levels of insect damage well, so overly protective or season-long insecticide treatments aren’t necessary.”

Pod Feeders

In the trial plots, populations of stink bugs and podworm never approached treatment thresholds. Greene notes that this is unusual, because stink bug populations in most soybean fields in the region will build to threshold levels if no broad-spectrum insecticides are applied. 

“The light pressure from pod feeders could have been an artifact of small-plot research, where some insects like stink bugs might avoid colonizing near treated areas,” he says. “Research can present some challenges to obtaining the answers we need.”

He reports that no statistical yield differences occurred in these trials, indicating that full-season protection and aggressive control was unnecessary for this insect complex alone under low pressure.

Stem Feeders

In these trials, Greene explains that populations of kudzu bug and threecornered alfalfa hopper never reached treatment thresholds in the research plots.

“We made an application near, but still below, the recommended threshold for kudzu bug, and we saw no statistical improvement in yield,” he says. “That suggests the threshold is still good and reinforces that we shouldn’t spray when insects are not at or above those thresholds.”

He believes most treatment thresholds are conservative, giving producers time to react before pest populations reach the economic injury level, the point where money will be lost due to insect damage. 

“I think the treatment threshold for kudzu bugs alone is conservative, for sure,” Greene adds.

Defoliators

The pressure from this category of pests also remained below thresholds throughout the season. Greene treated plots when defoliation levels got close to thresholds. However, because defoliation didn’t reach 30% before bloom or 15% after bloom, he observed no statistical yield differences. Cumulative defoliation at the end of the season showed some differences, but only in-season levels matter for potential yield loss.

“Despite the end-of-season differences in percent defoliation, the lack of statistical yield differences demonstrates the soybean plant’s remarkable ability to compensate for foliage loss,” he says.

Published: Jun 12, 2023