Research Highlights

Evaluating Profitable Stink Bug Thresholds and Treatment Timing

By Laura Temple

Stink bugs infest Alabama soybeans annually as the most damaging pest complex farmers face. That makes them the most important insects to manage, according to Scott Graham, assistant professor and Extension specialist of entomology and plant pathology for Auburn University.

“The Alabama Cooperative Extension System uses a much more aggressive economic threshold for stink bug treatment in soybeans, three times lower than other states across the south,” Graham says. “I wanted to evaluate that threshold to determine when our farmers should treat to achieve adequate stink bug control as economically as possible.”

Alabama Soybean Producers invested checkoff dollars into his research, which started in 2021. Graham monitored soybean plots at the Prattville Agricultural Research Unit in central Alabama for stink bugs during pod fill, treating at various levels of pressure.

Aggressive Treatment Thresholds

Graham’s trials compared stink bug treatments at three different thresholds to an untreated check. The thresholds apply to the entire stink bug complex, which includes different species: southern green stink bugs, brown stink bugs, green stink bugs, invasive but established brown marmorated stink bugs and the most damaging species, redbanded stink bugs.

The plots at the current Alabama threshold of three stink bugs per 25 net sweeps were treated twice. Plots at twice that threshold, six bugs per 25 sweeps, received one insecticide application. Plots at three times the current threshold, nine bugs per 25 sweeps, did not reach that threshold and were not treated. Nine stink bugs per 25 net sweeps is the threshold commonly recommended in other southern states.

“The results in 2021 surprised me,” he says. “Even when factoring cost of two insecticide treatments, the more aggressive threshold of just three stink bugs per 25 sweeps yielded enough to make this the most profitable treatment. We will repeat the study at least one more year to validate these results. If we see dramatically different results, we will continue the research longer.”

Graham adds that the study may show that stink bug thresholds should be adjusted based on soybean value. In 2021, his team figured profit based on an average cost of $8 per acre for insecticide treatment and application, and the average value of $13.50 per bushel for soybeans.

He plans to monitor multiple locations for stink bugs in 2022, as well as giving farmers the opportunity to participate in in-field trials to validate the economic threshold for stink bugs.

Scouting to Determine Treatment Timing

While evaluating stink bug complex thresholds, Graham is also considering ideal timing for treatment.

“Many farmers lack time or resources to scout regularly for insects in soybeans as pods develop and fill,” he explains. “Often they choose to automatically add an insecticide to a soybean fungicide treatment at the R3 growth stage, but insecticides may not be needed at that time.”

He believes stink bugs should be a focus of insect control in soybeans because of the yield damage they can do. However, they feed on seeds, so they are not an issue when fungicides are sprayed at the R3 stage, as pods are just starting to develop. He adds that invasive redbanded stink bugs may cause damage earlier in the season, but if they aren’t present, insecticide treatments would not be helpful.

“A broad-spectrum insecticide at R3 may eliminate beneficial insects, creating worse infestations later in the season, as pods are filling,” Graham says. “Often there aren’t many economically important bugs in soybeans yet at that stage, but if farmers really want to include an insecticide, they should consider an insect growth regulator that is softer on beneficials.”

Insect movements and interactions impact multiple species, and he adds that scouting is helpful because it reveals the entire insect complex in a field. For example, while green cloverworm can cause damage in soybeans, in some cases it can almost be considered a beneficial insect, because it attracts other beneficials to a field to feed on it. That results in higher populations of beneficials later, when other pests arrive.

Graham notes that scouting for the full insect spectrum in a field should also impact choice of insecticide.

“Pyrethroids control many species well, but they also control beneficials,” he says. “Soybean loopers are harder and more expensive to control than many other pests, and beneficial insects are helpful to manage this worm. Without beneficials in a field, they have a much better chance of doing economic damage.”

These are all reasons Graham encourages scouting and recommends waiting for stink bugs or other pests to appear in fields before treating.

“When scouting, farmers could consider sampling ‘sentinel’ fields to represent multiple fields in the area with similar planting dates,” he adds. “If that is the case, scouting could be more manageable. That would help farmers spray for insects just when needed.”

Published: Jun 27, 2022