When it comes to insect defoliation thresholds, it is important to examine the upper and lower levels of the plant canopy as well as know the pod growth stages.

Ben Thrash, Extension entomologist at the University of Arkansas System Division of Agriculture, spoke on insect defoliation thresholds in late-season soybeans, during the University of Arkansas' online soybean field day.

Insect defoliation thresholds

"A lot of people have trouble estimating how much defoliation is taking place out in a soybean field," Thrash said. "Oftentimes, a field will have some defoliation on the tops of the plants and not as much on the bottom."

With insects like bean leaf beetles, green cloverworms, and velvetbean caterpillars, these pests start feeding at the top of the soybean canopy.

"When you initially go out to a soybean field with these insect pests, it looks like you have a lot of defoliation, but throughout the rest of the canopy of the plant, there's still a lot of whole leaves there with not much defoliation," he said.

Some insects, primarily soybean loopers, start feeding at the bottom of the canopy, and they come to the top. If defoliation is found from soybean loopers in the top canopy, then there is already quite a bit of injury in the soybean field.

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"Often when people go to a soybean field to estimate defoliation, they'll only look at the top of the plant," Thrash said. "They see defoliation on the tops of the plants, and it looks serious. However, when you pull the canopy back and look at the leaves down lower, there's hardly any defoliation in some of the lower layers of leaves. Don't just pay attention to the upper leaves."

A grower or consultant needs to look at the entire plant to get an idea of how much defoliation is taking place in a field.

"Defoliation looks different depending on the insect," he said. "For instance, the bean leaf beetle gives the leaves a shot-hole appearance with a lot of little holes. Comparing a caterpillar with the bean leaf beetle, a caterpillar eats large sections out of the leaves."

Remember, the threshold is 25% defoliation.

"As an example, if a grower treats early, at the first sign of defoliation, he would have taken care of the defoliators," Thrash said. "However, if a threshold level of stinkbugs shows up in the field a couple of weeks later, that would turn into two insecticide applications. Many times, if the grower would have waited for the defoliation to reach the threshold, they could get away with a single treatment."

Not spraying when initial insect defoliation shows up could save a grower a lot of money as well as time by not needing to make two applications.

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"In this example, a single application would take care of both types of defoliating insects as well as the stinkbugs," he said.

Soybean growth stages

Knowing the growth stages of soybeans is also important.

"An R5 soybean pod is the beginning seed stage while an R6 is when the seeds are butted up in the pod and still have the membrane intact on the seeds when you split the pod open," Thrash said. "Our stinkbug threshold changes based on the growth stage. Our threshold doubles whenever you reach R6, so you are at nine per 25 sweeps before R6 and 18 per 25 sweeps after."

Whenever the R6.5 growth stage is reached, the membrane separates from the seed. This is when farmers can terminate insecticide applications for stinkbugs as well as defoliating insects.

"However, for red banded stinkbug, our threshold is four to five per 25 sweeps," Thrash said. "For them, we double the threshold at R6.5, and we have to continue spraying for it up through R7.

"With this year's soybean crop being late, the yield potential may be off on some soybean fields. Accordingly, growers need to be mindful of how they're going to spend their money on fields experiencing insect defoliation to decide if an application will give a good return on their investment."