Careful monitoring of insects in soybean fields and assessing how much threat they pose to crop yield could result in fewer insecticide applications and greater cost savings, says Angus Catchot, associate Extension professor of entomology and plant pathology at Mississippi State University.

And, he notes, preliminary studies in Mississippi indicate that defoliation in early growth stages may not be as damaging to soybean yields as once thought, which could result in revision of thresholds for treatments.

“It’s easy to overestimate defoliation — it always looks worse than it actually is,” he said at the annual conference of the Mississippi Agricultural Consultants Association. “The thing that always jumps out is that, if you look at a plant with 35 percent defoliation pre-bloom, it looks like a lot more — 50 percent or more.

“We have a guide to help calibrate defoliation levels, and you can look at photos and train yourself on what to look for before you go in the fields. But, because defoliation can have a big impact on yield at critical growth stages, it’s important that you be able to estimate the defoliation level.”

A 2010 study by graduate student Lucas Owen looked at soybean insect management, with an emphasis on the multi-pest defoliation complex that includes green cloverworms, velvetbean caterpillars, grasshoppers, and others.

“Last year was perhaps the worst fall armyworm situation we’ve ever had,” Catchot says. “There were pastures we sprayed four or five times, starting in June. We saw complete defoliation of soybeans in some areas.

“In the study, looking at early season defoliation of Group IV and Group V beans, we defoliated at V3 and V6 across both maturity groups at levels of zero, 33 percent, 66 percent, and 100 percent.

“Interestingly, at every defoliation level with the Group IVs, we actually saw a numeric yield increase, but no statistical differences at either growth stage at any defoliation level. These results were somewhat eye-opening.”

There were also no differences in yield for the Group IV beans at either growth stage regardless of defoliation.