There are an array of foliar feeding insects in soybean fields right now. It is sometimes hard to determine the percent of defoliation and to identify which insects are causing the defoliation in your soybean crop.
First, as soybeans begin to set pods, an insecticide treatment generally is not warranted until the combined defoliation from all pest approaches 20% and active feeding is still occurring. Most people tend to overestimate the defoliation. You need to assess the average defoliation of the entire plant, not just the upper leaves. Additionally, you need to get an average defoliation from several locations across the field, not just on the field borders.
To estimate the percent defoliation, follow these 4 steps:
- Remove a leaf from the top, middle, and lower third of 9 randomly selected plants.
- Discard the most and least damaged leaf. This will leave you with 25 leaves.
- Compare the 25 leaves to a reference image chart (figure below)
- Repeat these three step at several locations within a field.
Here is a list of foliar feeding insects I saw during the July 16 Dodge County Crop Condition Tour and other fields I visited this week. Caterpillars and larva included the thistle caterpillar, alfalfa caterpillar, green cloverworm, stalk borer (not a defoliator), and yellowstriped armyworm. Beetles included the bean leaf beetle, southern corn rootworm beetle, and grape colaspis. Grasshopper species including the redlegged grasshopper and probably others.
For more information about the crop tour, pictures of these insects, and a reference images for percent defoliation, please visit our local website at croptechcafe.org or give me a call at 727-2775.
Percent defoliation references for of soybean leaves
Thistle caterpillar
velvetbean caterpillar
Green cloverworm
Stalk borer
Yellowstriped armyworm
Bean leaf beetle
Southern corn rootworm or spotted cucumber beetle
Grape colaspis
Grasshoppers (redlegged and others)
