This article is from the 8/21/2000 Iowa State University ICM newsletter. For the complete article, go to this web address http://www.ipm.iastate.edu/ipm/icm/2000/8-21-2000/lblroof.html
Rice, extension entomologist, Department
Beetles injure pods by feeding on the outside layer of the soybean pod, leaving a thin layer of tissue still covering the seed. They very rarely chew through the pod and into the developing seed. Grasshoppers also feed on pods, but they bite completely through the pod and destroy the seed.
All soybean fields should be scouted now. Scout fields by walking 100 feet in from the field edge. Each field, and each variety within a field, should be scouted separately because bean leaf beetles sometimes concentrate in one variety while avoiding another variety. Scouting is no longer necessary after pods reach the R7 stage (yellow pod).
In 30-inch row soybeans, place a 3-foot-wide strip of cloth (stapled to two dowel rods) on the ground between the rows. Slide the cloth under the plants and try to keep plant disturbance to a minimum before the cloth is spread between the rows and you are ready to shake the plants. Bend the plants over the cloth, and shake them vigorously when the cloth is in place. Count the number of beetles on the cloth. Repeat this procedure four times for each 20 acres in the field. Determine the average number of beetles per foot of row and then consult the economic threshold table. The thresholds listed in the table are for fields that were not scouted in July or early August for first-generation beetles. These thresholds basically are for fields that are now being scouted for the first time.
In narrow-row soybeans, a sweep net will be easier to use than a drop cloth. Take 20 sweeps in each 20 acres across the field. Determine the average number of beetles per sweep and consult the economic threshold table. For narrow-row soybeans (8-inch rows) and a plant population of three plants per foot of row, multiply the economic thresholds by 0.7 to determine an approximate threshold in narrow-row fields.
If the average numbers of bean leaf beetles equals or exceeds the economic threshold, an insecticide application is necessary to prevent economic yield loss. The benefits (saved bushels of soybean) should exceed the costs (insecticide and application) and provide an economic return.
If the beetle population is less than the economic threshold, scout the field again 5 days later. More beetles could emerge from the soil, and the population could reach the economic threshold at that time. Stop scouting when 1) beetle counts start to decline, 2) soybean pods begin to turn yellow (R7 stage), or 3) the field is sprayed. A list of insecticides is in the companion article in this issue.
Table 1. Bean leaf beetle economic thresholds in reproductive-stage soybeans.*
*Economic thresholds are based on a row spacing of 30 inches and a plant population of eight plants per foot of row. For narrow-row soybeans (8-inch rows) and a plant population of three plants per foot of row, multiply the above economic thresholds by 0.7.
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