Bean leaf beetles damage soybean pods

Note: this article was republished in 1998, with an updated economic threshold and chemical control table. Bean leaf beetles are now being found in soybean fields. The late-summer generation can be one of the most damaging insects for soybean in Iowa. Large populations have reduced yields by 20-50 percent.

The yellow phase of the bean leaf beetle is the most common color variety.
Some bean leaf beetles are red instead of yellow.
Some bean leaf beetles lack the "rectangular" spots on the wing covers.

Bean leaf beetles will feed on soybean leaves throughout the season, but leaf feeding seldom causes yield loss. Most damage (economic yield loss) occurs when beetles feed on the developing pods. This yield loss can occur in several ways. Pods may be clipped from the plants, or fungal pathogens may enter the pod from the feeding sites, causing seeds to appear shrunken, discolored, and moldy. This injury reduces seed quality. Beetles injure pods by feeding on the outside layer of the soybean pod, leaving a thin layer of tissue still covering the seed. They do not eat into the developing seed. Grasshoppers also feed on pods, but they bite completely through the pod and destroy the seed.

Bean leaf beetle injury to a soybean pod.

Begin looking for bean leaf beetles during the R4 stage (full pod) of soybean development. Scouting is no longer necessary after pods reach the R7 stage (yellow pod). Scout fields by walking 100 feet in from the field edge. Each field, and each variety within a field, should be scouted separately.

Bean leaf beetles can be shaken onto a white cloth and counted.

In 30-inch row soybeans, place a strip of cloth (3 feet wide 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.

A sweep net can be used to sample for bean leaf beetles in drilled beans.

In narrow-row soybeans, a sweep net will be easier to use than a drop cloth. Take 20-25 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 3 plants per foot of row, multiply the economic thresholds by 0.7 to determine an approximate threshold in narrow-row fields. If the average number 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 beetle counts start to decline, soybean pods begin to turn yellow, or the field is sprayed.

Bean leaf beetle economic thresholds*

Crop value ($ per bushel) Treatment cost per acre (insecticide + application)
$7 $8 $9 $10 $11 $12 $13 $14 $15
No. of beetles per foot of row
$7.00 3.9 4.4 5.0 5.6 6.1 6.7 7.3 7.8 8.4
$8.00 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5
No. of beetles per sweep
$7.00 2.4 2.8 3.1 3.5 3.8 4.2 4.5 4.9 5.2
$8.00 2.2 2.5 2.8 3.2 4.1 4.5 4.8 5.2 5.5

* Economic thresholds are based on a row spacing of 30 inches and a plant population of 8 plants per foot of row. For narrow-row soybeans (8-inch rows) and a plant population of 3 plants per foot of row, multiply the above economic threshold by 0.7.

Common chemicals labeled for bean leaf beetle control.

Insecticide Amount per acre Harvest interval (days)
Ambush 2EC* 3.2-6.4 ounces 60
Asana XL* 4.8-9.6 ounces 21
Cygon 400 1 pint 21
Lorsban 4E 1-2 pints 28
Penncap-M* 2-3 pints 20
Pounce 3.2EC* 2-4 ounces 60
Sevin XLR+ 1-2 pints 0

This article originally appeared on page 161 of the IC-478(21) -- September 2, 1997 issue.

Updated 09/01/1997 - 1:00pm