Bean leaf beetles reduce soybean yield

Bean leaf beetles are now being found in soybeans. This late-summer generation of bean leaf beetles can be one of the most destructive insects in soybean in the Midwest. Large populations have reduced yields by 20-50 percent.

The bean leaf beetle often has four rectangular spots in the center of the wing covers.

How do bean leaf beetles cause yield loss?

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, causing seeds to appear shrunken, discolored, and moldy. This injury reduces seed quality.

Some bean leaf beetles are red instead of yellow.

What does bean leaf beetle injury look like?

Beetles injure pods by feeding on the outside layer of the soybean pod, leaving a thin layer of tissue still covering the seed. Grasshoppers also feed on pods, but they bite completely through the pod and into the seed.

Bean leaf beetle feeding on soybean pod.

When should a field be scouted?

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).

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

How is a field scouted?

Walk 100 feet in from the field edge. Each field, and each variety within a field, should be scouted separately. Place a 3-ft.-wide strip of cloth on the ground between the rows. Bend the plants over the cloth, and shake them vigorously. 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.

Is an insecticide application necessary?

This can be determined by using the economic thresholds below. These thresholds are for 30-inch row beans. No thresholds have been developed for narrow-row beans.

  1. Estimate the market value in dollars per bushel.
  2. Determine the treatment costs per acre (insecticide plus application cost).
  3. Look under the appropriate treatment cost per acre. The number in the bottom table below is the bean leaf beetle economic threshold.
  4. If the average number of bean leaf beetles per foot of row equals or exceeds the economic threshold, the benefits (saved bushels of soybean) should exceed the costs (insecticide and application) and provide an economic return.

What if the beetle population is less than the economic threshold?

Scout the field again five days later. More beetles could emerge from the soil, and the population could reach the economic threshold at that time.

When can scouting be discontinued?

Stop scouting when:

  1. beetle counts start to decline,
  2. soybean pods begin to turn yellow, or
  3. the field is sprayed.

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
Lannate 1.8L* 1-1.5 pints 14
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

* restricted-use chemical

Economic thresholds for bean leaf beetle

Crop value Treatment cost per acre
($ per bushel) (insecticide + application)
$7 $8 $9 $10 $11 $12
beetles per foot of row
$7.00 3.9 4.4 5.0 5.6 6.1 6.7
$8.00 3.5 4.0 4.5 5.0 5.5 6.0

This article originally appeared on pages 167-168 of the IC-476(23) -- September 3, 1996 issue.

Updated 09/02/1996 - 1:00pm