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[Home]Newsletter 2005 ] Field and Feedlot ] Stakeholder Report ] Soybean Aphid ] Soybean Rust ] Special Topics ] Crop Modeling ] Weather Data ] Yield Trials ] IA Crop Stats ] Subsoil H20 ][ISU Extension][IA State University]

Todd Vagts
Iowa State University Extension
Field Crops Specialist
1240 D. Heires Avenue 
Carroll, IA 51401 
Office: 712-792-2364; Cell: 712-249-6025;  Fax: 712-792-2366
Email: vagts@iastate.edu  

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

By Marlin Rice, extension entomologist, Department of Entomology

Many soybean fields contain noticeable, and sometimes dramatic, leaf defoliation from the second generation bean leaf beetles. This 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 plant diseases may enter the pod through 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 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.


Enlarge
Bean leaf beetle injury to a soybean pod.

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.*

Crop value
($/bushel)
Treatment cost per acre (insecticide + application)
  $7 $8 $9 $10 $11 $12 $13 $14 $15
  beetles per foot of row
$5.00 5.5 6.3 7.1 7.9 8.7 9.5 10.3 11.0 11.8
$6.00 4.6 5.2 5.9 6.5 7.2 7.8 8.5 9.2 9.9
$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
  beetles per sweep
$5.00 3.5 4.0 4.5 5.0 6.5 7.2 7.7 8.3 8.7
$6.00 2.9 3.3 3.7 4.1 5.4 6.0 6.4 6.9 7.3
$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 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.

 

[Home]Newsletter 2005 ] Field and Feedlot ] Stakeholder Report ] Soybean Aphid ] Soybean Rust ] Special Topics ] Crop Modeling ] Weather Data ] Yield Trials ] IA Crop Stats ] Subsoil H20 ]

This page last updated on 01/26/05

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