Economic Thresholds of the
Bean Leaf Beetle on Soybean
in South Dakota

Michael A. Catangui, Ph.D. - Associate Professor
 South Dakota State University

 
   The local ecology of bean leaf beetles in SD is not yet completely known.  We have been borrowing data from Iowa and Nebraska that may or may not apply to northern SD soybean growing conditions.  Remember when we mistakenly assumed that there was only one type of corn borer moth flight in the entire state that affected corn?  Since SD is a relatively new major soybean producer, our understanding of soybean insect pests is still evolving.

    Until we know more about bean leaf beetle flight patterns in SD, the best way to manage this insect is by regular monitoring of the field.  Management decisions must be based on actual injuries observed on the plants at any given time, and not on some predictive recommendations developed from historical data by our neighbors to the east and south.

    Bean leaf beetles can damage soybeans by defoliating the plants, and by feeding on and clipping the pods.  Economic thresholds are available for both types of damages and may be used whichever is most important on the field at the time of scouting.      

    Defoliation will be less important when the leaves start turning yellow.  Pod feeding by the beetles also becomes less critical once the pods themselves turn yellow.  Remember that pods tend to stay green after the leaves have changed colors.  And bean leaf beetles will feed on any green soybean tissues.

    SDSU Extension Fact Sheet 905 (Grasshopper and Bean Leaf Beetle Economic Thresholds in Soybeans) contain details on calculating economic thresholds based on percent defoliation or pod feeding.  You may also contact SDSU Extension Entomology to obtain a free printed material.

    During the pod-fill through "green bean" stages of soybeans the formulas below can be used to calculate economic thresholds.  These thresholds are based on the injuries caused by bean leaf beetles on soybean pods.

A) Per foot of row; 30-inch rows:

    Economic threshold = [(control cost)(156.70)] ÷ [(market value)(expected yield)]

    Example:  A grower raises soybeans in 30-inch rows in Redfield, SD.  Soybeans are already in the "green bean" stage of development.  The crop sprayer is charging $11 per acre to treat the field, and the year 2000 commodity loan rate for soybean in Spink County is $4.83 per bushel.  Expected yield is 45 bushels per acre.  How many bean leaf beetles (per foot of row) should be considered economically important?

    ET = [(11)(156.70)] ÷ [(4.83)(45)] = 8 bean leaf beetles per foot of row.

B) Per foot of row; 8-inch rows drilled:

    Economic threshold = [(control cost)(109.69)] ÷ [(market value)(expected yield)]

    Example:  A grower raises soybeans in 8-inch rows (drilled) in Aberdeen, SD.  Soybeans are already in the "green bean" stage of development.  The crop sprayer is charging $11 per acre to treat the field, and the year 2000 commodity loan rate for soybean in Brown County is $4.83 per bushel.  Expected yield is 45 bushels per acre.  How many bean leaf beetles (per foot of row) should be considered economically important?

    ET = [(11)(109.69)] ÷ [(4.83)(45)] = 6 bean leaf beetles per foot of row.

C) Per sweep of net; 30-inch rows:

    Economic threshold = [(control cost)(97.15)] ÷ [(market value)(expected yield)]

    Example:  A grower raises soybeans in 30-inch rows in Volga, SD.  Soybeans are already in the "green bean" stage of development.  The crop sprayer is charging $10 per acre to treat the field, and the year 2000 commodity loan rate for soybean in Brookings County is $5.02 per bushel.  Expected yield is 45 bushels per acre.  How many bean leaf beetles (per sweep of net) should be considered economically important?

    ET = [(10)(97.15)] ÷ [(5.02)(45)] = 4 bean leaf beetles per sweep of net.

D) Per sweep of net; 8-inch rows drilled:

    Economic threshold = [(control cost)(68.01)] ÷ [(market value)(expected yield)]

    Example:  A grower raises soybeans in 8-inch rows (drilled) in Tyndall, SD.  Soybeans are already in the "green bean" stage of development.  The crop sprayer is charging $10 per acre to treat the field, and the year 2000 commodity loan rate for soybean in Bon Homme County is $5.02 per bushel.  Expected yield is 45 bushels per acre.  How many bean leaf beetles (per sweep of net) should be considered economically important?

    ET = [(10)(68.02)] ÷ [(5.02)(45)] = 3 bean leaf beetles per sweep of net.

   Bean leaf beetles may also cause substantial defoliation in soybeans.  Certain levels
of defoliation may cause yield loss.  To calculate thresholds based on leaf feeding by the beetles please refer to SDSU Extension Fact Sheet 905.  A simple means of estimating percent defoliation can be found under "Estimating Percent Defoliation" in this Web site.

References:

Catangui, M. A.  2000.  Grasshopper and bean leaf beetle economic thresholds in soybeans.
                  Fact Sheet 905, South Dakota State University, Brookings.

Kalton, R. R., C. R. Weber and J. C. Eldredge.  1949.  The effect of injury simulating hail damage
                  to soybeans.  Iowa Agr. Home Econ. Exp. Sta. Bull. 359: 736-796.

Rudd, W. G. and R. L. Jensen.  1977.  Sweep net and ground cloth sampling for insects in
                  soybeans.  J. Econ. Entomol. 70: 301-304.

Smelser, R. B. and L. P. Pedigo.  1992.  Soybean seed yield and quality reduction by bean leaf
                  beetle (Coleoptera: Chrysomelidae) pod injury.  J. Econ. Entomol. 85: 2399-2403.

Stone, J.D. and L. P. Pedigo.  1972.  Development and economic-injury level of the green
                  cloverworm on soybean in Iowa.  J. Econ. Entomol. 65: 197-201.