Issue No. 9, Article 3/May 26, 2006
Scrutinize Early-Planted Soybean Fields for Bean Leaf Beetles
Entomologists at Iowa State University, who have been surveying for bean leaf beetles for several years, have reported large numbers of the pest in alfalfa fields in central Iowa. You can read the details for yourself in "Bean Leaf Beetles Return--With a Vengeance" in the Integrated Crop Management newsletter. They report that the numbers of bean leaf beetles they have found in 2006 are second only to 2002, the year with the largest densities of bean leaf beetles to date.
I received my first report of significant numbers of bean leaf beetles in soybean fields from Todd Price with Illinois Valley Ag Supply in Greene County. Although he had not assessed the densities of beetles in the fields he visited, he noticed as many as two to three beetles per VC to V1 stage plant in some areas of some fields. He planned a return trip to several fields to obtain a more quantitative estimate of the beetles' densities.
As I indicated in "Bean Leaf Beetles Deserve Attention on Early-Planted Soybeans" in issue no. 7 (May 12, 2006) of the Bulletin, yield loss from bean leaf beetle injury to leaves and cotyledons usually does not occur until densities of the beetles are relatively large (16 per foot of row and 39 per foot of row in VC-V1 and V2+ stage soybeans, respectively). For making control decisions, you may wish to refer to dynamic thresholds that vary with the value of soybeans and cost of control. Tables that provide these dynamic thresholds are included in the aforementioned article in Iowa State University's Integrated Crop Management Newsletter.
For example, if the cost of control is $10 per acre and the value of the soybeans is $6 per bushel, treatment with an insecticide to prevent further injury is warranted if the density of bean leaf beetles reaches 5.2 beetles per V1-stage soybean seedling. If the cost of control is only $8 per acre and all other values remain the same, treatment with an insecticide to prevent further injury is warranted if the density of bean leaf beetles reaches 4.1 beetles per seedling. Keep in mind that the thresholds presented in the Iowa State University table do not include the impact of bean pod mottle virus. However, bean pod mottle virus has been neither common nor widespread in Illinois, so the thresholds should be relevant in most fields.
Sharpen your scouting skills for bean leaf beetles in seedling soybeans. Experienced scouts have learned that bean leaf beetles tend to "play dead" and drop from seedling soybeans to the ground when they are disturbed. So approach your sampling area quietly. Examining plants by hand is the easiest method for seedling soybeans. As the plants grow, use of a beat cloth may help you assess numbers of bean leaf beetles per foot of row. Remember to scout in several areas of a given field to obtain a reasonable assessment of bean leaf beetle density for the field. A few high counts are offset when several areas of a field are not infested, lowering the average number of beetles per plant. There's no need to treat an entire field and spend more money than necessary if the field average of bean leaf beetles is lower than published thresholds.
If an insecticide to control bean leaf beetles is warranted, refer to Table 1 in the aforementioned article in the Bulletin (issue no. 7, May 12, 2006).--Kevin Steffey