Issue No. 8, Article 2/May 14, 2004
Watch for Bean Leaf Beetles in Fields of Emerging Soybeans
During the past couple of weeks, we have received a handful of reports from around the state indicating that bean leaf beetles have been observed, primarily in fields in which soybeans had not yet emerged. The beetles have been noticed crawling on crop residue, trucks, bags of seeds, planters, and other things. Bean leaf beetles have become active throughout the state, so it's time to start watching fields of emerging soybeans for signs of their presence. This is particularly important for early-planted soybeans. We recently learned that in some early-planted soybean plots (planted in mid-April) in Champaign County, defoliation of the small seedlings was an estimated 20%.
Bean leaf beetle adult on corn residue. (Photograph courtesy of Steven Doench, Pioneer Hi-Bred International.
The earliest report of bean leaf beetle activity I received was from Matt Montgomery, crop systems Extension unit educator in Springfield. On April 26, Matt noticed bean leaf beetles were active in alfalfa fields. As most of you are aware, bean leaf beetle adults end their "hibernation" for the winter relatively early in the spring, usually well before most soybean fields have been planted and the seedlings have begun to emerge. Adults leave their overwintering quarters--wooded areas, clumps of grass, leaf litter, crop residue--during April and fly into alfalfa fields and other suitable habitats. When soybean seedlings emerge, the beetles fly from alfalfa fields into soybean fields, where they feed and mate, after which the females lay eggs to begin the first generation of the year. Remember, the beetles you see first in the spring are the same ones you saw late last fall, the beetles that end the second generation.
Because bean leaf beetles seek sources of food early in the spring, the earlier that soybeans are planted, the greater the likelihood of economic infestations, especially if the adults survived the winter in respectable numbers. In an article in the most recent edition of Iowa State University's Integrated Crop Management (IC-492, May 3, 2004), extension entomologist Marlin Rice showed that the percentage mortality of overwintering bean leaf beetles in the central and southern regions of Iowa was lower during the 2003-2004 winter than during the 2002-2003 winter (http://www.ipm.iastate.edu/ipm/icm/2004/5-3-2004/blboverwinter.html). Consequently, soybean producers in those regions should expect to find bean leaf beetles in their fields. We can say the same thing for all regions in Illinois.
Reports from fields in Illinois thus far this season suggest that numbers of bean leaf beetles in the state this spring are significant in some areas. Some observers have noticed a little feeding injury in fields planted in April, and others have reported hearing the beetles "buzzing" as they flew around in fields in which the soybeans had not yet emerged. With these observations as background, we encourage scouting in soybean fields right now. Economic damage to seedlings caused by bean leaf beetles requires a fairly large number of beetles. However, if the beetles transmit the bean pod mottle virus to seedlings in the spring (and this is still debatable), few beetles are required to spread the virus from plant to plant.
Early signs of injury caused by bean leaf beetles include feeding scars on cotyledons and shot holes in unifoliate and trifoliate leaves. When growing conditions are favorable for soybeans, defoliation injury usually does not result in economic losses later in the season. However, if the injury is significant and soybean seedlings are growing slowly, an insecticide application may be justified.
Scars on soybean cotyledons caused by feeding by bean leaf beetles. (Photograph courtesy of Marlin Rice, Iowa State University.)
Shot holes in seedling soybean leaves caused by feeding by bean leaf beetles (University of Illinois).
Table 1 shows economic thresholds for bean leaf beetles feeding on soybean seedlings (growth stage VC through V2). Please note that these thresholds apply only to the damage resulting from leaf feeding; they are not applicable for management of the bean pod mottle virus. The thresholds are dynamic, depending on the cost of control and the value of the soybeans. Insecticides suggested for control of bean leaf beetles in soybeans are listed in Table 2. If an insecticide application is warranted, please follow all label directions and precautions.