We have received numerous questions regarding early-season control of bean leaf beetles, which potentially can transmit bean pod mottle virus to soybeans. In fact, we have heard that many growers are considering having their soybeans sprayed this spring as soon as bean leaf beetles are observed in the fields. Some have gone so far as to suggest that preventive insecticide applications are necessary. (More on this later.) The underlying premise of this suggested approach is that controlling bean leaf beetles will prevent the transmission of bean pod mottle virus (BPMV). Interest in (and possibly concern about) BPMV and bean leaf beetles has been heightened by research conducted by entomologists and plant pathologists at Iowa State University (ISU). This research, and associated management suggestions, has been published in several issues of ISU's Integrated Crop Management newsletter. However, soybean producers in Illinois need to understand that the situation in Iowa may not pertain to the situation in Illinois.|
Based on research conducted in Illinois, entomologists and plant pathologists believe that insecticides to control bean leaf beetles are warranted only to reduce feeding damage (if a threshold number of beetles has been reached). Few, if any, of the overwintering beetles are able to transmit BPMV, and we do not have enough information about the association between the beetles and BPMV in Illinois to suggest that insecticides would be beneficial. Therefore, application of insecticides to control of bean leaf beetles to manage BPMV is not warranted in Illinois. Exceptions to this generalization are possible. For example, if BPMV has been positively diagnosed in your area and has caused yield losses, and if soybean fields in the area are near woods or alfalfa fields where large densities of bean leaf beetles have been observed, spraying insecticides may be warranted.
Thresholds for bean leaf beetles feeding on soybeans and insecticides suggested for control of the beetles have been discussed in previous issues of the Bulletin (issue no. 6, May 3, 2002; issue no. 7, May 10, 2002). However, you should also know a little bit more about the virus that has caused all of the ruckus. Following is some information about BPMV that should add to your knowledge about the virus.
BPMV is one of the most common soybean-infecting viruses in Illinois. Infection by this virus causes a yellow-green mottling and distortion of leaves in the upper canopy of the plant and is most obvious during periods of rapid growth and cool temperatures. BPMV has been reported to decrease seed size and number, cause seed mottling, and increase susceptibility to Phomopsis seed infection. Yield losses attributable solely to the virus are not available for soybeans in Illinois. BPMV also has been associated with "green stem syndrome"; however, plants with green stems late in the season may not be infected with BPMV. On the other hand, some plants infected with the virus do not develop green stem. Thus, there is no clear association between green stem and BPMV in Illinois. BPMV infects many legumes, including soybeans, snap beans, and lima beans. Dr. Craig Grau, at the University of Wisconsin, has reported that alfalfa and red clover also can be infected with BPMV.
BPMV is transmitted by chewing insects that carry sap from infected plants to healthy plants. The most common vector for this virus seems to be the bean leaf beetle. Other beetles that can transmit the virus include the banded cucumber beetle, blister beetles, grape colaspis, Mexican bean beetle, and spotted cucumber beetle. In recent studies reported by USDA and University of Illinois researchers, the western corn rootworm was able to transmit BPMV, but Japanese beetles were not able to transmit the virus. BPMV also can be transmitted via seed but only at low levels--up to 0.1%.
Although BPMV-transmission studies have demonstrated that the aforementioned insects can transmit the virus, the frequency of transmission and importance in widespread dispersal are not known. Researchers at the University of Illinois have been evaluating the importance of bean leaf beetles in transmission of BPMV. Surveys conducted in 2000 and 2001 revealed more BPMV in soybeans in 2000 than in 2001. "Hot pockets" of BPMV infection in Illinois were detected during both years. The correlation between the frequency of bean leaf beetles (and western corn rootworms) infected with the virus and plants infected with the virus was low. Although a fairly high level (25%) of bean leaf beetles from two overwintering sites in April 2001 carried the virus, the beetles were able to transmit the virus to a very low number of plants (only 1 of 95 plants tested positive for the virus). Whether bean leaf beetles feed on early-emerging infected plants, acquire the virus in the spring, and transmit the virus to soybeans remains to be determined.
The bottom line regarding bean leaf beetles and bean pod mottle virus in Illinois is that we need to learn a whole lot more about this association before suggesting management tactics focused on the insect. Although we encourage everyone to watch for bean leaf beetles in early-planted soybeans, we strongly suggest that you focus on leaf-feeding injury rather than attempt to "stop the virus." The suggestion by some to apply insecticides before, at, or shortly after planting to prevent infestations of bean leaf beetles is inappropriate. We cannot condone this type of "insurance" insecticide application for bean leaf beetles. With commodity prices as low as they are and with thin farm operating margins, the needless expense represented by this approach makes no sense at all. Stick to IPM practices with bean leaf beetles--scout soybean fields, and apply an insecticide only if numbers of bean leaf beetles exceed economic thresholds.
One final note: Populations of bean leaf beetles will diminish if soybeans are planted late. Although some early-planted fields may be "magnets" for bean leaf beetles, many soybean fields planted this year will escape early-season infestations of bean leaf beetles because the beetles will die before soybeans emerge. Delayed planting is aggravating (at the very least), but we can count some blessings.--Dean Malvick and Kevin Steffey