The "insect of the week" award goes to bean leaf beetles in several areas of Illinois. We have reported a few times already that numbers of bean leaf beetles seemed to be greater than anticipated this year, and they have begun to find the soybeans that were planted earlier this spring. Numbers of bean leaf beetles reported range from significant to worth watching. So, if you have not already done so, please schedule some scouting trips to soybean fields in which seedlings have emerged and are growing. Bean leaf beetles that have been waiting for emerging soybeans in their vicinity almost certainly will find them.
The threshold we have used for bean leaf beetles feeding on seedling soybeans are 16 beetles per foot of row during the early seedling stage and 39 beetles per foot of row when soybeans are at stage V2+. As one astute observer noted, counting bean leaf beetles per foot of row is not easy when the beetles sense your presence and scurry into cracks in the soil. So, if you prefer to count beetles per plant, the guidelines from Iowa State University suggest 2.0 to 4.4 beetles per plant at growth stage VC, 3.1 to 6.8 beetles per plant at growth stage V1, and 4.9 to 10.7 beetles per plant at growth stage V2. The ranges in thresholds are the result of different values for soybeans and different costs of control. You can access the entire economic threshold table at http://www.ipm.iastate.edu/ipm/icm/2003/4-28-2003/blbmanagement.html. Please note that these thresholds pertain to feeding injury by bean leaf beetles and are not appropriate for making a decision regarding management of bean pod mottle virus.
Bean leaf beetle feeding on small soybean seedling. (Photo courtesy of Scott Stein, Monsanto.)
Bean leaf beetle feeding on unifoliolate leaf of soybean seedling. (Photo courtesy of Kevin Black, Growmark.)
Bean pod mottle virus? Are the current bean leaf beetles carrying the virus and transmitting it from plant to plant? Remember, the beetles we see now are the same ones we saw at the end of the season in 2002. These are the second-generation beetles from 2002 that overwintered and emerged this spring. Did these beetles harbor the virus overwinter and begin spreading it around this spring? Did the beetles pick up the virus from other hosts this spring and then carry it into soybeans?
Quite honestly, the answers to the aforementioned questions are not known, particularly in Illinois. There is much "word on the street" that there was a lot of bean pod mottle virus last year in Illinois, and apparently soybean growers are anxious about it in 2003. But do we really have a good assessment of the prevalence of the virus in Illinois soybeans in 2002? Lots of word of mouth and lots of testimonials, but very little verification. Certainly the virus affected some seed and food-grade soybeans last year, but the effects of the virus on commercial soybean cultivars in Illinois remain not well documented.
We are almost certain that people are overreacting to bean leaf beetles, and we fear the overreaction will escalate as rumors continue to spread. We urge you to assess the situation with bean leaf beetles and bean pod mottle virus thoughtfully before making a decision that an insecticide is warranted. Research from Iowa State University suggests that treatments for management of bean leaf beetles and, ultimately, bean pod mottle virus should be applied early in the season. Treatments late in the season are not very effective for managing bean pod mottle virus. However, just because early treatments are preferred over late treatments, the ultimate question is whether any treatment is necessary. Don't spend money foolishly if bean leaf beetles are not causing significant injury and if there has been no confirmation of bean pod mottle virus in your area.
All that having been said, if an insecticide for control of bean leaf beetles is warranted, consult Table 1 for suggested products and rates. Please read the label carefully, follow all directions, and comply with precautions.
As we learn more about bean leaf beetles and bean pod mottle virus, we will keep you informed. This pest situation has developed rather suddenly over the past 2 to 3 years, and we have much to learn.--Kevin Steffey and Mike Gray