Issue No. 7, Article 5/May 12, 2006
Bean Leaf Beetles Deserve Attention on Early-Planted Soybeans
As of May 7, 14% of Illinois's projected soybean acreage had been planted, compared with 34% planted by the same date in 2005. With fewer soybean acres to choose from, bean leaf beetles will undoubtedly concentrate in fields with the earliest-emerging seedlings. Consequently, soybean growers should be particularly alert for overwintered bean leaf beetles in those fields. We already have received a couple of reports that bean leaf beetles have been observed in some areas of Illinois, so their appearance in early-planted soybean is just around the corner.
Bean leaf beetles that emerge in the spring are the survivors that went into overwintering quarters (e.g., wooded areas) last fall. These beetles colonize soybean fields in the spring and feed on soybean seedlings, chewing on the cotyledons and the unifoliate and trifoliate leaves. Based solely on the consequences of leaf feeding, economic yield loss does not occur unless the numbers of bean leaf beetles are relatively large (16 per foot of row in the early-seedling stage, 39 per foot of row at stage V2+). Marlin Rice, extension entomologist at Iowa State University, recently published an article in Integrated Crop Management (May 1, 2006) that indicated that densities of bean leaf beetle populations in Iowa this spring seem to be low. However, he too points out that early-planted soybeans should be watched closely.
Bean leaf beetle feeding on soybean cotyledon (photo provided by Kevin Black, Growmark).
Bean leaf beetle feeding on unifoliate soybean leaves (photo provided by Kevin Black, Growmark).
In the past, discussions about bean leaf beetles and seedling soybeans focused solely on leaf-feeding injury. However, since the late 1990s, soybean growers have also been concerned with bean pod mottle virus, which is transmitted by bean leaf beetles. Entomologists at Iowa State University conducted research in central and northwestern Iowa in 2000 and 2001 and had some success in managing both the bean leaf beetle and bean pod mottle virus with early-season and midseason insecticide applications (R.K. Krell, L.P. Pedigo, J.H. Hill, and M.E. Rice. 2004. "Bean leaf beetle [Coleoptera: Chrysomelidae] management for reduction of bean pod mottle virus." Journal of Economic Entomology 97: 192-202). Application of Warrior after soybean emergence and again as first-generation bean leaf beetles emerged in northwestern Iowa in 2000 significantly reduced bean leaf beetle densities through midseason and significantly reduced field incidence of bean pod mottle virus. However, these same insecticide applications in 2001 produced fewer positive benefits, because the densities of bean leaf beetles were low. It's important, therefore, to determine bean leaf beetle densities before assuming that insecticide applications will "solve the problem." In fact, there is no problem if densities of bean leaf beetles are low.
Entomologists and plant pathologists at the University of Illinois conducted research in 2000 and 2001 to determine the distribution of bean leaf beetles and bean pod mottle virus in Illinois. They published their findings in the following paper: T.R. Mabry, H.A. Hobbs, T.A. Steinlage, B.B. Johnson, W.L. Pedersen, J.L. Spencer, E. Levine, S.A. Isard, L.L. Domier, and G.L. Hartman. 2003. "Distribution of leaf-feeding beetles and Bean pod mottle virus (BPMV) in Illinois and transmission of BPMV in soybean." Plant Disease 87: 1221-1225. In short, the researchers found that densities of bean leaf beetles were greatest in central Illinois in 2000, although they found the beetles more evenly distributed in 99 Illinois counties in 2001. Bean pod mottle virus was detected in bean leaf beetles in 37 of 41 counties assayed in 2000, with the percentage of individuals carrying the virus ranging from 5% to 100%. In 2001, bean pod mottle virus was detected in bean leaf beetles collected in 86 of the 99 counties sampled, with 5% to 100% of the beetles carrying the virus. The occurrence of BPMV-positive bean leaf beetles was greatest in counties near the Illinois and Wabash rivers. The authors speculated that wooded areas adjacent to river systems may provide protection and resources to overwintering bean leaf beetles.
So the upshot of this article is that early-planted soybeans should be scouted vigilantly for bean leaf beetles, but there is little reason to assume that the beetles will infect seedling soybeans with bean pod mottle virus. This is especially true in areas where bean pod mottle virus has not been commonly observed. However, if you find densities of bean leaf beetles that reach or exceed the aforementioned thresholds, an insecticide application may be appropriate. Insecticides labeled for control of bean leaf beetles are listed in Table 1.--Kevin Steffey