Although very few soybean fields have been planted thus far, growers who have planted or soon will plant soybeans should be aware that bean leaf beetles will pose an early threat. Early-planted soybeans are very attractive to bean leaf beetles that already have emerged from their winter dormancy. Based on some of the observational reports we have received this spring, bean leaf beetles are abundant in some areas of Illinois. Bean leaf beetles must have been sheltered from the cold temperatures this past winter by snow or other types of cover.
In issue no. 2 (April 4, 2003) of the Bulletin, I mentioned that Scott Isard, University of Illinois Department of Geography, had found fairly significant numbers of overwintering bean leaf beetles in a wooded area in Champaign County. Scott uses pieces of cloth as overwintering traps. Joe Spencer, entomologist with the Illinois Natural History Survey, has visited some of these overwintering sites with Scott and shared this report: "Scott Isard and I made a stop last evening [April 15] at his favorite woodland piles of discarded clothing (north of Urbana). As he has noted before, we found hundreds of bean leaf beetles among the castoffs. Yesterday's big winner was a pair of brown and yellow plaid, flair-legged, denim trousers (circa 1971) that yielded 30 or more beetles. Our local BLBs seem to have gone retro! I hesitate to think how many BLB we might catch if we were to scatter some Supertramp albums and a leisure suit at this site!" Interesting thoughts for future sampling protocols.
With another somewhat unconventional sampling approach, Mike Hellmer, field sales agronomist with Pioneer Hi-Bred International in east-central Illinois, observed large numbers of bean leaf beetles on April 29. Mike reported that large numbers of insects were hitting the windshield of his pickup truck. When he stopped to investigate, he found hundreds of bean leaf beetles on the grill of his truck. He indicated that other sales representatives with Pioneer also have observed large numbers of bean leaf beetles either flying around or moving about in fields of soybean stubble.
I can add to these reports by conveying that we, too, observed many bean leaf beetles in a field in Piatt County on April 28. During field preparations for a Japanese beetle grub trial, we encountered quite a few bean leaf beetles moving about in the soybean stubble.
So the stage is set for bean leaf beetles to cause more concern in 2003. Although bean leaf beetles have been the most consistent pests of soybeans in Illinois for decades, their larger numbers in recent years and the possibility of their transmitting bean pod mottle virus have elevated the level of concern among growers. Let's review what we know about this insect's life cycle, ability to cause injury to soybeans, and the potential for bean leaf beetles to transmit bean pod mottle virus.
Life cycle (in a nutshell). Bean leaf beetles overwinter as adults under litter in wooded areas and in crop fields. They become active early in the spring and move into nearby fields of alfalfa or clover, although they do not lay eggs in these fields. The beetles we observe in the spring are the same beetles we observed last fall (i.e., second-generation beetles from the previous year). As soon as soybean seedlings emerge, bean leaf beetles abandon forage fields and colonize soybean fields. They feed on the emerging seedlings, primarily on newly emerged leaves. After they finish feeding, the females lay eggs in the soil. Bean leaf beetle larvae feed on underground portions of soybean plants, including the nodules, although the larvae never have been associated with economic damage. The adults from the first generation of bean leaf beetles emerge in July and feed on the leaves. Females lay eggs for a second generation, and adults of the second generation feed on both leaves and soybean pods late in the summer. These beetles seek shelter for overwintering.
Injury caused by bean leaf beetles. Bean leaf beetle adults chew small, round holes in soybean leaves. Excessive feeding in the spring can cause plant death and stand loss. However, because soybeans can compensate for stand loss early in the season, control of bean leaf beetles in seedling soybeans usually is not necessary. Insecticides are warranted only when densities of bean leaf beetles reach 16 per foot of row in the early seedling stage or 39 per foot of row at stage V2+. These thresholds are based solely on the potential damage resulting from defoliation and have no bearing on the situation with bean pod mottle virus.
Bean leaf beetles and bean pod mottle virus. Based on what I have learned from the entomologists and plant pathologists at Iowa State University, the situation with bean leaf beetles and bean pod mottle virus is real in areas of Iowa, although still not thoroughly understood. As both Dean Malvick, Extension plant pathologist in the Department of Crop Sciences, and I have indicated in past issues of the Bulletin, the situation in Illinois is uncertain. We know that bean leaf beetles transmit the virus in Illinois, but we do not know the extent of the resulting infection in field situations, and we don't know the extent of the problem, if there is a problem.
Rather than paraphrase what the entomologists at Iowa State University are telling their growers about bean leaf beetles and bean pod mottle virus, I direct you to three articles published recently in their Integrated Crop Management newsletter--http://www.ipm.iastate.edu/ipm/icm/. The articles titled "Recent bean leaf beetle and bean pod mottle virus research," "Bean leaf beetles and soybean planting date," and "Management decisions for bean leaf beetles and bean pod mottle virus" summarize what the folks at Iowa State University know at the present time. Although we can't say for certain that their management recommendations are relevant in Illinois, soybean growers certainly are interested in learning more. We intend to embark on some of our own research to further investigate this issue in Illinois. In the meantime, we'll continue to relay important developments in both research (in Illinois or elsewhere) and field observations. As always, stay tuned.--Kevin Steffey