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 Bean Leaf Beetles are Active, Scout Emerging Soybeans Minimize
Location: BlogsWisconsin Crop ManagerInsects and Mites    
Posted by: WCM Staff 5/24/2007 12:21 PM
Bean leaf beetles have begun to emerge in the region. Bean leaf beetles overwinter as adults beneath leaf litter. In early spring, adults feed on cultivated (alfalfa) and wild (e.g. showy tick trefoil) legumes. As soybean seedlings emerge, beetles move into soybean fields to continue feeding and lay eggs.

Bean Leaf Beetles are Active, Scout Emerging Soybeans

Eileen Cullen, Extension Entomologist, UW-Madison Entomology Department


Bean leaf beetles have begun to emerge in the region. Bean leaf beetles overwinter as adults beneath leaf litter. In early spring, adults feed on cultivated (alfalfa) and wild (e.g. showy tick trefoil) legumes. As soybean seedlings emerge, beetles move into soybean fields to continue feeding and lay eggs.
 
Marlin Rice, Iowa State University Field Crop Extension Entomologist reported in the Iowa ICM newsletter overwintered bean leaf beetle mortality estimates from 88-92% in northern Iowa, to about 60% in southern Iowa http://www.ipm.iastate.edu/ipm/icm/2007/4-30/blbpop.html.
 
In Wisconsin, winter morality rates were calculated by the WI DATCP Pest Survey using two bean leaf beetle winter survival predictive models; the University of Minnesota model and the Iowa State University model.  The models were run for 12 Wisconsin localities including Arlington, Hancock, Spring Green, Beloit, La Crosse, Milwaukee, Boscobel, Green Bay, Madison, Eau Claire, Wausau and Rhinelander. Winter mortality estimates ranged from 36-60% using the UMN model, and 60-100% using the ISU model (Wisconsin Pest Bulletin 52(5) May 11, 2007 http://pestbulletin.wi.gov/).
 
Once soybeans begin to emerge in an area, bean leaf beetles will quickly leave alfalfa fields and other alternate hosts and concentrate on soybean. Fields planted earliest in an area are most at risk for high populations of the beetle. Concentrate initial scouting activities in these early-planted fields.
 
Bean leaf beetles feed on young, new tissue and can cause noticeable defoliation on seedlings. Fortunately, soybean plants are quite resilient in terms of the amount of defoliation that is tolerated: up to 40 per cent leaf area defoliation in the vegetative stages. Therefore, feeding by overwintered and first-generation beetles on soybean leaves seldom results in economic yield losses because the seedlings often recover. Monitor bean leaf beetle populations and defoliation levels, and make sure the cotyledons and growing point are still intact and developing well. Adults of the first generation of bean leaf beetles will show up in the summer after the current batch of beetles lays its eggs and the insects develop through larval and pupal stages. (Bean leaf beetle larval development on soybean roots is not of economic concern).
 
Bean leaf beetles tend to drop quickly to the ground from seedling soybeans when disturbed. Approach plants carefully and look on the soil around seedlings for beetles that may have dropped off the plant. Count these beetles in your sample number as well as those found on the plant. Examining plants by hand is the easiest method for seedling soybeans.
 
Remember to scout in several areas of a given field to obtain a representative assessment of bean leaf beetle density for the field. A few high counts are offset when several areas of a field are not infested, lowering the average number of beetles per plant. There's no need to treat an entire field if the field average of bean leaf beetles is lower than published thresholds.
 
University of Nebraska recommendations suggest a density of 16 adults per foot of row in the early seedling stage before economic injury from physical feeding will occur. By the V2 stage, 39 beetles per row foot are required before economic injury will occur. Examine 5 feet of row in each of five locations throughout the field. Divide the total number of beetles found by 25 to estimate the number per foot of row.
 
Alternatively, you can calculate the average number of beetles/plant from the five sampled areas in the field. ISU entomologist Marlin Rice has developed a table of dynamic early-season bean leaf beetle economic thresholds in soybean that incorporate crop value and treatment cost. Between VC and V2 growth stages, thresholds range between 2.0 and 10.7 beetles/plant depending on crop growth stage, crop value and insecticide treatment cost. Consult table 3-5 on p. 115 of UWEX Publication A3646 Pest Management in Wisconsin Field Crops to match your scouting information with numbers in the ISU table. Beetles/plant may be multiplied by 7.6 to obtain beetles per foot of row. More information on bean leaf beetle and insecticides registered for their control can also be found in UWEX Publication A3646, See Table 3-71, p. 121.
 
Bean leaf beetles are capable of transmitting a soybean virus called bean pod mottle virus (BPMV). While BPMV transmission can occur at any stage, early infection in the VC to V2 stage poses the greatest risk of yield loss. Field history of virus problems is an important indicator of whether BPMV may present a problem due to bean leaf beetle feeding.
 
Researchers at Iowa State University have constructed a flowchart to help growers with insect/virus management decisions. To view the chart in the article titled “Revisiting an integrated approach to bean leaf beetle and bean pod mottle virus management” please visit the ISU Integrated Crop Management newsletter, March 26, 2007 issue at http://www.ipm.iastate.edu/ipm/icm/2007/3-26/beanleafbeetle.html 
 
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