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Bean Leaf Beetles Are Active

May 3, 2002
Several entomologists have reported finding bean leaf beetles recently active in alfalfa and clover fields and in noncrop areas. Although very few soybeans have been planted yet this year, any early-planted fields should be monitored very carefully as soon as plants emerge. These early fields will be "magnets" for bean leaf beetles.

Bean leaf beetles overwinter as adults, becoming active in the spring soon after the temperature rises above 50 to 55 deg F. They fly first to alfalfa and clover fields where males and females mate. However, the females do not lay eggs in alfalfa and clover. As soon as soybean seedlings emerge, the beetles abandon the forage fields and colonize soybean fields. They feed on emerging seedlings and deposit eggs in the soil near the plants.

Because bean leaf beetle adults overwinter above ground, their survival through the winter hinges on winter temperatures. Research and Extension entomologists at Iowa State University have accumulated some really good stuff regarding survival (or mortality) of bean leaf beetles through the winter. Broadly stated, bean leaf beetle mortality is less during mild winters than during winters with plenty of cold temperatures. The entomologists at Iowa State University have compiled a table of bean leaf beetle mortality, from 1989 through 2001 ( icm/2001/5-7-2001/blbsurvival.html). In the article, they explain how they developed the model to predict mortality of bean leaf beetles. Average beetle mortality in central Iowa during this period was 71%. Average beetle mortality in central Iowa from the 2001-2002 winter was only 48% ( icm/2002/4-29-2002/blbwinter.html). If the information from Iowa is any indication of survival of bean leaf beetles in Illinois this year, we'd better be ready.

Bean leaf beetles are about 1/4 inch long, with considerable variation in color pattern. Most bean leaf beetles are light yellow to tan, but they may be red. Wing covers may have four main black spots and stripes, but these markings may be absent. However, a black triangle is always present behind the prothorax ("neck").

Bean leaf beetle adults exhibiting color and pattern variations.

If large numbers of bean leaf beetles feed on newly emerged soybean seedlings, the damage can be severe. Feeding on VE (plant emergence) and VC (expansion of the unifoliolate leaves) can result in plant death and reduced stands. As soon as the first trifoliolate emerges, the soybean plant is better equipped to compensate for injury to the foliage. Nevertheless, large numbers of bean leaf beetles can overwhelm small soybean plants.

Bean leaf beetle injury to early trifoliolate leaves.

Economic thresholds for bean leaf beetles are much higher now than they were a few years ago, based on research conducted at the University of Nebraska. Densities of 16 per foot of row in the early seedling stage or 39 per foot of row at stage V2+ are necessary before economic losses occur. I'll offer a list of insecticides suggested for control of bean leaf beetles in soybeans in a forthcoming issue of the Bulletin.

Much of the concern about bean leaf beetles in recent years has resulted from the discovery that the beetles can transmit bean pod mottle virus. However, although research regarding the disease and the insect is being conducted in several midwestern states, definitive results have yet to be determined. Following is information taken verbatim from the May 14, 2001, issue of Iowa State University's Integrated Crop Management:

"However, we do not know what percentage of beetles carry the living virus in their gut when they emerged. The virus may not overwinter very well inside the beetle. Studies conducted by Craig Grau, plant pathologist at University of Wisconsin, suggest that alfalfa may harbor bean pod mottle virus so it would be relatively easy for beetles to acquire the virus from alfalfa before moving to soybean.

"The rise of bean leaf beetle populations and bean pod mottle virus problems during the past 2-3 years also strongly suggests that we may need to manage our early-season bean leaf beetle population much differently than we did in the 1990s. We are researching the bean leaf beetle-bean pod mottle virus problem but we have few solid answers regarding management at present. However, some soybean fields, especially very early emerging fields, may benefit from an early-season insecticide application during the VC-V2 stages to control bean leaf beetles, which would then help reduce viral infection."

There's still a lot to learn about bean leaf beetles and bean pod mottle virus. Let's keep this in perspective.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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