No. 8 Article 1/May 25, 2012

Bean Leaf Beetle Thresholds: Where Do They Come From?

In a recent article (issue 6, May 10), I mentioned that densities of bean leaf beetles typically must reach 39 or more per foot of row in soybeans at V2+ stages for economic damage to occur. In essence, large numbers of beetles are generally required in the spring to warrant a rescue treatment. Early-planted fields are often the most at risk following mild winters. As more fields are planted, the bean leaf beetle population tends to spread out across the landscape, lessening infestation levels in any particular field.

So where did this threshold guideline come from? The paper "Bean Leaf Beetle Injury to Seedling Soybean: Consumption, Effects of Leaf Expansion, and Economic Injury Levels" was published in 1995 (Agronomy Journal, Vol. 87, pp. 183-188) by some University of Nebraska entomologists (Thomas E. Hunt, Leon G. Higley, and John F. Witkowski). They conducted some very thorough research that led to quantifying the precise level of soybean foliage that bean leaf beetles consume, a necessary component in calculating an economic injury level. The researchers determined that a single bean leaf beetle could eat 13.93 cm2 of leaf tissue over 14 days.

Other factors go into determining an economic injury level, such as market value of the crop (dollars per bushel), the cost of a potential rescue treatment (dollars per acre), and damage (actual estimated yield loss that occurs from insect feeding). The economic variables (value of the crop and cost to treat) have obviously changed since the Nebraska paper was published, but the principles still apply with respect to calculating an economic injury level.

For V1-stage soybeans at a market value of $7.50 per bushel and a treatment cost of $11 per acre, the authors estimated the economic injury level to be 6.08 beetles per plant. With current market prices being much higher, the economic injury level would be reduced significantly from this level, perhaps to 3 or 4 beetles per plant. Also, the economic threshold is set below the economic injury level, allowing for the timely application of a rescue treatment as needed. This being the case, an estimated threshold of 2 to 3 beetles per plant seems reasonable. For 30-inch-row beans (150,000 plants per acre) with 8.6 plants per foot of row, this estimated threshold equals 17.2 to 25.8 beetles per foot of row.

Even though this adjusted threshold is lower than the threshold suggested in 1995, many beetles are still required to warrant a rescue treatment. Producers are encouraged to continue scouting soybean fields to determine the necessity of a rescue treatment. Another factor that will lessen the likelihood of economic losses caused by bean leaf beetle feeding on seedling soybeans is insecticidal seed treatments, more commonly used today than in the mid-1990s, when the Nebraska research was conducted.

Through mid-May, some areas of central and southern Illinois were characterized by the USDA as "abnormally dry" (droughtmonitor.unl.edu). The forecast for the Memorial Day holiday period is for temperatures in the mid- to upper 90s throughout much of the state. Keep a watchful eye out for the twospotted spider mite, whose most significant outbreak occurred in 1988. A quick look back at some issues of the Bulletin revealed that by early June, infestations were beginning to occur in clover, soybeans, and even corn. Because we are well ahead of schedule this spring regarding the occurrence of several pests, twospotted spider mites should definitely be scouted for, particularly if hot and dry weather continues. Let's hope we don't have issues with this pest in 2012.--Mike Gray

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