Bean Leaf Beetles: Expectations for This Growing Season

May 12, 2000
Bean leaf beetle adults are being commonly reported throughout Illinois. Chuck Osmond, Dose Fertilizer, reported that soybean fields on May 6 supported "heavy" densities of bean leaf beetles in Marshall County. As seedling soybeans begin to emerge, don't be surprised if bean leaf beetle adults "swarm" into early-planted fields to dine on tender plants. The following questions and answers are intended to provide a brief review of the life cycle of bean leaf beetles and also to offer some insights on management strategies.

What's the best way to correctly identify bean leaf beetle adults?

Bean leaf beetle adults are about 1/4 inch long with considerable variation in color. The background color of most bean leaf beetles is light yellow to tan; however, some bean leaf beetles are green, and others are red. Their wing covers usually have four main black spots and stripes along the edges, but these markings may be absent. A black triangle is always present behind the "neck" region.

Bean leaf beetle adults (color variations).

Bean leaf beetle injury to soybean seedlings (two plants).

Where do bean leaf beetles spend the winter?

Bean leaf beetle adults overwinter throughout the Midwest primarily beneath leaf litter in woodlots surrounding soybean fields. Dr. Larry Pedigo, professor of entomology, Iowa State University, reports that approximately 80% of adults spend the winter in wooded areas, 20% beneath soybean residue in fields, and fewer than 1% in protected areas of alfalfa stands, cornfields, and other grassy areas. Bean leaf beetle adults become active in April and are commonly observed in stands of alfalfa. While in alfalfa, some feeding and egg laying takes place; however, bean leaf beetles are not considered to be of any economic importance to this crop.

When do bean leaf beetle adults abandon alfalfa?

As the first cutting of alfalfa is taking place and the earliest-planted soybean fields begin to emerge, bean leaf beetle adults leave alfalfa and colonize soybeans. Females that fly to soybean fields have typically mated already and have abdomens full of eggs. Females are capable of laying between 130 and 200 eggs in the upper 1 to 2 inches of the soil near soybean plants. The eggs hatch in about 1 week, and larvae feed on the roots and nodules of soybean plants. The larvae closely resemble corn rootworm grubs. This shouldn't be too much of a surprise since they belong to the same family of beetles (Chrysomelidae). Many folks the past few seasons have mistakenly assumed that corn rootworm larvae were feeding on the roots of soybean plants. Although we know that western corn rootworm adults can feed on the leaves of soybean plants in east-central Illinois, the larvae are unable to utilize soybean roots as a food source.

Are bean leaf beetle larvae considered to be of any economic importance?

No, at least not directly. While feeding on roots and nodules, bean leaf beetle larvae molt three times during a 15- to 30-day period. The pupal stage requires an additional week, after which the second "flush" of adults begins to emerge in July.

How many generations of bean leaf beetles occur in Illinois?

In Illinois, we have two generations annually. The bean leaf beetle adults that emerge in July feed on soybean foliage, mate, and lay eggs to begin a second generation. Some entomologists consider the larvae of the second generation to have the most effect on soybean nodules. Adults of the second generation are common in late summer, densities peaking from late August to mid-September. In the north-central region of the United States, second-generation bean leaf adults do not mate in late summer or early fall. Adults of this generation feed on tender soybean leaves and pods and by early fall return to alfalfa. As cool fall temperatures become more common, adults begin to seek out suitable overwintering sites.

What effect will the mild winter have on bean leaf beetle infestations this spring?

Research conducted by Drs. Wai-Ki F. Lam and Pedigo, Iowa State University, suggests that this spring will most likely yield impressive densities of bean leaf beetles. These researchers suggest that for central Iowa, survival of adults through the winter should be approximately 60%. This survival is greater than the previous 11 winters. So bean leaf beetles are likely to "come out swinging" this spring. Producers should keep a watchful eye out for those earliest-planted soybean fields.

What are the suggested economic thresholds for bean leaf beetle adults on seedling soybeans?

We suggest that an insecticide treatment for seedling soybeans is rarely justified. 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 begin to accrue. Although rescue treatments are typically not needed for seedling soybeans, the following products are labeled for use: *Ambush (3.2 to 6.4 oz), *Asana XL (5.8 to 9.6 oz), dimethoate (see product label), Lorsban 4E (1 to 2 pt), *Penncap-M (2 to 3 pt), *Pounce 3.2EC (2 to 4 oz), Sevin XLR Plus (1/2 to 1 qt), and *Warrior T or 1E (1.92 to 3.2 oz). (Products that are preceded by an asterisk are restricted for use to certified applicators.)

Do bean leaf beetles vector the bean pod mottle virus?

Dr. Marlin Rice, Extension entomologist, Iowa State University, reported that the bean pod mottle virus was confirmed in several central and western Iowa counties. To date, the virus has primarily been of most concern to soybean producers in the southern United States. Soybean plants that are infected with the virus may have a mottled appearance and crinkled leaves, and some may be stunted. Plants with these symptoms may be misdiagnosed as injured by herbicide drift or infected with soybean mosaic virus. Marlin indicates that bean leaf beetles are considered the most important vector of bean pod mottle virus in Iowa. However, he indicates that uncertainty remains regarding when the virus is vectored to soybean plants by bean leaf beetle adults. Apparently, earlier infections are associated with the greatest potential yield reductions. Until more is learned about the relationship between bean leaf beetles and transmission of this virus, we cannot offer firm management recommendations.--Mike Gray and Kevin Steffey

Author: Kevin Steffey Mike Gray