Entomologists in the midwest recommend including bean leaf beetle in the early season scouting schedule. Injury due to beetle feeding is easily recognized. Photo: Kevin Black, University of Illinois.

BEAN LEAF BEETLE

The bean leaf beetle (Cerotoma trifurcata) is a common soybean pest that feeds on cotyledons, leaves and pods.

Overwintered bean leaf beetles feed on alfalfa and other legumes in the early spring. As soybeans emerge, the beetles move from legumes to soybeans to continue feeding and lay eggs in the soil.

Early planted soybeans and especially the first fields to emerge in an area is where seedling injury due to feeding by overwintering beetles is most likely to occur. Feeding injury is recognized by small round or oval holes in the middle of young leaves or on the leaf edge.

Populations of bean leaf beetles have been increasing in the North Central region — it's thought that back-to-back mild winters have favored the survival of overwintering adults.

Second-generation beetles are most damaging

Overwintered beetles feed on soybean during May and June in the midwest. Overwintered females lay eggs that develop into first-generation beetles which emerge in July. The first- generation population usually peaks in the early reproductive stage of the soybean crop. Feeding by the over-wintering population or the first-generation beetle does not usually cause yield loss.

However, a second population of beetles peaks during the pod-fill stages, and feeding by this population can cause extensive damage to pods, seed yield, and seed quality. Damaged pods are also more susceptible to rotting and discoloration.

Bean leaf beetles can transmit viruses

In addition to the physical injury to the plant, bean leaf beetles are known to transmit the Bean pod mottle virus (BPMV) from plant to plant as it feeds. The increasing numbers of bean leaf beetles in the region is thought to be the cause of a higher incidence in BPMV observed in recent years.

  • Agronomic impact

    Bean leaf beetle

    Damage to pods by second-generation bean leaf beetles can lead to loss of yield and seed quality. Photo: South Dakota State University. Click on image to view enlargement.

    Early feeding
    Bean leaf beetles feed on young, new tissue and can cause noticeable defoliation on seedlings. Fortunately, feeding by first-generation beetles on soybean leaves seldom results in economic yield losses as seedlings can often recover.

    Recommendations developed at the University of Nebraska suggest a density of 16 adults per row foot 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.

    Transmission of viruses by first generation beetles
    Transmission of soybean viruses is a separate issue. Early feeding by the overwintered and first-generation beetles in early (May) and mid-season (June-July) can cause the initial introduction and spread of Bean pod mottle virus into soybean fields.

    Feeding by second-generation beetles can reduce yield and quality
    Feeding by second-generation bean leaf beetles in August is not important in the spread of soybean viruses, but can cause significant damage to seed yield and quality. Complete pod loss can occur when adults feed at the base of the pod. This type of injury is referred to as "pod clipping." Feeding on the outer pod wall leads to pod lesions and seed damage.

    Damaged pods are predisposed to secondary infection by bacteria and fungi which may cause rotting and discoloration. Infections may increase in severity during periods of cool and wet weather.

  • Life cycle

    Overwintering adults feed on emerging seedlings. Their eggs become the first generation of bean leaf beetles - Photo: Kevin Black

    Overwintering population
    Adult beetles overwinter under leaf debris adjacent or near soybean fields. Once they become active in the spring they will feed on wild legumes, alfalfa and clovers.

    When soybeans emerge they quickly leave these alternate hosts and concentrate on soybean. They feed on emerging seedlings and deposit eggs in the soil near the plants. These overwintering adults generate the first generation of bean leaf beetles.

    First and second generation
    In addition to the overwintering adults, two generations of bean leaf beetles occur in the North Central region. The first adult generation usually peaks in the late vegetative or the early reproductive soybean stages, whereas the second generation peaks during the pod-fill stage. In mid- and southern parts of the region, this is usually mid-July for the first generation and late-August or early September for the second generation.

    The adults eventually leave soybean and feed in alfalfa and other legume hosts before seeking overwintering sites under crop residue and leaf debris in fence rows and wood lots.

  • Scouting

    Bean Leaf Beetle
    Note the prominent black triangle at the front of the wing covers. This is the most reliable diagnostic feature of the bean leaf beetle. The marking is always present and it distinguishes the bean leaf beetle from other beetles in soybean - Photo: ISU Entomology Image Gallery

    Also see: Bean leaf beetles and others: correctly identifying the pest, Iowa State University

    Start scouting early in the season. Kevin Steffey, entomologist at the University of Illinois, recommends sharpening your scouting skills for bean leaf beetles in seedling soybeans. Experienced scouts have learned that bean leaf beetles tend to "play dead" and drop from seedling soybeans to the ground when they are disturbed. So approach your sampling area quietly.

    Examining plants by hand is the easiest method for seedling soybeans. As the plants grow, use of a beat cloth may help you assess numbers of bean leaf beetles per foot of row. Remember to scout in several areas of a given field to obtain a reasonable 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 and spend more money than necessary if the field average of bean leaf beetles is lower than economic thresholds.

    Drop cloth method

    • Walk 100 feet in from the field edge and scout each field and each variety separately.
    • Place a 3-foot wide strip of cloth on ground between the rows.
    • Bend the plants on one row over the cloth, and shake them vigorously.
    • Count the number of beetles on the cloth.
    • Repeat the procedure four times for each 20 acres of the field.
    • Determine the average number of beetles per 3-foot of row.
    • If the number of beetles is below the economic threshold, sample your fields again the following week, or a third week if necessary.

    Sweep net method

    • Walk 100 feet in from the field edge and scout each field and each variety separately.
    • Take 20 sweeps.
    • Repeat the procedure four times for each 20 acres of the field.
    • Determine the average number of beetles per 20 sweeps.
    • If the number of beetles is below the economic threshold, sample your fields again on following week, or a third week if necessary.
    • For suggestions on sweep net equipment, read If you scout soybeans, you need a net in the ICM newsletter.

    Adapted from
    New Concept for Bean Leaf Beetle Management
    Wai-Ki F. Lam, Marlin Rice, Larry Pedigo, and Rich Pope
    Integrated Crop Management Newsletter , Iowa State University

  • Distribution

    Entomologists in the midwest recommend including bean leaf beetle in the early season scouting schedule. Injury due to beetle feeding is easily recognized.

    The bean leaf beetle is native to the U.S. and is widely distributed in soybean-growing regions of the U.S., including the north central region.

    High numbers of beetles in recent years have been attributed to milder winters or adequate snow cover that insulated and protected overwintering adult populations.

    The beetle is an occasional pest of snap beans, clover, dry edible beans, and several leguminous weeds

  • Management

    Second generation bean leaf beetles feeding on pods. Feeding at this growth stage can cause loss of yield and seed quality - Photo: South Dakota State University - Click on image to view larger version

    Sampling for beetle thresholds
    Feeding by the second generation beetle at the beginning of pod growth and development has the greatest potential for yield and quality loss in the crop.

    Therefore, researchers at Iowa State University have developed a research-based management concept based on sampling the first generation of beetles to predict the size of the second generation.

    If first generation numbers reach densities that indicate the second generation may cause economic losses, treatments can be made in August as soon as the second-generation beetles are found. This prevents the situation of the second generation beetles feeding for several weeks before economic levels are reached.

    The model is based on degree days and is described in detail in the following article:

    Predicting first-generation bean leaf beetles
    Integrated Crop Management Newsletter, Iowa State University. (2004 but still current)

    Early-season management decisions for management of just the beetle, or for beetle and virus interactions, are outlined in the following two articles:

    Bean leaf beetle and bean pod mottle virus management: An integrated approach. Integrated Crop Management Newsletter (2005 - but still current),

    Revisiting an integrated approach to bean leaf beetle and bean pod mottle virus management
    Integrated Crop Management Newsletter (2007 - but still currrent).

    Soybean varieties
    Soybean varieties differ in their maturity rate and may be more or less susceptible to bean leaf beetle feeding.

    An early-maturing variety may be susceptible to late first generation pod feeding while another may set pods after first generation activity has peaked. Similarly, a late-maturing variety may be more vulnerable to bean leaf beetle injury when the second generation peaks in the late summer or early fall.

    Early planting can impact beetle populations
    The relative proportion of soybeans planted early in an area may have a significant impact on the establishment and development of bean leaf beetle populations.

    Fields planted earliest in an area are most at risk for high populations of the beetle. First-generation populations are limited in late-planted soybeans if the stand emerges after the overwintering adults have concluded egg-laying.

    Managing beetles for virus control
    A separate issue with bean leaf beetles is the physiological stress caused by transmission of the bean pod mottle virus (BPMV). Although bean leaf beetles transmit BPMV all season long, soybeans are most affected when plants are infected in the seedling stage.

    Researchers at Iowa State University have constructed a flowchart to help growers with insect/virus management decisions. The flowchart is a dynamic, two-pronged decision guide based on field history. If bean pod mottle virus has not been present in a soybean field, then the only reason to consider early-season bean leaf beetle management is if populations reach extremely high levels (more than 2.5 beetles per plant). Insecticide treatment will not be cost-effective if the virus is not present.
    » view flowchart (pdf)

    Right now, most of the information for predicting economically important variables for beetle feeding damage is available. Much more information is still needed regarding these variables and bean pod mottle virus. As more research is conducted, the results can be added to the decision guide.

    View the entire article at:
    Revisiting an integrated approach to bean leaf beetle and bean pod mottle virus management
    ISU Integrated Crop Management Newsletter

  • Resources

  • Photo Gallery

    Bean leaf beetle on seedling. University of Illinois