Soybean Stem Borer
The bean leaf beetle (Cerotoma trifurcata) is a common soybean pest that feeds on cotyledons, leaves and pods.
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. 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.
Populations of bean leaf beetles have been increasing in the North Central region — back-to-back mild winters may be favoring the survival of overwintering adults.
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.
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.
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 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.
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 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.
Entomologists in the midwest recommend including bean leaf beetle in the early season scouting schedule. Examining plants by hand is the easiest method for seedling soybeans. By midseason, either sweep nets or drop cloths are effective sampling tools. When disturbed, adult bean leaf beetles will drop to the ground or onto lower leaves. 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.
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
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.
For guidance on scouting thresholds and treatment recommendations, see also
Bean Leaf Beetle – Ohio State University
Bean Leaf Beetle – University of Missouri
Bean Leaf Beetle – Northern Plains IPM Guide
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.