The bean leaf beetle (BLB) Cerotoma trifurcata (Forster) is a chrysomelid leaf beetle that varies in color from golden brown to green, generally has 4 black spots on the wing covers, and always has a black triangle on the area centrally behind the thorax. Larvae develop below ground and can be found feeding on soybean nodules. BLB is distributed throughout Ohio, but BLB population activity tends to be heavier in areas of concentrated soybean production.
Bean Leaf Beetle
Early planted soybeans and especially the first fields to emerge in an area may exhibit seedling injury due to feeding by overwintering adult BLB populations. Foliar injury due to adult BLB feeding will again appear in late June and continue until fall as a succession of 1st and 2nd generation BLB adults emerge and feed on the crop.
Feeding injury by the larval stage on the root nodules may be detected, but such injury is not regarded as economic.
When pod set occurs, BLB adults will feed on both the foliage and the succulent pods - especially the pods in the upper canopy. As soybean foliage begins to turn dry and discolor in late August or early September, pod feeding may intensify as the pods become more succulent than the foliage.
Pod injury caused by BLB enables infection of the developing bean seeds by micro-organisms leading to moldy beans that (1) may exhibit incomplete development, (2) stick to the pod and are lost during harvest, or (3) appear in the final harvest causing dockage at the time of sale.
Recognition of stages of pod injury is necessary to determine if BLB pod feeding has just begun or peaked. Fresh BLB pod injury initially appears green. After a couple days the scars appear white. Old BLB scars on the pod appear dry and brown. The relative number of fresh vs. old scars provides an indication of whether BLB pod feeding activity is currently an increasing problem.
As the level of pod injury increases, the probability for multiple feeding scars increases and the potential for infection also increases. The relationships found between percent pod injury and percent damaged (discolored & moldy) and percent moldy seeds from surveys conducted in 1988 and 1991 are illustrated in Table 1.
|Table 1. Seed damage expected from levels of pod injury based on regression analysis of 1988 and 1991 survey observations.|
|(a) Lower levels of moldy seeds in 1991 were presumably due to dryer field conditions.|
The incidence of moldy and discolored seeds increases as the level of pod injury increases. As the level of moldy seed development increases, the potential for yield reduction may become significant especially if the moldy seeds adhere to the pods during the harvesting process. The presence of discolored seeds may not affect yield, but a loss in seed quality may result in dockage of price at the time of marketing. It should be noted that the relative proportion of seeds developing mold in 1991 was less than that of 1988 due to dryer weather conditions prevailing in the late summer and fall months of 1991.
The bean leaf beetle overwinters in the adult stage, which resumes activity in the spring and can be found feeding on soybean foliage soon after emergence. The overwintering BLB adults feed on foliage and deposit their eggs in the soil. If a soybean field is late planted (after June 1st), the first generation may not become established in the field and the probability of early season BLB damage is minimal.
The BLB larval stage feeds on the root system and after a few weeks the larvae pupate and adults emerge to feed on the soybean foliage. During the growing season, BLB passes through 2 generations with the 1st generation of BLB beetles appearing in early summer and the 2nd generation appearing around late August or early September (see Figure 1). The time of peak occurrence of BLB adults per generation may differ from field to field depending on the date of planting, since the time of initial egg laying in a field depends on the time of initial emergence of the crop which attracts the overwintering beetles to the site.
Late planted fields in which the 1st generation does not develop may be colonized later by migrating 1st or 2nd generation adults. Either late activity of the 1st generation adults or the 2nd generation adults may cause pod injury depending on the timing of BLB development and soybean crop development.
Figure 1. Life Cycle of Bean Leaf Beetle Activity and Occurrence of Foliar and Pod Injury
The severity of BLB on early or late planted soybeans from year to year may vary depending on climatic conditions. Growing seasons characterized by early plantings and above normal summer temperatures exhibit high levels of BLB activity and early appearance of 2nd generation BLB adults which first feed on early plantings and then migrate into later plantings. Growing seasons characterized by late plantings and below normal summer temperatures exhibit low levels of 1st generation BLB activity and late appearance of 2nd generation BLB adults which may cause significant pod injury to late plantings.
When the soybean crop is dry, the beetles leave the crop and overwinter in protected places until the following spring when they resume activity on early green foliage such as weeds and forage crops until soybeans emerge.
Early planted soybeans may attract large numbers of overwintering bean leaf beetles. Periodic inspection of early planted fields is advised especially if the field in question is the first to emerge in the area. A heavily infested field may serve as a trap crop attracting many beetles to a single location and providing an opportunity for early control of a potentially injurious BLB population. Rules and conditions applicable to such a situation are given in Figure 2.
|Figure 2. Rules to determine need for rescue treatment for control of bean leaf beetle following soybean emergence.|
If seedling injury and early defoliation are severe, and stand
loss appears evident, and soybean planting is first to emerge
then consider rescue treatment to protect stand and possibly reduce BLB population via trap cropping.
In late June and July, when the 1st generation of BLB adults begin to appear, periodic sweep net sampling for determination of BLB population abundance should be initiated plus periodic assessment of defoliation injury (see IPM Circular # FC-22). Periodic sweep net sampling will provide a measure of BLB abundance and facilitate an awareness of trends in first and second generations of adult BLB activity. Sweep net sampling should be taken as sets of 10 sweeps at 3 to 5 locations in a field.
Decisions to apply rescue treatment to prevent severe defoliation after bloom and prior to podset should be based primarily on observed defoliation levels, which should not be allowed to exceed 15% during the reproductive stages of soybean development. Since defoliation may be due to one or more insect pest populations (BLB, Japanese beetle, Mexican bean beetle, etc.), periodic sweep net sampling will provide an indication of the presence and relative trend of defoliating insect populations. Rules applicable to determination of the need for rescue treatment prior to pod set are given in Figure 3.
Pod injury due to adult BLB feeding may be detected following pod set. Evaluation of pod injury should be based on inspection of all pods on 10 randomly selected plants. On each plant sampled, count the number of total pods and the number of pods exhibiting pod injury, and then determine the percent pod injury based on the 10 plants inspected. It is important to estimate percent pod injury on inspection of the entire plant. Pod injury tends to be greatest on the upper portion of the soybean plants, and pod injury estimates based on visual inspection of the top canopy of the stand will lead to misleading estimates of pod injury.
|Figure 3. Rules to determine need for rescue treatment for control of bean leaf beetle prior to pod set.|
If defoliation is less than 15% and BLB abundance is less than
2 BLB adults per sweep|
then plan to recheck field again in a week or two.
If BLB abundance is 2 or more adults per sweep, and defoliation
is 15% or more during the reproductive stages of soybean
then consider application of rescue treatment to reduce BLB adult activity and damage potential.
Initial pod injury will appear in the upper canopy and will be equivalent to defoliation levels. When the foliage begins to turn color, the pods may become more succulent than the foliage and the level of pod injury may increase if BLB adults are present in abundance.
The development of pod injury resulting from adult BLB feeding activity is an accumulative process starting with early pod set and terminating with leaf drop when the adult beetles exit the field to locate more favorable feeding sites. Destination of pod injury present will provide an estimate of expected seed damage (see Table 2) that cannot be reversed. Sweep net sampling will provide an estimate of additional pod injury and related seed damage that will occur in the forthcoming week (see Table 3). It is important to note that the period of time remaining to leaf drop is a significant factor affecting pod injury development. For example, collection of 6 BLB adults per sweep one week prior to leaf drop may not cause less yield loss than an average catch of 4 BLB adults per sweep in a soybean stand with 3 weeks remaining until leaf drop.
|Table 2. Seed damage expected from levels of pod injury based on regression analysis of 1988 and 1991 survey observations.|
|60||30.0||18.1||(a) Damage = discolored plus moldy seeds|
|Table 3. Estimated additional soybean pod injury and seed damage anticipated for Forthcoming week from levels of BLB abundance based on average sweep net catch. (a)|
|10||11.3||4.2||2.6||(a) Note: If more than 1 week reamins until leaf drop injury will increase proportional to time remaining and fluctuations in BLB population activity.|
Given the relationship presented between pod and seed injury presented in Tables 1 and the relationship presented between BLB sweep net catch and injury presented in Table 2, assessment of BLB activity during soybean pod development should include the following steps:
Step 1: Inspect all pods on 10 plants and determine the percent of pods exhibiting pod injury.
Step 2: From Table 2, determine the estimated levels of seed damage and moldy seeds that is likely to develop given the level of pod injury found.
Step 3: Sample BLB activity by taking 30 sweeps with a standard sweep net and determine the average BLB catch per sweep.
Step 4: From Table 3, determine the additional level of pod injury, seed damage, and moldy seeds likely to develop during the forthcoming week given BLB catch per sweep.
Step 5: If more than 1 week remains until leaf drop, multiply the predicted additional injury levels by the number of weeks remaining until leaf drop.
Step 6: Add the anticipated seed damage determined from the sweep net catch to predicted seed damage level determined from current level of observed pod injury to estimate total seed damage if no action is taken.
Step 7: If predicted additional seed damage exceeds the cost of corrective treatment or total predicted seed damage exceeds market standards of crop, then rescue treatment may be warranted.
The potential for severe damage to soybeans by BLB depends on the presence of abundant BLB adult populations at a time when pods are susceptible to BLB feeding. Since BLB abundance varies from field to field and the cycles of BLB activity over time depends on the planting and emergence of a given soybean field, field monitoring is needed to determine when BLB populations are most abundant.
Soybean varieties differ in their maturity rate and one variety may be susceptible to BLB populations when another is not in any given point in time. For example, one variety may mature early and be susceptible to late 1st generation BLB pod feeding while another may set pods after 1st generation BLB activity has terminated. In contrast, a late maturing variety may be more vulnerable to BLB injury when 2nd generation BLB activity peaks in the late summer or early fall.
The annual area-wide abundance of BLB on soybeans is related in part to soybean planting dates. Early planting of soybeans enables establishment and development of the 1st generation of BLB which in turn generates the 2nd generation of BLB. In contrast, late planted soybeans supports limited development of 1st generation BLB since the stand may emerge after overwintering BLB adults have concluded egg laying. Therefore, the relative proportion of soybeans planted early in an area may have a significant impact on the establishment and development of BLB populations.
Information on natural factors controlling BLB abundance is limited. Occasionally, soybean fields with significant 1st generation BLB activity may exhibit potential for BLB problems, but severe problems do not develop later in the season. Thus, preventive chemical treatment should be questioned unless a combination of pod injury observations and sweep net catches of adult BLB clearly indicate a potential problem.
Copyright 1993, The Ohio State University
OPMS Circular #FC-23 BLB
Prepared by: Harold R. Willson, Dept. of Entomology
Updated September 1992
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