Bean Leaf Beetle Management in Soybeans

By Steve Butzen

Summary

  • Due to recent mild winters, bean leaf beetle (BLB) populations in soybeans have increased dramatically, resulting in increased leaf and pod feeding and virus incidence.
  • Just-emerged soybeans are at risk to significant BLB feeding damage when beetle populations are high, especially when planted early and emerging first in an area.
  • During vegetative growth from the V2 stage to flowering, soybeans can tolerate from 40% to 60% defoliation without yield loss.
  • The second generation of beetles usually peaks during soybean pod-fill stages, resulting in "clipped" or damaged pods. Significant yield loss can occur at this time (usually during August in Midwestern states).
  • Scouting regularly for BLB and treating if necessary are recommended to address this problem insect. A new strategy to treat second generation beetles based on first generation beetle numbers has been proposed.
  • This Crop Insights will discuss the bean leaf beetle, including identification, life cycle, plant damage, relationship to viruses, scouting and management.

Introduction

Due to extremely mild winters for several years, bean leaf beetle (BLB) populations have increased dramatically in many soybean growing areas, particularly in the Midwest U.S. The result has been increased leaf defoliation and pod damage to the soybean crop, and a much higher incidence of soybean viruses in affected areas.

Higher Winter Survival Leads to Higher Infestations

Because bean leaf beetles overwinter above ground, their survival is highly dependent on winter temperatures. Researchers at Iowa State University have determined that winter survival can be predicted by a model that uses accumulated daily average subfreezing temperatures from Oct. 1 to April 15 (Lam and Pedigo, 2001). This model shows that beetle mortality averages about 70% in Iowa over the long term, but has averaged less than 60% the last 5 years. Mortality in the winter of 2001-2 averaged only 48% (Rice and Pope, 2002). Many other Midwestern states with similar winter temperature patterns would expect the same effect on BLB populations. This helps explains the high levels of bean leaf beetles the last few seasons in the major soybean growing areas of the Midwest United States.

Bean Leaf Beetle Identification and Life Cycle

BLB adults are just over 1/4 inch in length, and are variable in color (yellow to tan or red) and markings. Most often, the wing covers have four black spots and distinct black margins, but these are not always present. However, all bean leaf beetles have a black triangle (pointing backward) just behind the head. The most common color is yellow. BLB larvae, white segmented grubs with a brown head and an anal shield, may reach 1/3 inch in length.

Bean leaf beetle adults are 1/4-inch in length Bean leaf beetle coloration may vary
Photo on left: Bean leaf beetle adults are 1/4-inch in length. Photo on right: Bean leaf beetle coloration may very from yellow to tan or red.

Bean leaf beetles overwinter as adults beneath plant debris in woods, grassy areas and cropland near soybean fields. When spring temperatures reach 50-55°F., adults become active and seek available host plants for feeding, such as alfalfa, clover and certain grasses or weeds. When soybeans emerge, these colonizing populations leave the alternate hosts and move to soybean fields to feed and lay eggs. Eggs are deposited in the top two inches of soil near soybean plants over a three to four week period. Egg hatch may take one to three weeks, depending on the soil temperature.

BLB larvae remain in the soil, feeding on soybean root hairs and then nodules. The effect of this feeding is largely unknown, but is generally considered non-economic. The length of the larval stage is dependent on temperature, ranging from two weeks at 75°F. to 7 weeks at 65°F.

Pupation occurs in earthen cells below the soil surface. Adults, comprising the first in-season generation, emerge about a week later, usually in July in Midwestern states. As this cycle is repeated, a second in-season generation of beetles will emerge in late August or September. The beetles move from soybean fields to alternate hosts as soybeans mature, and eventually move to overwintering sites later in the fall.

Feeding Damage to Soybeans

Bean leaf beetles possess chewing mouth parts and feed on soybean plants at all stages of crop development. When overwintering populations are high, just-emerged soybean stands can be economically damaged by beetle feeding on cotyledons and leaves. The next-generation beetles emerge in July and feed primarily on soybean leaves. The final generation of beetles emerges in late August or September and feeds on leaves and pods.

Leaf feeding by the bean leaf beetle can be identified by small round holes between the veins. Although leaf feeding is damaging to plants, soybeans can withstand a surprising amount of defoliation without incurring economic losses. In the vegetative stages, soybeans can usually sustain 50% leaf area loss or more without appreciable yield reductions.

Although bean leaf beetles do not feed directly on soybean seeds, they reduce soybean seed yield and quality by feeding on pods. Entire pods may be "clipped" when feeding occurs at the base of the pod. Beetles also consume the outside layer of pod tissue, leaving a thin layer still covering the seed. Moisture and diseases can enter the pod through this lesion. Secondary infection by fungal pathogens such as Alternaria results in shrunken, discolored and moldy seeds.

The Virus Connection

The bean leaf beetle is also a vector of several soybean viruses, including yellow cowpea mosaic, cowpea chlorotic mottle, southern bean mosaic, and bean pod mottle virus (BPMV). Bean pod mottle has been identified at increasingly high levels in Illinois, Iowa and other major soybean-producing states in the last few years. Bean pod mottle virus can reduce yields by about 10-15%, and by much more when other viruses also occur with it.

BPMV causes mottling and distortion of the upper soybean leaves. The crinkled leaves and stunted plants can resemble injury from herbicide drift or soybean mosaic virus. Death of new terminal leaf growth may also be exhibited. BPMV also gives rise to "green stem" symptoms in some soybean plants. Affected plants do not mature normally, and stems remain green throughout the harvest period. (However, factors other than virus are implicated in green stem syndrome as well.)

Bean pod mottle virus may also affect the seed, causing a light purplish discoloration of the seed coat. Seed mottling may also occur, resulting from pigments diffusing from the hilum of the seed.

Feeding damage to hypocotyl. Feeding damage to cotyledons.
Photo on left: Bean leaf beetle feeding damage to soybean hypocotyl. Photo on right: Bean leaf beetle feeding damage to soybean cotyledons.

Scouting and Management for Feeding Damage

Bean leaf beetles are present throughout the soybean growing season, so all crop stages from emergence to R7 are exposed to feeding. In addition, the beetles are known vectors of viruses, including BPMV. This management section will focus on feeding damage only, and virus control will be addressed at the end.

Emerging Soybeans

Just-emerged soybeans are at risk to significant BLB feeding damage when beetle populations are high, especially when planted early and emerging first in an area. The period from emergence through establishment of the first trifoliolate leaf is one of the most critical for soybean damage. If the cotyledons (seed leaves) are destroyed before the unifoliolate leaves fully emerge, or if the growing point is severely damaged, stands and yields may be reduced.

Scouting of bean leaf beetles on just-emerged soybeans is by direct observation, as beetles are easy to see and count at this stage. Each state has developed its own treatment thresholds for BLB feeding at various stages of crop development. Several states' recommendations for VC and V1 stage soybeans are shown below:

Table 1. Economic thresholds (beetles per plant) for bean leaf beetles in the VC stage of soybean development, by state.
State Crop Value
($/bu)
Pest Management cost ($/acre)
  $6 $8 $10 $12
NE $5 3 4 4 6
IA $5 2.4 3.2 4.0 4.8
IN $5 3 4 5 6

 

Table 2. Economic thresholds (beetles per plant) for bean leaf beetles in V1 stage of soybean development, by state.
State Crop Value
($/bu)
Pest Management cost ($/acre)
  $6 $8 $10 $12
NE $5 4 5 7 8
IA $5 3.7 5.0 6.2 7.4
IN $5 3 4 5 6


Other states' management recommendations can be found in their university and extension publications or on websites, or can be obtained by contacting your extension entomology specialist. Insecticides labeled for management of bean leaf beetles in soybeans are listed in Table 3.

Soybeans in Vegetative Stages

Once the trifoliolate leaves have unrolled, soybeans can tolerate from 40% to 60% defoliation without yield loss. University recommendations vary during vegetative development. For example, University of Illinois thresholds for V2 and later vegetative stages are 39 beetles per foot of row. Iowa State thresholds for V2 soybeans range from 45 to over 80 beetles per foot of row, depending on the cost of treatment.

Scouting may be done by direct observation at V2 or V3, but this method will become impractical as canopy development progresses. At this point, use of a drop cloth or sweep net is necessary. Scouting procedures and treatment thresholds vary by state - check your state's publications, website, or extension entomologist's recommendations.

Soybeans in Reproductive Stages

Both the first and second in-season generation of BLB may feed on soybeans during reproductive development. The first generation populations usually peak in the late vegetative and early reproductive soybean stages. Feeding at this time seldom causes economic losses.

The second generation usually peaks during pod-fill stages, resulting in "clipped" or damaged pods. It is essential to scout fields regularly for BLB at this time. Management decisions are based on beetle densities, which can change rapidly. During times of BLB activity, fields should be scouted every 5-7 days. Counts can be stopped when any of the following conditions apply (Rice, 2000a):

  1. Beetle populations start to decline.
  2. Soybean pods begin to turn yellow (R7 stage).
  3. The field is treated.

For scouting at this time, entomologists recommend using a drop cloth between soybean rows, shaking the soybeans vigorously, and counting the beetles as they hit the cloth. A sweep net can also be used, and is recommended by some entomologists for narrow-row soybeans. When using the sweep net, sweeping technique is important for accurate sampling and valid use of economic threshold tables.

Early planted soybeans damaged by bean leaf beetle feeding.
Bean leaf beetles and feeding damage to early planted soybeans.

States have different sampling techniques and economic thresholds. Some internet publications with information on sampling and treatment of bean leaf beetles are listed below:

Iowa: http://www.ent.iastate.edu/ipm/icm/2000/8-21-2000/lblroof.html
Illinois: www.ipm.uiuc.edu/publications/infosheets/40-blb/blb.html
Kentucky: www.uky.edu/Agriculture/Entomology/entfacts/fldcrops/ef131.htm
Missouri: muextension.missouri.edu/xplor/agguides/pests/g07150.htm
Nebraska: http://www.ianr.unl.edu/pubs/insects/g974.htm

Check with your local Pioneer representative if you need help in downloading these documents. In addition to these internet publications, hard-copy extension publications are available from other states. Insecticides for BLB management are shown in Table 3.

New Control Strategy Proposed

Second generation BLB may feed on pods for several weeks before population densities reach the economic threshold. In such situations, some loss of yield and quality is inevitable prior to insecticide application. A new approach that would attempt to prevent such damage before it occurs has been proposed by Iowa State University (Rice and Pope., 2001). This system is radically different from other management approaches that use economic thresholds. The new concept is to sample first-generation beetle density and use thiinformation to make management decisions regarding the more damaging second generation. This strategy requires the use of degree days from planting, as well as weekly sampling to time a possible insecticide application. The researchers hope that this system may be a good fit for areas where regular scouting by crop consultants is routine. However, other states have not adopted this practice (Also see Lam et al. 2000 and Rice et al 2001 for more information on this new strategy.)

Managing Bean Pod Mottle Virus

Many growers who have had BPMV symptoms in their fields in recent seasons (particularly green stem syndrome) are concerned about controlling this new soybean virus. However, much about the relationship between the bean leaf beetle, BPMV and soybeans remains unknown. It is commonly known that the earlier soybeans are infected, the greater the potential reduction in yield. But researchers are uncertain about whether the virus overwinters in the beetles, and when they transmit the virus to soybeans.

One strategy for controlling BPMV calls for planting soybeans later (mid- to late-May) to deter colonization by BLB. Another idea is to spray early to control first generation BLB and thereby reduce plant contact and virus spread (Krell, et al, 2002). According to the Iowa State researchers who initiated this strategy, the early spraying option should be limited solely to fields with a history of BPMV infection and bean leaf beetles present this year. In fact, this early spraying concept is relatively unproven and is not recommended in some other states such as Illinois (Malvick and Steffey, 2002) and Ohio (Hammond and Eisley, 2002). Research will continue to help clarify control measures in the future.

Insecticides for Management of BLB Adults

Insecticides registered for BLB in soybean are shown in Table 3. Effective insecticides should have good initial knockdown as well as residual control. Growers should also consider the preharvest interval when selecting an insecticide. Some, such as Asana XL, have an interval of 21 days or less, but others have a much longer interval. For example, Warrior is labeled at 45 days, and Ambush and Pounce are currently labeled at 60 days.

Table 3**. Common chemicals labeled for bean leaf beetle in soybean (Krell, et al, 2002.)

Insecticide Amount / acre Harvest interval
Ambush 2EC* 3.2-6.4 ounces 60 days
Asana XL* 5.8-9.6 ounces 21 days
Dimethoate* See label See label
Lorsban 4E* 1-2 pints 28 days
Mustang* 3.0-4.3 ounces 21 days
Penncap-M* 2-3 pints 20 days
Pounce 3.2EC* 2-4 ounces 60 days
Sevin XLR Plus 1-2 pints 0 days
Warrior* 1.92-3.2 ounces 45 days

*Restricted use insecticide.
** Labels may change. Read and follow all current label instructions.

 

Acknowledgements

Picture of yellow BLB courtesy of S.C. White, Kansas State University Photo Gallery.

Red BLB image courtesy of Jim Boersma, Pioneer Hi-Bred.

Picture of BLB feeding damage to hypocotyl and cotyledons courtesy of Kirby Wuethrich, Pioneer Hi-Bred.

Picture of BLB feeding damage courtesy of Jody Gander, Pioneer Hi-Bred.

References

Hammond, R. and B. Eisley. 2002. Bean leaf beetle early season control and soybean viruses. In Crop Observation and Recommendation Network Newsletter, April 15-22 issue. http://corn.osu.edu/archive/2002/apr/02-09.html

Krell, R., M. Rice and L. Pedigo. 2002. Early-season management of bean leaf beetle and bean pod mottle virus. In Integrated Crop Management Newsletter, May. 6, 2002 issue. Iowa State University, Ames, Iowa. http://www.ipm.iastate.edu/ipm/icm/2002/5-6-2002/blbearlyman.html

Lam, W.F. and L.P.Pedigo. 2001. Bean leaf beetle and winter survival. In Integrated Crop Management Newsletter, May 7, 2001 issue. Iowa State University, Ames, Iowa. http://www.ipm.iastate.edu/ipm/icm/2001/5-7-2001/blbsurvival.html

Lam, W.F., M.E. Rice, L.P. Pedigo and R. Pope. 2000. New concept for bean leaf beetle management. In Integrated Crop Management Newsletter, July 10, 2000 issue. Iowa State University, Ames, Iowa. http://www.ipm.iastate.edu/ipm/icm/2000/7-10-2000/newconcept.html

Malvick, D. and K.Steffey. 2002. Bean leaf beetles and bean pod mottle virus (a 2002 management perspective from Illinois). In Pest Management & Crop Development Bulletin, May 17, 2002 issue. University of Illinois at Urbana-Champaign. http://spectre.ag.uiuc.edu/cespubs/pest/articles/200208d.html

Rice, M.E. and R.Pope. 2001. Scout first generation bean leaf beetles now. In Integrated Crop Management Newsletter, July 16, 2001 issue. Iowa State University, Ames, Iowa. http://www.ipm.iastate.edu/ipm/icm/2001/7-16-2001/scoutblb.html

Rice, M.E. and R.Pope. 2002. Bean leaf beetle winter survival. In Integrated Crop Management Newsletter, April 29, 2002 issue. Iowa State University, Ames, Iowa. http://www.ipm.iastate.edu/ipm/icm/2002/4-29-2002/blbwinter.html

Rice, M.E., R.K. Krell, W.F. Lam, and L.P. Pedigo. 2001. New strategies for an old pest: rethinking bean leaf beetle management. Proc. Univ. of Illinois Crop Protect. Tech. Conf. p. 49-55. Champaign-Urbana, IL.

Other Resources

Wedberg, J.L. and C. Grau, Bean leaf beetle, soybean, and soybean viruses. Online pub., U. of Wisconsin, Madison. http://www.plantpath.wisc.edu/soyhealth/blbeetle.htm


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