UNL CropWatch April 16, 2010: Scout Early Emerging Soybeans for Bean Leaf Beetles

UNL CropWatch April 16, 2010: Scout Early Emerging Soybeans for Bean Leaf Beetles

April 16, 2010

Closeup photo of a bean leaf beetle on a soybean plant.

Bean leaf beetle

There is no sure way to accurately predict spring bean leaf beetle infestations. While air temperatures were low enough to cause high overwintering mortality, the deep snow cover likely helped protect overwintering beetles, reducing the effect. Many soybeans were treated last year for soybean aphid about the same time that bean leaf beetles would have been present, which may have reduced the number of overwintering beetles in some areas. However, we usually have at least some fields each year that have problems with this insect and early planted soybean fields always attract the most beetles. It is best not to rely on predictions. Simply scout seedling soybean for bean leaf beetle.

Bean Leaf Beetle Life Cycle and Appearance

Bean leaf beetles have two generations a year in Nebraska. Since they overwinter as adults, three periods of beetle activity are seen in the growing season: Overwintering colonizers, F1 generation (offspring of the colonizers, the true first generation) and the F2 generation.

Bean leaf beetles overwinter as adults in leaf litter (woodlots) and soybean residue. They become active fairly early in the year (April-May), and often can be found in alfalfa prior to soybean emergence. As soybeans emerge, the beetles quickly move to the seedling plants, feeding on cotyledons and expanding leaf tissue. These overwintered beetles, called colonizers, mate and begin laying eggs. Females live about 40 days and lay from 125 to 250 eggs. After egg laying is complete, the colonizing population dwindles as the beetles die. A new generation of beetles (F1) will begin to emerge in late June to early July. The F1 beetles mate and produce a second generation of beetles (F2) that begin to emerge in mid to late August.

Bean leaf beetles vary in color, but are usually reddish to yellowish-tan. They are about ¼ inch long and commonly have two black spots and a black border on the outside of each wing cover. These spots may be missing, but in all cases there is a small black triangle at the base of the wings near the thorax.

Risk In Early Planted Soybeans

Because they move to soybean fields so soon after seedling emergence, early planted fields will usually have more beetles and suffer the most injury, particularly if they are the only beans up and available for the beetles to move into. This has become more of a problem in recent years because farmers are planting soybeans earlier to take advantage of the associated yield benefits.

Although the defoliation they cause can appear quite severe, research in Nebraska and elsewhere has shown that it usually does not result in economic damage. Soybean plants can compensate for a large amount of early tissue loss, so it takes a considerable amount of beetle feeding to impact yield. Generally, soybeans planted during the normal soybean planting window in Nebraska are not colonized by enough beetles to cause economic injury.

Effect on Weed Management. Early season defoliation can result in a need for earlier weed management. For example, with no defoliation, weeds can remain in the crop up to the V4 stage (third trifoliate) without significantly affecting the yields. However, at 30% and 60% defoliation, weeds require removal by the V3 and V1 stages, respectively.

Table 1. Economic threshold for treating bean leaf beetle in soybean at the VC growth stage.
 
Management Costs

Crop Value

$6

$8

$10

$12

$5

3

4

4

6

$6

2

3

4

5

$7

2

3

3

5

$8

2

2

3

4

$9

2

2

3

3

$10

1

2

2

3

$11

1

2

2

2

$12

1

1

2

2

Table 2. Economic threshold for treating bean leaf beetle in soybean at the V1 growth stage.
 
Management Costs

Crop Value

$6

$8

$10

$12

$5

4

5

7

8

$6

3

4

6

7

$7

3

4

5

6

$8

3

3

4

5

$9

2

3

4

4

$10

2

3

3

4

$11

2

2

3

4

$12

2

2

3

3

Economic Thresholds and Treatment Recommendations

Tables 1 and 2 present economic thresholds for bean leaf beetle on seedling soybean. Be aware that these thresholds are for defoliation of beans at VC - V1. If beetles enter the field right at or during seedling emergence, the thresholds will be lower because the beetles do not have leaf tissue to eat and they will feed on the growing point, stem, and cotyledons.

We do not have a good research base for bean leaf beetle injury to newly emerging soybean, but if the beetles appear to be significantly injuring or clipping the cotyledons and growing points, an insecticide treatment may be warranted. Research has indicated that early loss of both cotyledons can result in about a 5% yield loss. If control costs or crop values are lower or higher than those presented in the table, change the thresholds accordingly.

Remember that early planted, temporally isolated soybeans are the most susceptible. If economic thresholds are reached, many insecticides are available for bean leaf beetle control. All will do an adequate job if applied according to label directions. For those who plant early, regularly have economic levels of colonizing bean leaf beetles, and /or have a history of bean pod mottle virus (a bean leaf beetle vectored disease), neonicotinoid insecticide seed treatments may be warranted.

Some producers treat bean leaf beetle on seedling soybeans to reduce the subsequent F1 and F2 generations; however, UNL Extension does not recommend this practice. There are many environmental factors that can impact beetle populations throughout the growing season, making it impractical to use spring beetle numbers to accurately predict if beetle populations will reach economically damaging levels in August.

Regular scouting and the use of the appropriate economic thresholds are the best way to manage late season bean leaf beetle in soybean. Late season economic thresholds will be included in CropWatch later this summer.

Tom Hunt
Extension Entomologist, Haskell Agricultural Lab
Keith Jarvi
Extension Educator in Dakota, Dixon, and Thurston Counties