Scout Early Emerging Soybeans for Bean Leaf Beetles

Scout Early Emerging Soybeans for Bean Leaf Beetles

May 1, 2009

With all of the spraying for soybean aphids last summer, we may not see a lot of bean leaf beetles this spring; however, we usually have at least a few areas each year with problems. Since early-planted, temporally isolated soybeans attract the most beetles and are the most susceptible to early damage, scout these fields first.

 

Life Cycle

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

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 forty 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.

Appearance

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.

Damage Potential

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 planting dates seem to be getting earlier each year.

Although the defoliation the beetles 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.

Economic Thresholds

Table 1. Economic thresholds for treating bean leaf beetle in soybean at the VC growth stage (number of beetles per plant).

 

Management Costs

Crop Value

$6

$8

$10

$12

$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 thresholds for treating bean leaf beetle in soybean at the V1 growth stage (number of beetles per plant).

 

Management Costs

Crop Value

$6

$8

$10

$12

$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

 

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 will feed on the growing point, stem, and cotyledons. Remember, early-planted, temporally isolated soybeans are the most susceptible.

We do not have a good research base for bean leaf beetle injury to newly emerging soybean, but if 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, adjust the thresholds accordingly.

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. If you plant early and regularly have economic levels of colonizing bean leaf beetles, insecticide seed treatments such as thiamethoxam or imidacloprid may be warranted.

Impact on Weed Control

Our research shows that 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.

Another reason some producers treat bean leaf beetle on seedling soybeans is 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 Ag Lab, Concord
Keith Jarvi
Extension Educator in Dakota, Dixon, and Thurston counties