Bean Leaf Beetles Numbers Down, but Hot Spots Still Possible - UNL CropWatch, April 2013

Bean Leaf Beetles Numbers Down, but Hot Spots Still Possible - UNL CropWatch, April 2013

April 25, 2013

Heavy, early spring bean leaf beetle infestations have been less frequent the last few years. One reason is the increasing acreage planted with insecticidal seed treatments. These insecticides are very effective at controlling spring colonizing bean leaf beetles in individual fields. Another reason may be that the overall population of beetles is lower because of the widespread use of insecticidal seed treatments coupled with insecticide treatment of other pests later in the year, such as soybean aphid (or spider mites last year). However, do not let this lull you into a false sense of security, particularly if you grow soybean without an insecticidal seed treatment. Bean leaf beetles are still present, and pockets of relatively high populations can cause problems.

Appearance and 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. Because 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 (the generation that will overwinter).

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.

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. Although defoliation caused by the beetles 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.

Tables 1 and 2 present economic thresholds for bean leaf beetle on seedling soybean. If control costs or crop values are lower or higher than those presented in the table, change the thresholds accordingly. Be aware that these thresholds are for defoliation of soybeans 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. Also, these thresholds were developed from studies of soybean seedlings that had relatively large leafs. You may want to take the size of your soybean seedling leaflets into consideration — seedlings with small leaves may be more susceptible to beetle injury and a lowering of the thresholds may be justified. Again, we do not have a research base that considers seedling leaflet size, so experience will have to be your guide.

Sampling seedling soybeans can be somewhat difficult. Wind, shadows, and movement often cause beetles to drop from the plant and hide under dirt clods or in cracks in the soil. We suggest walking carefully along 10-15 feet of row, being careful not to let your shadow cover the row, count beetles on or near the plants, and calculate how many you have per plant. If you can stay at least 3 feet to the side of the row, you would be less likely to alarm the beetles. Sample at least five locations in the field.

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), insecticide seed treatments may be warranted.

How Defoliation Affects Weed Management

We also have found seedling 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 yield; however, at 30% and 60% defoliation, weeds need to be removed by the V3 and V1 stages, respectively.

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 published in CropWatch later this summer.

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

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