May 11, 2007

 

The soybean aphid has taken most of the attention of soybean producers lately, but do not forget to think about other soybean insect pests. Although bean leaf beetles have not been a big problem the last couple of years, populations are up from a few years ago. We usually have at least some areas each year that have problems with the bean leaf beetle, and early planted soybean fields always attract some beetles.

Bean Leaf Beetle Life Cycle

Bean leaf beetle

Bean leaf beetles have two generations a year in Nebraska; however, 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.

Identification

Bean leaf beetles vary in color, but are usually reddish to yellowish-tan. They are about 1/4 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 these beetles 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 them to move into. This has become more of a problem in recent years because planting dates seem to be getting earlier each year.

Table 1. Economic thresholds for bean leaf beetles in soybeans at the VC stage.   Management costs Crop Value $6 $8 $10 $12   Threshold number of beetles/plant $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

Table 2. Economic thresholds for bean leaf beetles in soybeans at the V1 stage.   Management costs Crop Value $6 $8 $10 $12   Threshold number of beetles/plant $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

Treatment Thresholds

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, unless insect populations are large enough to cause more than 50% defoliation of seedling soybeans, it is unlikely that treatment would be economically justified.

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 likely be lower because the beetles do not have leaf tissue to eat and 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 the thresholds are probably about 1 to 1.5 beetles lower than the VC thresholds; however, we do not expect any economic damage to occur below about 0.5 beetles per plant. If control costs are lower than those presented in the table, reduce the thresholds accordingly.

Remember that early planted, temporally isolated soybeans are the most susceptible.

Treatment Options

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.

We are also finding that early season defoliation can affect weed management. 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 yields. However, at 30% and 60% defoliation, weeds should be removed 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
Keith Jarvi
Extension Assistant, Integrated Pest Management