April 22, 2011

Bean leaf beetle

Accurately predicting spring bean leaf beetle infestations is not possible in Nebraska; however, because of persistent snow cover, overwintering beetle survival was probably above average this winter.  Growers should be vigilant.

In the north central U.S. bean leaf beetles overwinter best in regions with large tracts of wooded areas, persistent snow cover, and moderate winter temperatures. In regions with few tracts of woodland and large fields (for example. much of Nebraska), persistent snow cover and moderate winter temperatures are even more important for overwintering beetles. In Nebraska many beetles spend the winter in soybean stubble, not woodland leaf litter.

Another factor which may lead to an increase in spring bean leaf beetles in northeast Nebraska is last year’s lack of soybean aphids. When we have widespread soybean aphid infestations, soybeans are treated about the same time that bean leaf beetles are present, which in turn reduces the number of beetles that would overwinter. We did have some spraying for late season defoliators, but not at the high levels we observe when soybean aphids are present.

BLB Life Cycle

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 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.

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

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 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.

Treatment Threshold

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 that 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.

We have also found 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 the yields. However, at 30% and 60% defoliation, weeds require removal 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 included in CropWatch later this summer.

Tom Hunt
Extension Entomologist, Haskell Agriucltural Laboratory, Concord
Keith Jarvi
Extension Educator, Dakota, Dixon and Thurston Counties