September 1, 2009

Bean leaf beetle

We are receiving questions about second generation bean leaf beetles, which are appearing in some Nebraska soybean fields. Let's discuss basic bean leaf beetle biology and management.

Biology

Two generations of bean leaf beetles develop in Nebraska. The second generation overwinters as adults and are the beetles seen early in the year feeding on seedling soybeans. These beetles feed, mate, lay eggs and die in early to mid-June. There is usually a period from mid-June to early July when few if any beetles are in the field. This is the period before the first generation emerges. However, with today's broader range of planting dates, this period is less distinct than in the past.

Total developmental time from egg to adult can range from 25 to 40 days. Because of this range of development, it is common to see adults from the first generation and the second generation in the field at the same time. Because the generations can overlap, beetles can be present at some level from mid-July until the end of the growing season. That's why it's important to monitor beetles regularly to determine population shifts to aid in management decisions.

Injury and Management

Bean leaf beetles will feed on soybean leaves throughout the season, but leaf feeding seldom causes yield loss. Most damage (economic yield loss) occurs when second generation beetles feed on the developing pods. This yield loss can occur in several ways. Pods may be clipped from the plants; however, this is not the primary cause of yield loss. Many flowers and pods are aborted naturally and to blame pod loss on bean leaf beetle feeding may be a costly mistake.

Preharvest Intervals Most insecticides for control of bean leaf beetles have preharvest intervals of 14 or more days. Insecticide selection information is available on the UNL Entomology Department Web site.

There are no thresholds that consider pod-drop. Beetles normally injure soybean pods by feeding on the outside layer of the pod, leaving a thin layer of tissue covering the seed. They do not usually eat into the developing seed, although this may occur on very small pods. Fungal pathogens may enter the pod from the feeding sites, causing seeds to appear shrunken, discolored, and moldy, which can result in dockage. After full pods are formed and seeds begin developing, soybeans are most susceptible to yield loss from pod feeding.

The best time to sample is before significant pod feeding occurs, but after second generation beetles have emerged. Second generation bean leaf beetles are emerging and beetle numbers will build to a peak, which is usually in mid-August through early September, depending on location. Because temperatures have been relatively cool this year, we are seeing beetles emerge a little later than normal in many fields. Beetle numbers will slowly decline as beans continue to mature and move to overwintering sites. Economic thresholds have been developed for two sampling methods: drop cloth (beetles per foot of row) or sweep net (beetles per sweep).

Although the most accurate way to sample beetles is with a drop (or shake) cloth, almost all sampling is done in Nebraska soybean with a sweep net. In addition, a drop cloth is not well suited to narrow row or drilled beans.

Drop Cloth Method

A drop cloth is a 3 x 3 foot piece of muslin or plastic attached on each side to dowel rods. Hold one rod against the base of the plants and lay the cloth between the rows. Shake the plants against the cloth to knock off the insects, and count the beetles. Remember to estimate the number per row foot. If you use a three-foot cloth, divide the total by three. Sample in several locations throughout the field (at least five locations) to get a good estimate of the population. If bean leaf beetle populations are four or more beetles per foot of row, consider an insecticide treatment.

Sweep Net Method

If sampling with a sweep net, sweep at least five randomly selected sites. Walk through the field at an even pace, performing about 25 sweeping 180° arcs. The best sweeping action for bean leaf beetle is a consistent motion using as much force as needed to move the net smoothly through the foliage. Bean leaf beetle activity varies during the day. Activity patterns suggest the best times to sample are around mid-morning or in the afternoon. Try to maintain a similar sampling time in each field to eliminate variability. If bean leaf beetle populations are three or more beetles per sweep, consider an insecticide treatment.

If the beetle counts are below the economic threshold, scout the field again about five days later. Stop scouting when the beetle counts begin to decline, the soybean pods begin to yellow (R7), or the field is sprayed.

Treatment Recommendations

Several insecticides can be used to control bean leaf beetles. Be aware that most have preharvest intervals of 14 or more days. Information on insecticide selection and use can be found on the UNL Entomology Department web site.

Tom Hunt
Extension Entomologist
Haskell Ag Lab, Concord
Keith Jarvi
Extension Educator
Dakota, Dixon, and Thurston Counties