Start Watching for Second Generation Bean Leaf Beetles

Start Watching for Second Generation Bean Leaf Beetles

Photo of a bean leaf beetle feeding on a pod
Bean leaf beetle
Photo of bean leaf beetle damage of a soybean leaf
Bean leaf beetle leaf damage
Photo of bean leaf beetle damage to soybean pods.
Bean leaf beetle pod damage

August 17, 2007

We have begun to receive questions about second generation bean leaf beetle, which are now appearing in fields. Let's review biology of the beetle and management.

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. Usually from mid June to early July few if any beetles are present in the field (before the first generation emerges), but 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 and 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. It is therefore important to monitor beetles regularly to determine shifts in population, which will aid in management decisions.

Damage 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.

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 or will be emerging and beetle numbers will build to a peak, which is usually in mid August through early September, depending on location. 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).

Table 1. Economic thresholds in beetles per row foot for R6 (full seed) soybeans in 30-inch rows.
 
Soybean Value
Pest Management Costs Per Acre

 
$6
$8
$10
$12
$5
6
7
9
10
$6
5
6
8
9
$7
4
5
7
8
         
Table 2. Economic thresholds (beetles per sweep) for R6. Numbers in parenthesis are for drilled soybeans with 7-inch row spacing.
 
Soybean Value
Pest Management Costs Per Acre

 
$6
$8
$10
$12
$5
4 (3)
5 (4)
6 (5)
8 (5)
$6
3 (2)
4 (3)
5 (4)
6 (5)
$7
3 (2)
4 (3)
4 (3)
5 (4)

Sampling Methods

Perhaps the most accurate way to sample beetles is with a drop (or shake) cloth, although this method is not well suited to narrow row beans. A drop cloth is a 3 x 3 ft 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, so if you use a three-foot cloth divide your total by three. Also, sample throughout the field in several areas to get a good estimate of the population.

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 arcs. The best sweeping action for bean leaf beetle is a consistent upward motion through the foliage, 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 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. Thresholds are based on the number of beetles per foot of row (Table 1) or beetles per sweep (Table 2), which varies according to total management cost and the crop value per bushel.

Treatment

The tables show economic thresholds for beans in 30-inch rows (7-inch rows in parenthesis). To use the tables find the number that fits both crop value and application costs. For example, if you set the value of your soybeans at $5 per bushel and your application costs at $6, you would need six or more beetles per foot of row to justify an application in 30-inch row beans.

Economic thresholds for reproductive stage soybeans other than R6 are probably higher (more beetles are needed to justify a treatment). This is because pods on plants past R6 are maturing and there is less green pod tissue available for beetle feeding, and plants in earlier reproductive stages have greater yield compensation potential than those in R6 or older. Several insecticides can be used to control bean leaf beetles. Be aware that most have pre-harvest intervals of 14 or more days. Information on insecticide selection and use can be found at http://entomology.unl.edu/instabls/beanlfbt.htm on the UNL Entomology Department Web site (www.entomology.unl.edu).

Tom Hunt
Extension Entomologist
Haskell Ag Lab
Keith Jarvi
Extension IPM
Northeast REC, Norfolk