Bean Leaf Beetles Now In Many Iowa Fields

Second generation of this insect pest eats pods on soybean plants mid-August through early September.

Published on: Aug 21, 2006

Second generation bean leaf beetles are now prevalent in many local fields in Iowa. "When this pest attacks soybeans this time of year it eats on the pods," says Palle Pedersen, Iowa State University Extension soybean agronomist. "Depending on how many you have in the field and whether or not they have reached the economic threshold - it will pay to spray an insecticide."

 

Pedersen says he's walked through some fields this past week that have bean leaf beetles so thick that you don't necessarily have to count them. "Once you do some scouting of your fields and make a couple of counts as a spot check, you get an eye for how many beetles are present," he notes.

 

"I have been getting a lot of calls about second generation bean leaf beetles," says Clarke McGrath, an ISU Extension field crop specialist in southwest Iowa who works with the Iowa Corn and Soybean Initiative.

 

Does it pay to spray beans in August?

 

"I hate spraying in August, but if the bean leaf beetle infestation in your field is above the economic threshold, you do what you have to do," says McGrath.

 

Some farmers are concerned that what yield gain they get by killing the beetles, they may lose yield by driving through beans with the sprayer and knocking down some soybean plants. McGrath says he gets asked that question often.

 

"Yield loss due to sprayer tracks depends on many factors," he says. "Those include the soybean row spacing, and the sprayer or tractor's tire size, boom width, soil conditions, bean architecture, driver skill, if there are old tracks to follow, growth stage of bean plants, etc."

 

The rule of thumb based on Midwest research says that "we can expect a low of around half of a bushel per acre to a high of around 3 bushel per acre on the very high end of loss--narrow rows, short boom," says McGrath. "My experiences have typically been around the one bushel per acre range. Ask your local chemical dealer for his opinion based on each field and the spray rig used."

 

Yield loss occurs from pod feeding

 

Bean leaf beetles will feed on soybean leaves throughout the season, but leaf feeding seldom causes yield loss--unless it is severe. Most 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, nor only, cause of yield loss. "I don't think there are any local thresholds that consider pod-drop," says McGrath. "More importantly, beetles normally injure soybean pods by feeding on the outside layer of the pod, leaving a thin layer of tissue covering the seed. They don't usually eat into the developing seed, although this may occur on very small pods."

 

Diseases caused by fungal pathogens may enter the pod from the feeding injury, causing seeds to appear shrunken, discolored, and moldy, which can result in dock on price when you deliver beans to the elevator. "After full pods are formed and seeds begin developing, soybeans are most susceptible to yield loss from pod feeding," says McGrath.

 

Mid-August through early September

 

The best time to sample is before significant pod feeding occurs, but after second generation beetles have emerged, he adds. Second generation bean leaf beetles are emerging and beetles numbers will build to a peak, which is usually in mid August through early September, depending on location. Beetle numbers will then slowly decline as beans continue to mature and beetles move to over- wintering sites.

 

Economic thresholds have been developed for two sampling methods: drop cloth (beetles per foot of row) or sweep net (beetles per sweep). Using the sweep net method is much more functional for various row widths, says McGrath.

 

Sampling for bean leaf beetles

 

Walking along with a sweep net in your hands, you should sweep at least five randomly selected sites. Walk through the field, performing about 25 sweeping arcs, advises McGrath. The best sweeping action for bean leaf beetle is an upward motion through the foliage, using as much force as needed to move the net smoothly through the foliage.

 

"Try not to let anyone see you doing this," he jokes.

 

Bean leaf beetle activity varies during the day, so the best times to sample seems to be mid-morning or in the afternoon. McGrath says you should try to maintain a similar sampling time in each field to eliminate variability.

 

If the beetle counts are below the economic threshold, he says you should scout the field again about five days later. Stop scouting when the beetle counts begin to decline, or when the soybean pods begin to yellow (R7 growth stage), or if the field is sprayed with an insecticide.

 

Thresholds are based on the number of beetles per sweep, which varies according to total management cost and the crop value per bushel. The table below shows economic thresholds for beans in 30-inch and 7-inch rows. These are R6 Economic Thresholds (beetles per sweep). Numbers in parenthesis are for drilled soybeans with 7-inch row spacing.

 

Soybean Value

Pest Management Costs Per Acre

$6.00

$8.00

$10.00

$12.00

 

$5.00

4 (3)

5 (4)

6 (5)

 

$6.00

3 (2)

4 (3)

5 (4)

 

$7.00

3 (2)

4 (3)

4 (3)

 

 

Economic thresholds for reproductive stage soybeans other than R6 are probably higher. 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.

 

Beware of harvest interval for insecticide

 

Several insecticides can be used to control bean leaf beetles. Most have pre-harvest intervals of 14 or more days. Read the label to find out how long you have to wait after spraying before you can harvest the soybeans. "Work with your local dealer on product choices if you have any questions," advises McGrath.