Soybean South

 - Insect Pests-

What’s Bugging You?

In the southern soybean insect arena, continue to scout and treat the usual suspects according to your state’s recommendations. Also, be on the lookout for new pests,
such as the red-banded stinkbug, as well.


By Carroll Smith

Early in the season, insects are attracted to young, juicy soybean plants. Fortunately, that’s also the time when it’s a bug-eat-bug world out there. Beneficials and parasites can usually keep harmful insects under control until the populations increase to the point that insecticides have to be applied.

For example, last year in Missouri when aphids swarmed in, it took about two weeks for the beneficials to wipe them out.

“The traditional threshold that all the states use is 250 aphids or more per plant,” says Wayne Bailey, a University of Missouri entomologist. “When we approach that number, then we have to use an insecticide. Last year the numbers came in fairly slow – about 50 or 60 per plant – but they never moved up to the 250 mark. If we had not had the beneficials here, the populations may have exploded more.”

Another pest that can appear in a cold, dry spring when plants are not growing rapidly is the bean leaf beetle. If early populations are high enough, farmers have to treat for them, Bailey says. They also have to treat for this pest as a pod feeder late in the season.

In Bootheel Missouri and along the southern counties, the corn earworm is prevalent.

“It’s one of those defoliators that we watch every year,” Bailey says. “Typically, beneficial insects and parasitic wasps will keep them in check, but we had such a drought in the state last year that the beneficial insect numbers were almost non-existent. At the end of the season when we started getting rains, the larvae, which were still present in low numbers, exploded and started feeding on pods.”

The Missouri entomologist says most of the chemistry that’s labeled for corn earworm control does a good job. He notes that these pests tend to feed on pods at the middle to the top of the plant, so it’s relatively easy to get the insecticide to them by air or with ground rigs.

“As long as the applicator uses enough water, coverage is not a problem,” Bailey says. “For aerial application, the two-gallon rate works well. On the ground, we recommend anywhere from five to 15 gallons per acre.”

A new pest that appears to be moving into Missouri soybean fields is the green stinkbug. Bailey says they see the highest numbers along the edge of the field, so he recommends that growers first scout the first 25 rows into the field to see if stinkbugs are present.

“We don’t have good thresholds on that pest yet because we have a lot more drilled soybeans than row soybeans,” he explains. “However, the threshold for row soybeans is more than one nymph or adult per foot of row or per square foot, which is really a recommendation for row soybeans that are on 36-inch centers or 15-inch centers versus drilled soybeans.”

Louisiana’s ‘new critter’
“Insects most frequently requiring the use of insecticides on soybeans in Louisiana include the soybean looper, velvetbean caterpillar and stink bugs,” according to the LSU AgCenter’s publication Control Soybean Insects 2006. And, of those, the stinkbug is the most serious.

The different species that cause economic damage include the southern green stinkbug, the green stinkbug and several species of brown stinkbugs. The LSU AgCenter publication notes that historically the southern green stinkbug was the most abundant, but in the late 1990s, “the brown stink bug complex became more common and was harder to control with insecticides.”

In the past few years, a new species called the red-banded stinkbug has become a serious soybean pest. In 2004 and 2005, this insect was prevalent in the “southern and south-central soybean parishes and is currently distributed throughout many northeastern soybean parishes.”

According to Control Soybean Insects 2006, the red-banded stinkbug’s early season behavior is not completely understood at this time.

Southeast stinkbugs still a staple
Although parts of the Mid-South soybean-growing areas are seeing some new pests appear, South Carolina Extension/research entomologist Jay Chapin says “not much has changed” in his part of the world.

“The biggies are still the same,” Chapin says. “Southern green stinkbugs are the primary issue, followed by corn earworms and loopers and velvetbean caterpillars.

“Stinkbugs are the main concern because they are difficult to control,” he explains. “You can kill them, but they will re-infest the field, and application costs are killing us.”

As far as the timing of major soybean pest infestations in South Carolina, the corn earworm usually appears toward the end of July through August and the looper from mid-August through mid-September. Stinkbugs and velvetbean caterpillars show up later in the season – September and October.

“Stinkbugs are mainly a September problem,” Chapin says. “Stinkbug damage is much less obvious than worm damage but usually more costly. Stinkbug feeding causes shriveled seed with reduced germination and can cause small pods to abort.”

The South Carolina entomologist notes that insecticide recommendations are based on what insects you actually have in the field.

“Farmers should consult the specific control recommendations for their state,” Chapin says. “Scout, determine which pests are present in the field and find out specific recommendations for that pest or combination of pests.”

Scouting For Insects

Following are tips from the LSU AgCenter Research and Extension Service to help you scout for pests correctly and in a timely manner.

• Scout all fields for insects and their damage at least weekly after blooming begins. Keep scouting until two weeks before harvest.

• Sample at least five different locations to get an average insect count that is representative of conditions in the field.

• Use a shake cloth or an insect sweep net where rows are 30 inches or wider. Narrower row spacing will require an insect sweep net.

The shake cloth – heavy cloth or plastic – should be three feet long and wide enough to cover the middle between rows. Attach a 4-foot length of 1/2-inch doweling to both ends. Place the cloth between two rows and vigorously shake the three feet of plants on each row over the cloth.

Count the insects that fall, starting with stinkbugs and three-cornered alfalfa hoppers. Count corn earworms separately from leaf-feeding worms like soybean loopers, velvetbean caterpillars and green cloverworms, which should be counted together as a group. Record the number of each insect per foot of row.

For sweep net sampling, use a heavy duty insect sweep net 15 inches in diameter. Count insect numbers on the basis of the number caught in 100 sweeps with the net. One sweep, which resembles a horizontal figure 8 motion, is the passage of the net across one row of soybeans. The bottom edge of the net should pass through the plants about 12 inches below the top of the plants.

Take sweeps throughout the field to get a representative sample. Count the various kinds of insects caught and record the number of each insect pest per 100 sweeps.