Four Insect Pests of Southern Soybeans

STARKVILLE, Miss. (DTN) -- The saltmarsh caterpillar, a hairy and often hungry insect that can defoliate soybeans and cause yield loss, is rarely a problem in Mississippi, but in 2021 it was, entomologist Don Cook said Tuesday.

It was an unusual year for insect activity for row-crop farmers in Mississippi and other Southern states, according to the Mississippi State University (MSU) insect researcher. In addition to saltmarsh caterpillars, fall armyworms and soybean loopers surprised growers with large populations, in addition to the growing problem of redbanded stink bugs.

Cook provided information of why some pests surprisingly affected soybeans and how best to control them in the future at the annual MSU Row Crop Short Course in Starkville, Mississippi. The three-day conference ends Wednesday.

Cook said the weather, changing management strategies and other reasons contributed to the strange year.

"From a crop (insect) standpoint in 2021, it was somewhat unusual," he said.

SALTMARSH CATERPILLAR

Often called "woolly worms," saltmarsh caterpillars feed on soybean leaves and other crops while in the larval stage. Their colors may be black, rust or yellowish-brown.

Eggs are laid in masses on the soybean leaves, and infestations often start around field borders. MSU's Insect Control Guide said the pest seldom reaches treatable levels, but large numbers can devastate plants.

"Quite a number of acres (were) sprayed for this pest," Cook said, though he doesn't know why. "This is something I've seen twice in almost 30 years."

MSU recommends farmers treat for saltmarsh caterpillars when 35% foliage loss has occurred and worms a half-inch long or longer are present prior to bloom, or when 20% foliage loss has occurred, and worms are a half-inch long or longer after bloom.

Cook said pyrethroids tend not to work well since they work through contact, and the insect's hair can prevent that. He said products such as Besiege, Prevathon, Intrepid and Intrepid Edge work well at labeled rates. Terminate insecticide applications seven days into the R6 growth stage.

FALL ARMYWORMS

Fall armyworms are occasional pests of soybeans, according to MSU. Cook expected a cold snap in the late winter would delay adult moths from migrating from overwintering spots in Central and South America or the southernmost U.S., but that didn't happen. He started getting reports that fall armyworms were already in fields in late May and early June.

Five to seven generations of fall armyworms can occur in a single season thanks to fast reproduction. There are two host strains of fall armyworm. The corn strain typically infests corn, cotton and sometimes soybeans. The grass or rice strain typically infests rice, grain sorghum, Bermuda grass, and other grasses.

Most fall armyworm issues in soybeans result from them infesting grass in soybean fields. Weather can delay herbicide applications, which allows grasses to get established. The larvae infest the grass, and when the grass starts dying, fall armyworms need something to eat and move to the soybeans. A lot of the worms are big, and they can defoliate small vegetative stage soybeans fairly fast.

"Typically, when soybeans are in a vegetative state, they are not much of an issue," Cook said. "You can have 100% defoliation in V3 and V6 (growth stages) that doesn't affect yield. But it will scare you to death seeing a field of only stems out there."

What is concerning, though, are reports of grass strain fall armyworms surviving pyrethroid applications or total failures.

"It's something to watch," Cook said.

MSU lists nearly 20 insecticides to control fall armyworms, such as Orthene, Baythroid XL and Vantacor. Typically, pyrethroids are used against grass strain fall armyworms. Products such as Prevathon, Besiege, Intrepid and Intrepid Edge are for the corn strain. Diamond is also very effective against fall armyworms.

Read a recent DTN story explaining how this pest became a national problem in 2021 here: www.dtnpf.com/agriculture/web/ag/crops/article/2021/10/06/fall-armyworms-bad-year-happen.

SOYBEAN LOOPERS

Soybean loopers are migratory insects that fly in from Central and South America each year and infest soybeans in the mid-to-late part of the growing season. They are leaf feeders that can cause extensive defoliation at high numbers.

Cook said that during the last several years, looper infestations have crashed after seven to 14 days due to unusually early cold fronts that have occurred during late August to early September. This year that didn't happen. Farmers dealt with them in some areas for up to six weeks with rolling populations. Soybeans in the early reproductive stages are more susceptible to damage.

Soybean loopers have developed resistance to some insecticides. "When diamides first came out about 10 to 12 years ago, they would provide control for up to 30 days," Cook said. "I don't anticipate any control of loopers now beyond 14 days."

A few insecticides used to control loopers include Vantacor, Besiege, Intrepid and Intrepid Edge. Prior to bloom, MSU recommends applications when eight or more worms a half-inch or longer are present per foot row when using a drop cloth or 38 worms of the same length per 25 sweeps of a sweep net. After bloom, the worm threshold is cut to four or more worms using a drop cloth or 19 worms per 25 sweeps. Alternately, growers can treat when defoliation reaches 35% prior to bloom or 20% after bloom.

REDBANDED STINK BUG

Cook initially expected problems with redbanded stink bugs in Mississippi this year due to an overall mild winter that would help some of the 2020 population survive. But a February cold snap killed many of them off.

But thanks to a February snow that acted as an insulating blanket, some bugs survived the cold temperatures.

"In most places, it was enough to know that they were there, and some reached treatment levels. However, they were not a huge problem," Cook said.

Farmers should keep tabs on redbanded stink bug levels, though. Damage from this pest can prevent plants and seed from properly maturing and can render seeds low quality and unmarketable and hurt yields. "They can be very devastating," Cook said.

Damaged seed is also susceptible to additional damage from diseases and weather.

MSU recommends treatment when numbers reach four bugs per 25 sweeps or two bugs per 6 feet of row with a drop cloth. After the R6.5 growth stage, treat populations that reach or exceed 10 bugs per 25 sweeps.

For management, MSU recommends acephate, bifenthrin, Engigo or bifenthrin mixed with acephate, Belay or imidacloprid.

Read a past DTN story on this increasingly problematic pest at: www.dtnpf.com/agriculture/web/ag/crops/article/2017/08/18/redbanded-stink-bugs-threaten.

PARTING THOUGHTS

Farmers need to pay attention to threshold levels to determine when to spray, Cook said. Despite high commodity prices, farmers can't afford a zero-tolerance attitude toward insects or damage.

Also, he urged farmers not to be the last ones with green soybeans.

"They will catch everything," Cook said, referring to attracting insects. "It won't be pleasant."

See the MSU Insect Control guide here: http://extension.msstate.edu/….

Matthew Wilde can be reached at matt.wilde@dtn.com

Follow him on Twitter @progressivwilde

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