If you’ve been growing soybeans the same old way year after year, things might be about to change.
“The kudzu bug — in my opinion — has the potential to change everything we’ve ever had to do in managing soybean insects,” says Auburn University Extension entomologist Tim Reed.
“It’s one of those things that make it difficult for coming up with thresholds.”
But after consulting with colleagues and others in the field, Reed has come up with what he calls a “tentative” economic threshold for treating kudzu bugs in soybeans in 2014.
“Since 2009, this insect pest has spread to eight Southeastern states, and it has spread very rapidly. Next year, I think we’ll see kudzu bugs reach economic damaging levels in even more fields unless parasites can keep up with them,” says Reed.
The Alabama Cooperative Extension System’s economic threshold for kudzu bugs in soybeans this year, he says, call for treating prior to first bloom when there is an average of five kudzu bugs per plant for the entire field.
“After first bloom through R6, apply an insecticide when a sweep-net sampling catches 10 adults per sweep or one nymph per sweep. When using a sweep net, a sweep is defined as one sweep across two rows using a 15-inch diameter sweep net.,” says Reed.
“If immature kudzu bugs are easily and repeatedly found on petioles and main stems during visual inspections of the canopy, treatment is likely warranted.”
He advises that growers should not limit all sampling to border rows where populations build initially.
“Border treatments in some cases have slowed the movement of adults across fields. If immature kudzu bugs are easily and repeatedly found on petioles and main stems during visual inspections of the canopy, treatment is likely warranted,” he says.
This threshold, he says, is based on one year’s experience and will be adjusted as more research is conducted and experience is accrued.
“Due to the tendency of this pest to congregate on the field borders, initially it is important that growers use an average population estimate for whole fields before making a whole-field insecticide application.”
During 2013, kudzu bugs continued to migrate into soybean plots at Prattville through the third week of July, adds Reed.
“Re-treatment may be necessary when a treatment is applied before migration into soybeans stops. Spraying for kudzu bugs will significantly reduce beneficial insects which could result in economic infestations of caterpillars.”
The kudzu bug, he says, has sucking mouthparts, feeding mainly on the stem and leaf petioles. It primarily will be on the main stem of the plant.
“The effects of this pest mimic drought conditions. They form lesions on the main stem and petioles, and they do feed on pods occasionally. They’ll reduce the number of pods per plant, the seeds per pod, and the seed size. They’ll make the plants shorter if they get on them when the plants are small.”
The immature stage is flat, hairy and fuzzy and looks like nothing else, says Reed. A high percentage of the over-wintering females are already fertilized when they move into kudzu and soybeans.
“I’ve seen them on kudzu at the Atlanta airport during the first week in March. You will see this pest in all soybean fields in the state in the not-too-distant future. They’re better at over-wintering and surviving harsh winters than the boll weevil.”
The problem is if growers start spraying for kudzu bugs early, they’ll end up having to spray for them three or four times, he adds. “When they’re bad, they’re bad. We don’t want kudzu bugs to cause us an economic loss.”
Many specialists in other states won’t have an adult threshold at all, says Reed. “If immature kudzu bugs are easily found on petiole and main stems during visual inspections of the canopy, the treatment is likely warranted. They’ll mainly start at the border rows and then move across the fields. Border treatments in some cases have slowed the movement of adults across the field.”
Can cost growers five bushels per acre
There’s a lot to consider when you’re dealing with this pest and you’ve got a problem, cautions Reed.
“In Prattville, the number per plant varied from zero to six, so they’re not consistent initially on the plant. They start out aggregating on some plants and not getting on others.
“Work done in Georgia indicates that if you infest V-3 soybeans — vegetative soybeans with five per plant — you get a reduction in plant height. Five bushels per acre is a pretty common yield loss for kudzu bugs in a lot of studies that we’ve done.”
With a threshold of 10 per sweep, growers will be spraying some soybeans in early July, and that’ll flare worm problems, but Reed says he doesn’t see any way to avoid it.
“We may be reaching the point on soybeans that we are on cotton in the Mid-South — having to spray to the point to where we’re eating up some of our profits.”
Research at Auburn has discovered a parasitic wasp — the paratelenomus saccharalis — that might be helpful in battling kudzu bugs, he says.
“This is the parasite that I’m optimistic will provide us some help with the kudzu bug,” says Reed.
“It lays its eggs inside the eggs of the kudzu bug. Research at Auburn has shown that 85 percent of the eggs were parasitized in late summer in Auburn. The parasitation rate is highest where kudzu bugs have been the longest.”
Some double-crop soybeans will be planted early enough that kudzu bugs will still be a factor in pest management decisions, says Reed.
“We’re going to have a complex that not only will include stink bugs, pod worms, and defoliating caterpillars, but now we throw the kudzu bug into the mix. We’ve also got Asian Soybean Rust to think about. When in doubt, spray. Kudzu bugs can be controlled with a large number of insecticides. Mostly, growers will use a pyrethroid.”
Looking at Alabama crop acres from 2013, soybeans were the largest acreage of any crop, with the total row-crop acreage pushing 1.5 million acres, says Reed.
Alabama row crops generated $1 billion in receipts for the first time in 2012, he says.
“This year, we expect soybeans to generate $179 million. Soybeans have become an important crop for our producers, and we need to make sure we try and do everything possible to protect this investment and keep the money coming in.”
The first insect pest growers will see on soybeans is thrips, but they don’t do the damage on soybeans they do on cotton, says Reed.
“They can require insecticide treatments on rare occasions. My main worry with thrips is that they vector diseases. Early on, soybean vein necrosis virus can form, and it seems to be more of a problem in Alabama than in other states.”
The three-cornered alfalfa hopper will be the next pest you’ll be concerned about on soybeans, he says. The adults and immatures have sucking mouthparts, and they’ll girdle the plant. The immature stage will be lower on the plant than the adult, and you won’t pick them up very well in a sweep net.
“The main issue we have with this pest from the farmer’s perspective is when the girdling damage is done really early, and the plants fall down making them difficult to combine.”
The three-cornered alfalfa hopper, says Reed, is likely the first pest to cause economic loss in soybeans.
“It’s hard to detect in the very early vegetative stages of the plant. Insecticide seed treatments will reduce the girdling damage for about two to three weeks, so that’s probably the best reason and the only reason to use a seed treatment on soybeans in Alabama. The Mississippi recommendations for three-cornered alfalfa hoppers are probably some of the best I’ve seen.”