Georgia soybean producers made a record crop this past year with 37 bushels per acre, but yields might be improved even more by controlling insect pests like the relatively new kudzu bug.
This pest was first observed in the United States in the fall of 2009, in northeast Georgia,” says Phillip Roberts, University of Georgia Extension entomologist.
“Now it’s easy to find — if you grow soybeans in Georgia, you’ve probably seen it, and you’ve probably seen a lot of differences among populations in your fields,” said Roberts at the 2013 Georgia/Florida Soybean/Small Grain Expo in Perry, Ga.
It is a “true” bug, he says, with a sucking mouth part. “Both adults and immatures will use that sucking, piercing mouth part to feed on a plant. They feed primarily on the main stem, and they also feed on petioles. The adults have a hard shell and the immatures are soft and almost fuzzy in appearance,” says Roberts.
The one good thing that can be said about the kudzu bug is that it usually doesn’t feed on pods — it’s primarily a stem feeder, he says.
“If this insect fed on pods, like stink bugs, we probably would not be growing soybeans. It would be very intensive in terms of management with insecticides. I like to think of it as a stress-inducing plant — it just adds more stress to the plant.”
In the past three years, Roberts and other researchers have learned the kudzu bug impacts soybean yield in three ways: it can reduce the number of pods set on the plant, it can reduce the number of seed set in the pod, and it can impact the size of the seed and affect how the bean itself actually sizes up.
The type of yield loss, he says, depends on when the bugs put enough stress on the plant to cause yield loss.
“Over the last three years, we’ve conducted a lot of different trials on this insect in soybeans. On average, we’ve observed about a 20-percent yield loss in untreated plots compared to where we treated for the insect. We’ve seen a range of yield loss from zero up to 60 percent. It can be a very serious pest if left uncontrolled,” says Roberts.
The kudzu bug initially was found between Athens and Atlanta, he says. To date, it has been observed in every county in Georgia with the exception of Camden County.
“No soybeans are produced in that particular county, and we could not find any kudzu. It was observed in 2012 in the Florida Panhandle. If you’re from north Florida, history has shown us that once the insect is detected, you can expect economic damage the very next year.
Widespread in three years
“In three years, it has spread over a large area, and we’ve actually got a couple of sites in Mississippi where the bug probably hitchhiked in on some type of vehicle.”
It appears, says Roberts, there are two generations of kudzu bug each year. The insects over-winter as adults, and they can be found in loose leaf litter and other protective areas on the ground. They also can be found in tree bark.
“Two generations per year is a long life cycle,” says Roberts.
“From eggs to adults is about six to eight weeks. The cooler the temperature, the longer it takes to get to the adult stage.
“Adults also live for a long time, meaning once they’re in soybean fields, they will be there for awhile. We typically see higher infestations at the edges of fields. So when you’re making treatment decisions, don’t base the entire field on what you see in the first 10 feet.”
As its name implies, the kudzu bug likes kudzu, he says.
“So kudzu is a very important part of the picture when we consider managing the pest in soybeans. We know a lot of these insects are produced in kudzu patches. Ultimately, those insects will move to soybeans in the latter part of June and into July. A lot of these bugs originate in a kudzu patch.”
This past year, he says, over-wintered bugs were seen moving directly to soybeans and not stopping in kudzu.
“If we plant in April, for example, we may see kudzu bugs come directly from over-wintering. That may not be important to us in Georgia, but it could be important in terms of where this insect will go in the United States. We may not need to have kudzu for this insect to be established.”
In 2012, says Roberts, it was confirmed that early-planted soybeans are at much higher risk to kudzu bugs than later planted soybeans.
“We planted a Group V and a Group VII soybean in Tifton in April, May, June and July. A kudzu bug egg mass usually will contain from 15 to 20 eggs. We counted the number of egg masses on five plants, when the soybeans reached the R2 stage, about 60 days after planting.
“Our early beans had many more egg masses on them at R2 compared to May, so expect more pressure the earlier you plant.”
The yields of treated and untreated soybeans were compared at various planting dates, he says.
“We treated with Indigo from Syngenta. We weren’t trying to treat these beans economically — we treated every two to three weeks to keep them as clean as possible.
“With our early planting date, we had 76-percent yield loss, and it might as well have been 100 percent. There just wasn’t much there.
“In May, we lost 54 percent, and the difference between the untreated and treated continued to get smaller as the planting date became later.
Worse in early-planted beans
“I’m not trying to say we need to plant soybeans late to manage kudzu bugs. You need to plant soybeans when you have the greatest yield potential. But expect kudzu bugs if you plant early.
“Also at Midville, our greatest yield loss came in the earliest planted soybeans.”
Roberts also observed kudzu bugs feeding on seedling soybeans, in the earliest plantings.
“Where we had bugs on seedling soybeans — and we tried to get an infestation of about 10 adults per plant — we did see a reduction in plant height, so they are impacting seedlings. But I think that will be a rare occurrence on the farm.
Roberts notes that the kudzu bug is not a difficult insect pest to kill, and that using a sweep net is a good way to monitor the insect in soybeans.
Data from 27 trials in Georgia, South Carolina and North Carolina has been summarized in which several different compounds were used to treat kudzu bugs, he says. Some of the treatment gave better than 80 percent control.
Multiple classes of insecticides have shown activity on kudzu bugs. Insecticide treatments containing bifenthrin, cyhalothrin, zetacypermethrin, carbaryl or acephate provided greater than 80 percent control two to five days after treatment in insecticide efficacy.
Growers actively treating kudzu bugs with broad spectrum insecticides should consider using a preventive application of Dimilin at the R2/R3 growth stage for control of velvetbean caterpillar and green clover worm.
Knowing when to spray is still the primary question to ask when treating for kudzu bugs, says Roberts.
“Over the last two years, we’ve compared a lot of different thresholds or triggers. Looking at yields from our trials, as long as you spray kudzu bugs at some point, yield is about the same.
“We had two threshold verification trials in Tifton. We settled on a threshold of one immature kudzu bug per sweep. Our threshold called for only a single spray, and there was a difference in yields with just that one spray.”
If you have a problem with this insect pest, you need to address it or it can hurt you, says Roberts. “But we have noticed we can get good control with a single spray. I’m not ready to say we can do it every time, but we’ve done it over the last three years.”
A sweep net can provide you a lot of valuable information for kudzu bugs and other pests, he says.
You can also scout with visual observation, but you need to trigger any treatment with the immatures.
“If you can find immatures easily, then a spray is definitely needed. A lot of times we don’t see this migration back into the field after we clean it up.”
If you have early maturing beans — Group V and Group VII on the same farm — when the Group V beans dry down, the bugs could leave those and jump to the Group VII, he says.
“You’re never really out of the woods with kudzu bugs, but if you use the recommended threshold, you can usually get them with a single spray.”
Soybeans need to be protected from kudzu bugs at least to the R7 stage, says Roberts.
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