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Increasing insect pressure changes soybean management

The Mid-South's move to earlier soybeans hasn't been without cause. Over the last several years, insect pressure has been steadily building in soybeans.

“Twenty or 25 years ago, there were occasional problems with insects in beans,” says Gus Lorenz, Arkansas Extension entomologist. “It was rare that the trouble they caused required treatment.”

More recently, however, “increasingly the message is: ‘You need to budget at least one insecticide application when figuring soybean budgets.’”

A lot is tied to production systems changing so much. The Roundup Ready technology, reduced-till systems, and planting earlier maturing varieties have all played roles. Producers have almost completely moved away from the Group 6s to avoid insect problems.

“Not only do growers need to budget for one insecticide application, they need to scout more. And if they're not scouting their soybean fields regularly — at least from bloom on — they need to reevaluate that. They need to be scouting at least once a week or have someone scout for them.”

With the price of soybeans jumping, it's easier to justify protecting soybeans. A price of $9-plus per bushel compared to $4 or $5 “eases farmers' minds about spending the extra money.”

The worsening situation with stinkbugs has been a trend since the late 1990s. The stinkbug complex has become a major pest, rising to the point that in some late-planted/late-maturing soybeans, particularly in south Arkansas, the pest is uncontrollable.

Lorenz likens the situation to “biomagnification.” As the soybean crop begins to mature and there are fewer acres of green soybeans, the stinkbugs are increasingly attracted to what's left. All the stinkbugs across all those acres concentrate on the green plants.

Lorenz says with all the wheat currently planted in the South, “we're going to have some late-planted soybeans. We need to be planting the early-maturing soybeans as much as possible — Group 4s and early 5s. That helps avoid the late-season situations with high levels of stinkbugs moving in.

“Suppose you have 20 stinkbugs per acre on 3 million acres. Then, at season's end, there's still 100,000 acres with stinkbugs that have to go somewhere. They're heading for any green beans left.

“That's where biomagnification plays into it: there are more and more pests and fewer acres to feed them. That concentrates the pest population.”

To deal with stinkbugs, Lorenz and colleagues have been studying the timing of insecticide application termination.

“Some of the most frequently asked questions by farmers have to do with that. When beans begin to mature everyone wants to know ‘when can I quit spraying?’

“Basically, the data shows you need to spray through R-7. That's almost to full maturity. Actually, R-7 is already known as physiological maturity, when the plant tops begin to yellow and the pods begin to turn brown. We've found as long as the bean is soft, the stinkbug can cause damage.”

Certainly, greater stinkbug damage is done when they get into an early crop. “They can cause more yield losses when they hit fields in reproduction like R-3/R-4. But late-season, they can still damage a crop with quality losses and prevent beans from filling to full potential.”

The stinkbugs can also cause problems with green bean syndrome. “We know from studies done in Arkansas, there is a correlation between green bean syndrome and high levels of stinkbugs.

“We've tested our threshold of one per row foot/nine per 25 sweeps. We have found that is a good threshold and spraying should occur when those are reached.”

When stinkbugs are building, “the time when numbers go from one per row foot to, say, six per row foot, is surprisingly rapid. Research data show when numbers reach three per row foot, the yield and quality will be damaged significantly and the susceptibility to green bean syndrome leaps.”

Lorenz acknowledges it can be very difficult to keep populations at one per row foot. “We've had situations where stinkbugs are coming in from surrounding fields that are drying down. There have been fields that were treated with an insecticide and five days later the field is re-infested with very high stinkbug numbers.”

Do seed treatments help control the pests? “I began looking at seed treatments in 2003 because growers in the Arkansas Grand Prairie — Woodruff, Monroe, Lee counties — have chronic problems with grape colaspis (lespedeza worm) feeding on soybean seedling roots. There are operations that had to replant three times.”

In such situations, Lorenz wondered if Cruiser might be helpful. The first tests in 2003 showed extremely good results in small plots.

“There were some outstanding yield returns with the treatment. When I showed some of my Mid-South colleagues, they were skeptical at first. Around 2005, others did similar tests — by now, something like 50 studies have been done on seed treatments.”

On average, those studies show yields have increased between 3 and 4 bushels per acre. And in certain situations where grape colaspis or grubs — wireworms or rootworms — occasional yield results are 10 to 11 bushels higher.

“Long-term, with seed treatments we are looking to pin down where that yield response is coming from — because the bump isn't seen every time in every field.”

Perusing all the data, “if you're planting beans early — say, prior to April 15 — and you have any history of grape colaspis or wireworms, seed treatments may be beneficial. And seed treatments are even more critical for those areas considering seed supplies are short”. Two products are labeled: Gaucho and Cruiser. “We've seen positive results from both.”

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