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Late-season pests moving in grains

Over the last few years, stink bugs have been a major bother for Louisiana soybean producers. Of all the stink bug species, the southern greens, greens and browns are the most common in the state.

However, “over the last five years, we’ve had a species called the red-banded (show up),” said Roger Leonard, LSU AgCenter research entomologist at the recent Dean Lee Research Station field day outside Alexandria, La. “It’s considerably smaller than the southern green and has a light red or yellow band behind the head that runs transverse to the body.

“The red-banded is a bear, very tough to kill. We believe it is capable of flying farther than and capable of damaging beans much more than other species.”

The problem is further compounded because the red-banded stink bug is more difficult to control with current insecticides.

“We’ve lowered the threshold over the last two years from nine per 15 sweeps for other stink bugs to six per 15 sweeps for the red-banded. The reason: we don’t have as many effective insecticides and need to begin treating a bit earlier.

“Our research shows that when red-banded stink bugs make up 50 percent of the population, you need to use the lower threshold.”

Later-planted soybeans usually suffer the heaviest stink bug populations. That’s especially true at R-5/R-6 when “all the stink bugs in the area are moving from beans that have finished up. Fortunately, red-banded don’t like to move from one crop to another. Currently, they’re only found in soybeans. That’s good for growers who have a mosaic of crops.”

Another pest, the three-cornered alfalfa hopper, is considered a problem at seedling stage. It damages the main stem after the beans emerge and that initial injury can carry through the season with few signs. In fact, plants often don’t show symptoms of the feeding until they lodge after pods begin to fill.

“You can see this when using a sweep net or when the plant breaks over as you brush them with your hand. At that point, there’s no sense in treating for something that happened when the plant was 4 inches tall.

Three-cornered alfalfa hoppers require “better scouting — especially in later-planted beans, like those double-cropped with wheat. We have very effective insecticides, but we have to find them in order to treat.”

And farmers should know that “most of the insecticide seed treatments we recommend, like Cruiser and Gaucho, are very effective through V-3 on three-cornered alfalfa hoppers and can significantly reduce injury.”

Leonard hasn’t been a proponent of using such seed treatments on late-planted soybeans. “But at this point, it’s probably more important to deal with the hopper problems.

“Later in the growing season, this pest is girdling petioles. Many plants with just a couple of pods may be having trouble with three-cornered hoppers feeding in the field. They’ll clip pods and flowers and new populations of the pest turn over very rapidly.”

It wasn’t long ago that farmers could get by treating soybeans once or twice a year. Presently, LSU agriculture economists “have budgeted from three to five insecticide applications for the pest complex. In 2006, several thousand acres of soybeans were not harvested in Louisiana. Part of that was due to insects that remained out of control during late growth stages, R-6 and even R-7.

“So if you’ve got a late-season problem, especially with stink bugs, it needs to be managed. The pests need to be dealt with or you’ll suffer yield losses and certainly lessen seed quality and be docked at the elevator. In some cases, more than one application will be needed to clean up the problem.”

Grain sorghum

This year, Louisiana’s grain sorghum crop has seen variation in insect pressure. The early-planted crop got by without many insect problems or insecticide applications and produced very high yields.

Unfortunately, “the later-planted sorghum has had tremendous insect pressure. Some midge applications have exceeded five. And populations of web worms greater than 20 per head with significant yield losses (have been seen).”

There haven’t been many corn borers in Louisiana’s grain sorghum or corn. Fortunately, the weather — “which was good for corn development” — suppressed early populations and the pests haven’t been a factor until recently.

“Several years ago, we lost a lot of late-planted grain sorghum fields to corn borers. In fact, fields were never harvested. At this point, I’ve only heard of several (similar situations this year).”

Even so, corn borer populations are on the upswing and late-planted sorghum is very susceptible. Such fields need to be scouted “about twice a week. Look just below the head — that’s where they enter first. They’ll keep heads from filling out and, under high populations, there will be no grain to harvest.”

Midge applications are effective only while the plant is flowering. As long as the midge is present, depending on the variety planted, “you need to continue spraying at least on a weekly interval if you don’t get a rain to wash off the residual.

“You can’t stop scouting after midge. Web worms will move in. And the populations in Louisiana are resistant to pyrethroids. We must use other chemistries.”

The problem is a pyrethroid is likely to be used when spraying for midge. That means it won’t be effective on head worms and web worms and is the reason scouting is so critical.

“In a short time, web worms can be just as devastating as the sorghum midge. The good thing is they’re much easier to find — after shaking a few heads, you’ll know you’ve either got them or not.”

Late-planted sorghum remains in a very susceptible stage. Yields are going down not only because of planting date but also insect pressure.

“If you’re going to make a (decent) yield in those fields, continue scouting and spraying, as needed. You can’t quit at this point.”


Louisiana’s corn crop “is successful and will go down in the record books.” Leonard believes part of that is due to lack of insect pressure on two fronts: stalk-feeding insects and those that feed on the kernels and ears.

The sugarcane borer and southwestern corn borer populations were very light.

“Again, I think that’s due to weather. They’re coming on now and I anticipate they’ll be a problem in future when we have more normal, or conducive, weather conditions for their development.”

There aren’t very many years when a producer can step into a non-Bt (or unsprayed conventional) cornfield, and find kernels that go all the way to the ear’s tip. This year, “it was hard to find a field that had good pollination where there wasn’t grain to the end of the ear. That added several bushels to the yield and contributed to the high yields we’re currently harvesting.”

But the crop has had to deal with another late-season pest — one without a common name. “If you walk a mature grain field, pull back shucks, and see a small shadow between the kernels, break it open. You’ll find a small caterpillar feeding beneath the kernel on the edge of the cob.”

The caterpillar feeds on dry grain and is found on both Bt and non-Bt corn. It arrives after farmers quit scouting corn and everyone thinks the crop is safe from insects.

“We are seeing some injury to corn. We don’t know the extent of any yield losses yet. That’s what we’re studying now.

“This caterpillar is an exotic pest brought into the state. We aren’t sure how bad it is. We know it isn’t in every field but it has been found from Lake Providence to the Crowley Research Station. So it’s scattered across the state.”

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