Cotton insect shift likely to continue

EDITOR'S NOTE — In this and future issues of Southeast Farm Press, we'll take a look at cotton insect control trends and recommendations from the various states in the region. Presentations in this series of articles will come from Extension entomologists in Virginia, North Carolina, South Carolina, Georgia and Alabama.

The shift of insect populations adjusting to Bollgard and other genetically modified cotton varieties continues. In addition to reducing the number of sprays in the field, the technology has generated a shift in insect population — moving once-secondary insects to primary concern.

In fact, says one North Carolina State University Extension entomologist, the shift is likely to become even more pronounced than it already is.

When Bt cotton was introduced, a shift occurred from caterpillars to stinkbugs and plant bugs. The idea behind the technology was to reduce sprays. On average, cotton farmers now spray .8 times, or on average, less than one spray.

When the second generation of Bollgard cotton grows in farmer fields — expected date of release is 2005 — that insect shift will become more pronounced, making sprays for bollworms virtually unnecessary.

Cotton thrips and aphids have remained about the same as before the advent of Bollgard cotton.

The shift in insect importance makes scouting even more important than it is today, says Jack Bacheler, North Carolina State Extension entomologist.

Bacheler told farmers attending the Southeast Cotton Conference recently in Raleigh that stinkbugs and plant bugs require close scouting. Extension has fine-tuned its threshold levels in an effort to help farmers get a better handle on a secondary pest that has become a primary concern.

While external damage may be an indication of internal problems, there's not a clear relationship that points to the problem being stinkbugs or plant bugs.

“There's not much correlation between external damage and internal damage,” Bacheler says. “If you see internal damage after cutting the boll, then you know you've got stinkbug or plant bug damage.”

That uncertainty led Extension to make internal warts part of the diagnosis criteria for stinkbugs this season. In extreme cases, heavy bug feeding can make the small bolls fall off the plant, and hard lock can occur.

To determine economic stinkbug reach threshold levels in Bollgard cotton, Bacheler recommends looking at a minimum of 50, quarter-sized bolls. “For these bolls you can't go on the basis of external damage alone,” he says.

“You have to crush open or cut open the bolls and count the internal damage on the bolls,” Bacheler says. “Our threshold is to treat when the damage is 10 percent.”

In 2001, the stinkbug damage didn't reduce yields nearly as much as compared to the damage resulting from a wet fall.

“Waiting until the bolls are quarter-sized gives you enough time for the stinkbug damage to accumulate,” he says.

The older the cotton gets, the less likely it will have economic stinkbug or plant bug problems.

“I hope consultants will think about the first four and a half weeks after bloom as being the critical time for stinkbug or plant bug damage,” Bacheler says. Researchers across the Cotton Belt are working toward adjusting stinkbug and plant bug thresholds.

Bolls that are over an inch and a quarter in diameter are generally outside of the damage reach of stinkbugs, Bacheler says.

While the green stinkbug is predominant in North Carolina, cotton farmers have begun to notice brown stinkbugs in the past couple of years. “With brown stinkbugs, you may need to use Bidrin along with the normal rate of a pyrethroid. You need to determine what color the stinkbugs are.”

In the course of scouting, visual observations can help you determine green versus brown stinkbugs. In the immature stage, brown stinkbugs are flat and are a uniform pale greenish yellow, while the green stinkbug nymphs have areas of black and are more robust.

Plant bugs have also benefited in the shift toward Bt cotton. Early in the season plant bugs damage small squares, causing them to fall of the plant. Later in the season, plant bugs still damage smaller squares, but they also damage larger squares, feed in blooms and damage small to medium sized bolls.

In some years, it's difficult to tell the difference between stinkbug and plant bug damage. Plant bugs, however, feed on the squares when the bloom opens up. The pollen anthers of an open or white bloom turn brown or black from plant bug damage.

The threshold level for plant bug treatment is 15 percent “dirty” blooms, Bacheler says. Growers in Alabama have successfully determined when to spray by looking for dirty blooms. A sweep net can also be used. “If nymphs are present, that means reproduction has been successful.”

Bottom line

Stinkbugs and plant bugs will continue to be a concern as insect pressures adjust to the new technology.

The near-term introduction of Bollgard II should eliminate the need to spray for caterpillars. This further reduction in caterpillar sprays will likely translate into even higher levels of bugs and their damage.

“It looks to me that we may be scouting primarily for plantbugs and stink bugs,” Bacheler said. “I think this whole thing with bugs is going to shift more. Stinkbugs are already a problem and they probably become more of a problem.”

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