In Alabama and other parts of the Southeast, the stink bug has become the dominant cotton insect pest, and that isn't likely to change anything soon. “We're talking about more than 1.5 million acres of cotton now where the stink bug is dominating, and it's becoming more important each year. As we grow more peanuts and cotton together, the problem will increase,” says Ron Smith, Auburn University Extension entomologist.
There are three stinkbugs — the green stink bug, the brown stink bug and the Southern green stink bug, he says. In most years, Southeastern cotton growers see the Southern green stink bug.
The damage threshold is currently 20-percent injury to the internal part of the boll,” says Smith. “We select 10 to 12-day-old bolls, when the bolls are still soft and can be crushed in your hands. You have to peel back the boll and look inside for internal damage. When we target brown stink bugs, we have to switch chemistries and go with phosphate materials. With green stink bugs, we have a lot of choices, including pyrethroids and products like Orthene or Bidrin.”
The migration of stink bugs from other crops has become more important as an increasing number of Alabama growers plant peanuts adjacent to cotton, says Smith. “Stink bugs have a clumping effect. If about 50 come from an egg mass, then you know they will be clumped together there for awhile. They spread as they get older. Most of the damage to cotton is done by the almost mature or the last instar of the immature stink bug and the adult. It takes them from the time they hatch until they are about four weeks old before they can do damage. This is a long life span, and the adult then lives for four or five more weeks,” he says.
If a cotton producer has adult stink bugs in his field in July, they will be there until the youngest bolls that are harvested are mature, says Smith. They will live for another month or more.
“In looking for internal damage, we look for a brown seedcoat and warts and calluses on the inside of the boll where stink bug punctures have healed. Yellow lint isn't as common as these other symptoms. What you end up with is one or more locks on a half boll that is completely rotted and won't pick. Sometimes, you'll have a hardlocked boll, and it'll be white. It won't be totally rotted, but you can't pick it because it isn't fluffed out.”
If cotton is planted near corn or peanuts, there could be a border effect, says Smith. “Stink bugs move only about 75 feet into a field in the beginning, but then they eventually scatter across the field. We need to think in terms of doing more border spraying, especially if we're next to a crop from which stink bugs are moving.”
If stink bugs damage a young boll, or if they feed on a boll long enough, they'll cause the entire boll either to abort or to come off the plant, he says. Another insect — the leaf-footed plant bug — causes boll damage similar to the stink bug.
“We conducted a study at Headland in southwest Alabama this past year on stink bugs and thresholds, to try and get a better handle on how many stink bugs are needed before we begin to spray. We had three thresholds planted in 90-foot strips and replicated four times. We planted eight rows of cotton in the middle of a peanut field so stink bugs would be coming in throughout the season. It's not that different a situation than the border of any field that is adjacent to peanuts.
“There were three threshold levels — untreated, where we didn't spray at all; 20-percent damage or the university recommendation; and a treatment aimed at making the strip free of any stink bugs.”
The bug-free strip, he says, was sprayed about seven times, from July 19 to Sept. 8. The 20-percent threshold was sprayed five times, but it still wasn't enough. Researchers recorded the amount of damage and the number of stink bugs per 12 row feet in each strip. The untreated had 91-percent boll damage from July 19 through the remainder of the year, with an average of 8.1 stink bugs per 12 row feet.
The 20-percent threshold damage strips averaged 30-percent damage even with the five sprays, with two and a half stink bugs per 12 row feet. The bug-free strips still averaged 16-percent boll damage.
“The point is you can't look for stink bugs and try to evaluate whether or not to spray. You'll get about 10-percent damage before you detect them in your field. We think we can live with that 20-percent damage threshold, but there will be key weeks during the season when that won't be good enough. We think the third, fourth and fifth weeks after bloom are critical when treating for stink bugs.”
As for yield averages, the untreated cotton averaged 263 pounds per acre, the 20-percent damage threshold averaged 1,375 pounds, and the cotton that was sprayed seven times averaged 1,714 pounds of lint per acre.
There are now pockets in the Southeast, including 100,000-plus acres in Georgia, where bollworms could not be controlled by over-spraying Bt cotton, says Smith. If you get enough escapes, they can do a lot of damage. They were getting up to 15-plus percent boll damage in Bollgard cotton in Georgia due to sheer numbers. Three pyrethroid sprays in five days didn't do the job.
“There is a major problem throughout the United States, and especially in the Midwest where they're growing sweet corn — those growers can no longer use pyrethroids when spraying for bollworms. After 30 years of good control, the day is coming when pyrethroids won't do the job on the bollworm species.”
If growers have such a failure, Smith advises that they switch chemistries. “There are a few things you can do, including going with the higher labeled rates, and getting it on as quick as you can after the worms hatch. That'll buy you some time. If you're not planting Bt cotton, and the resistance comes in, it'll force you to plant Bt cotton.”
Look at wheat
If you want an indication of what kind of stink bug year you'll have, Smith says you can look at your wheat crop after it has been combined.
“Otherwise, look on corn before it's time for the stink bugs to move into cotton. Stink bugs will move from corn in one large, sharp movement, as the corn dries down. But stink bugs can move from peanuts into cotton throughout the year. They'll also move from pecan trees.”
As for residual control of stink bugs in cotton, you'll get about three or four days of residual control with products like pyrethroids, Bidrin and Orthene, he says. “A cotton boll is safe from stink bugs when it is about 25 days old. You should sample about 25 to 50 bolls in a field to get a damage assessment.”