Life cycle increases stinkbug threat

Many cotton producers in the South accept too much stinkbug damage when it can be easily and inexpensively avoided, says Ron Smith, Auburn University Extension entomologist.

“We must consider the long life cycle of the stinkbug,” says Smith. “The adult stinkbug likely will be in the field for almost the entire growing season, and no other cotton insect pest has such a long life cycle.”

Stinkbugs have become a major insect pest of Southern cotton, primarily due to a low spray environment, he says. “In Alabama, we're hardly spraying cotton at all. We averaged less than two sprays last year. And some of those are worm sprays with materials that won't kill bugs,” he says.

This trend, he adds, likely will continue, especially in the Coastal Plains area of the Southeast, including south Alabama and about 1 million acres in Georgia. This area also has mild winters, he continues, which contribute to the increase in stinkbugs.

“We knew this would happen based on growers' experiences in the Carolinas after boll weevil eradication. It's actually a greater problem than we imagined because of the lack of winterkill in our region. In areas such as Baldwin County, Ala., farmers can't even grow corn without applying stinkbug controls. Stinkbugs will destroy the stalks,” says Smith.

Growers need to be more conscious of thresholds when treating for stinkbugs, he says. “Consultants are reporting that our thresholds are way out of line — that we're waiting too late before spraying. And part of this may be due to the stinkbug's long life cycle. It takes about 30 to 35 days for the stinkbug to develop into an adult, and the adults can live for two months. In other words, when stinkbugs get into cotton, they don't need to go anywhere else because they can feed on the bolls.

“And, basically, they're only a problem in cotton during the boll set period. That would begin in July and continue until the youngest bolls are about 25 days old. At that point, they're almost too hard for a stinkbug to do much damage.”

Surveying for stinkbugs isn't easy, notes Smith. One method is to look for stinkbugs using the drop cloth method.

“You can lay down a drop cloth and beat the plants to see if the stinkbugs drop off. But stinkbugs are very sensitive to noise, and this method can scare them away. So, if you're sampling with this method, you need to walk a distance and get away from your last sample to find the next one. You've just about got to sneak up on them. If you disturb the cotton or make noise, they'll move.”

The most accurate method for detecting stinkbugs is by slicing quarter-sized bolls, says the entomologist. A boll of this size still is soft and is most preferred by stinkbugs, he says.

“You need to look for calloused areas on the inside of the boll wall, deteriorating areas inside the boll or yellow-stained lint. All of these are symptoms indicating that part of that boll is going to rot before harvest.”

The reason for slicing bolls is that not all bolls with outside damage also will have internal damage, says Smith. On the other hand, some bolls that appear to have internal stinkbug damage will show no evidence of damage on the outside.

Stinkbugs often don't damage the entire boll, says Smith, and only one or two locks may be damaged.

“Many consultants in the coastal areas are going strictly by physical sightings. If they sight multiple stinkbugs in a field, they feel they have a problem. Stinkbugs aren't easy to see. And, if you see several, there are many more that you're not seeing.”

Stinkbugs, says Smith, do not clump early after hatching but instead clump for several weeks as they're going through immature instars, he says.

“They don't fly — they only crawl, and they'll pretty much stay in the same general area before dispersing.”

Stinkbugs also can move into cotton from adjacent crops, says Smith. They can come out of wheat early in the growing season, from corn at mid-season and from peanuts later in the year. If cotton matures on time, peanuts probably won't be a problem, he adds. In addition, there can be three or more generations of stinkbugs per year.

Almost everyone, says Smith, uses the same threshold for sliced boll damage — 15 to 20 percent damage. “That may seem like a lot of bolls, but it's not all of the bolls in the field. It's just the ones that are quarter-sized in diameter during that particular week.”

Smith says he disagrees with the other threshold measurement used by most states when sampling for immature or adult stinkbugs.

“Most states are using one per 6 row feet. If you calculate the number of row feet in an acre, this threshold translates into about 2,000 stinkbugs per acre. I don't think anyone with a reasonable knowledge of stinkbug damage would want to allow 2,000 stinkbugs per acre, especially considering the pest may live for two months. So, we're using one stinkbug per 20 row feet as our threshold in Alabama.

“Our growers need to be triggering quicker for stinkbug sprays. Basically, the damage is done from about the third week of bloom to maturity. That's a time period during which we're not so concerned with beneficial insects. We feel that many growers in Alabama are waiting too late before spraying for stinkbugs in an attempt to conserve beneficial insects. Once you spray for stinkbugs, you should get about two-plus weeks of control before another treatment is needed.”

The stinkbug, says Smith, is one of the most inexpensive cotton insect pests to treat, with controls costing $3.50 to $4 per acre. “We can achieve nearly 100 percent control for the $3.50 to $4 per acre. At this price, we shouldn't be afraid to spray for stinkbugs.”

Weather also can play a role in stinkbug damage, he says. “If we have a wet, humid fall, more of these stinkbug-damaged bolls will rot. If we have a perfect fall, we'll see less rot from stinkbug damage.”

Phosphate insecticides are most effective on stinkbugs, he says. These include Bidrin, methyl parathion and the encapsulated methyl parathion Penncap-M. These products, says Smith, will give 95-plus percent control of stinkbugs. Pyrethroids do an adequate job, with about 80 percent control of the Southern green stinkbug.

e-mail: [email protected].

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