Many Georgia cotton producers will remember this past growing season as one of spectacular yields. But Extension Entomologist Phillip Roberts will remember it as “the year of the stinkbug.”
“The ‘who dids’ and the ‘who did nots’ were very obvious this past year in Georgia as far as controlling stinkbugs,” says Roberts. “Some of our cotton this past year resembled cotton from back in the boll weevil days. We saw extremely high stinkbug populations in some parts of the state.”
Roberts reviewed Georgia's most recent cotton insect year at this winter's Georgia Cotton Workshop, held in Statesboro.
The year 2003 was a historic one in Georgia, he says, as no boll weevils were captured in the state. “This was the first year that boll weevils have zeroed out in Georgia. This has occurred for several reasons. One is that the Boll Weevil Eradication Program has expanded and pushed back the threat of boll weevils.
Also, growers and ginners have been more diligent in insuring that equipment is clean when it comes back into the state,” says Roberts.
Even better news, he adds, is that other states in the Southeast — such as the Carolinas and Virginia — also have zeroed out in boll weevil captures. Alabama reported only one boll weevil capture in 2003.
“We all know what boll weevil eradication has meant to our industry,” says Roberts. “We declared ourselves boll weevil-free in 1992, and we saw a big decrease in the number of insecticides being used. Bt cotton came on the scene in 1996 and we saw another big drop in insecticide applications. That technology, along with boll weevil eradication, have had a great impact on insect management in Georgia cotton.”
Georgia growers have been averaging two to two and one-half cotton insect sprays per season in recent years, he says. At the same time, however, treatments for stinkbugs have increased.
“It's important that we understand why this is occurring. Since boll weevil eradication and Bt cotton, we have used fewer broad-spectrum insecticides such as pyrethroids. Pyrethroids were knocking down or coincidentally controlling stinkbugs. When we removed those sprays from the system, the stinkbugs were left.”
Understanding why the stinkbug problem has developed will go a long ways towards helping growers manage the problem effectively and efficiently, notes Roberts.
“We're not just talking about the stinkbug. We're talking about a complex of boll-feeding bugs, whether they be the brown stinkbug, Southern green stinkbug, tarnished plant bug, leaf-footed bug or clouded plant bug.
“We talk a lot about beneficial insects and natural enemies. But, unfortunately, they don't help us much on the bug pests like they do on the caterpillars. That's just another reason why this problem has evolved.”
There are three primary species of stinkbugs, he explains, including the Southern green, green and brown stinkbug. In most years, the Southern green stinkbug will be most common in Georgia cotton fields. This past year, growers saw high populations of the brown stinkbug.
Due to their differing susceptibility to certain insecticides, the species of the stinkbug will help determine management strategies, he says.
Stinkbugs, says Roberts, have piercing, sucking mouthparts, and they damage cotton by feeding on the seeds of developing bolls. Stinkbugs feed by piercing the boll wall with their beak and injecting a digestive enzyme into the boll in or near the seed to soften or dissolve tissues, enabling the bug to remove them.
In addition, this process allows for entry of rot organisms, contributing to the degradation of bolls.
“When we treat stinkbugs in a cotton field, we should be treating adults that migrate from other host plants. These hosts may be weeds on the edge of the field or on the side of the road, brush in nearby woods, or other row crops.
“You or your scout typically will be detecting adult stinkbugs. If you start seeing a lot of immature stinkbugs in your cotton field, that should be a warning that you've dropped the ball at some point.”
An interesting point about stinkbugs, says Roberts, is their longevity. “If we make mistakes in trying to control corn earworms or tobacco budworms, the worm will cycle out in 12 days or two weeks at the most. A stinkbug may live for more than a month. Once a stinkbug establishes in your cotton field, it'll be there until the end.”
Stinkbugs also will move from a neighboring peanut field into cotton, he says. A good indicator of impending problems will be when stinkbugs are found on the edge of an adjacent peanut field, he says.
“We have the knowledge base to do a decent job of managing stinkbugs, but it's a matter of getting the job done.”
In southwest Georgia this past year, extremely high numbers of stinkbugs were found in some fields on pre-bloom cotton, says Roberts.
“Based on our experience and observations during the last few years, we did not believe a stinkbug would damage pre-bloom cotton. Research conducted in Louisiana demonstrated that stinkbugs do not damage pre-bloom cotton. We start having damage potential when we get to bloom.”
The recommended economic threshold using internal boll injury has been that when 20 percent of quarter-sized bolls display internal signs of stinkbug feeding, and stinkbugs are observed in the field, it's time to begin treatments, says Roberts.
“We'll change that recommendation this year to state that boll sampling should begin when bolls are present. If no bolls the diameter of a quarter can be found, the largest bolls present should be sampled. Once cotton begins to flower, it becomes susceptible.”
Pyrethroids and organophosphates will do an excellent job on green stinkbugs, he says. The organophosphates also work well on brown stinkbugs, but pyrethroids aren't as effective on this particular species, he adds.
“In some situations — like in late August — bugs will be your only target, and organophosphates will be your material of choice. There are other times, however, when you might have corn earworms on Bt cotton, in addition to fall armyworms and bugs. That's a great fit for pyrethroids.”
Stinkbugs can't be controlled without disrupting beneficial insects, says Roberts. “Fortunately, stinkbugs primarily are a problem late in the year.
We can walk a fine line and keep beneficials in the system as long as possible. But we have to bite the bullet when the time comes to treat stinkbugs. It's much more important that we don't disrupt the system in June and the early part of July. But we need to protect cotton as we move into August.”