Improving Georgia's cotton quality can be compared to putting together a jigsaw puzzle, with several pieces fitting together to form a solution. One of those pieces might be controlling boll-feeding bugs, says Phillip Roberts, University of Georgia Extension entomologist.
“This is just one piece of the puzzle,” says Roberts. “We're trying to understand if it fits as part of the solution to our cotton quality problem.”
In Georgia, any discussion on boll-feeding bugs focuses on stink bugs, he says. “But we also know that other bugs, like plant bugs, are capable of feeding on a developing boll,” he says.
Preliminary data from University of Georgia researchers shows the impact such pests can have on fiber quality, says Roberts. “The fiber quality issue is a complex one. Most likely, there will be a lot of different factors in solving this problem, and perhaps controlling stink bugs is one of those factors,” he says.
Most growers, says Roberts, are familiar with stink bug damage and are aware that stink bugs have become a primary insect pest of cotton in Georgia.
“The year 2003 was a difficult one for stink bug problems in cotton. This year, for the most part, much of the cotton in Georgia has been treated for boll-feeding bugs,” he says.
The University of Georgia has been estimating cotton losses to stink bugs since the early 1990s, according to Roberts. “The university has been making cotton insect loss estimates since the early 1970s. But stink bugs didn't show up on our radar and weren't even mentioned in cotton until 1992,” he says.
Stink bugs didn't become a real problem for Georgia cotton producers until about 1997, he adds. “In 1996, we began planting Bt cotton. We eliminated a lot of the pyrethroid sprays we had been making for worm pests, and stink bugs were allowed to continue existing. This is a problem that appeared to have stabilized. However, 2003 was a difficult year.”
The national report of cotton insect losses for 2003 — sponsored by the National Cotton Foundation — shows that stink bugs were the primary insect pest of cotton in Georgia, in terms of acres treated and bales lost.
In 2003, an estimated 1,150,000 acres of Georgia cotton were infested with stink bugs, with 900,000 of those acres treated for the pest. It also is estimated that 71,875 bales of the state's crop were lost to stink bug damage.
To make a connection between stink bug damage and fiber quality, it's helpful, says Roberts, to understand cotton fiber development.
“Fiber in a developing boll lengthens for about the first three weeks of that boll's life. After about three weeks, the majority of our length has been attained. A lot of the thickening of the fiber occurs after the first three weeks.
“We feel that the ideal time for a stink bug to begin feeding on bolls is when the bolls are about 12 days old. That's when we've advised growers to begin sampling bolls for internal damage — when they're about the diameter of a quarter. We know stink bugs feed on small bolls that are just a few days old, and they also can feed on bolls that are about 22 to 23 days of age.”
Stink bugs, he says, are looking to feed on the fibrous part of the seed. If the stink bug damages the seed, then fiber quality will be impacted, he adds.
“We're still confident that you can use our current threshholds to maximize cotton yields,” says Roberts.
Research trials conducted a couple of years ago looked at extreme cases of stink bug damage, says the entomologist, in untreated versus treated cotton.
“We took the plants to yield, and we looked at the fiber properties. We had two locations, and we harvested the first-position bolls at nodes 5-8, 9-11, 12-14, 15-17 and 18-plus. In this trial, we did see some difference in the cotton treated for stink bugs versus the untreated cotton. The percent of short fibers was much higher in the untreated compared to where we treated. This is a worst-case scenario where stink bugs were left completely untreated.”
More recent data from researchers at Clemson University showed a similar trend, says Roberts. “They went into a field of cotton late in the year and hand-picked bolls with stink bug damage versus bolls that appeared to be normal. Again, in this trial, if the boll was damaged by stink bugs, the fiber quality was less.
“But we need to ask ourselves if fiber quality is being negatively affected by stink bugs in commercial production fields. We're not sure, but we're looking at it very hard. When we manage stink bugs, do we need to be more aggressive? We don't know the answer yet, but we're looking closer at it this year.”
Researchers have demonstrated that in worst-case scenarios, stink bug damage does impact quality, he says.