Research conducted in Georgia has shown consistently that stink bug damage does affect cotton quality, says Phillip Roberts, University of Georgia Extension entomologist.
"There has been a lot of discussion about fiber quality within the past couple of years, especially for cotton being produced in Georgia," says Roberts. "And several factors in producing a crop can impact fiber quality, from variety to timely harvest."
Research in Georgia in recent years, he says, has shown a relationship between fiber quality and stink bug damage. "Everyone is familiar with stink bugs — they're boll feeders. They look to feed on developing seeds inside a boll. When they feed, you can have physical damage to that seed and to the lint associated with that seed."
Pathogens, he adds, also may be introduced into that boll. Pathogens that cause boll rot may enter through the wound in the developing boll.
"In Georgia, stink bugs are probably our most important pest, particularly in our stacked varieties where we're using Bollgard technology," says Roberts. "The type damage you see from stink bugs can vary dramatically from little or no damage where a stink bug is found on a boll to bolls where we may see an individual lock with discoloration, or individual locks that fail to fluff and open — locks that have hard lock."
In worst-case scenarios, boll rot can occur, he says. "We don't understand how this damage occurs. It probably has something to do with the environment, such as rainfall."
Stink bugs are a serious pest in Georgia, says Roberts. "In 2003, there were tough lessons learned as we had very high pressure. Coincidentally, that was a tough year in terms of fiber quality. In 2004, our growers responded and were very effective in managing stink bug damage."
For about the first three weeks of a cotton boll's life, individual fibers are elongated, he says. After fibers elongate, they begin to thicken.
"When do stink bugs feed on developing bolls, and how long do you need to protect that developing boll from stink bugs in terms of yield? You need to protect the boll in terms of yield for about 45 days. That's the time frame when stink bugs can damage developing bolls. That corresponds very closely with when those fibers are elongated. Stink bugs prefer to feed on a developing boll when it is about 12 to 14 days old."
Georgia researchers looked at treated and untreated stink bug plots at four different locations, says Phillips. "Of those four locations, we can see the yield response from treating for stink bugs. It ranged from 2 percent or little yield response to more than 30 percent where we had severe pressure.
"We hand-harvested first-position bolls on each of the plots by node. We basically got all of the first-position fruit from nodes 5,6, and 7, all the way up the plant, and we got individual quality data at each node in the treated and untreated plots."
The treated cotton was ginned on a table-top gin, separating the seed from the lint. Then, the fiber was sent to Cotton Inc. for HVI and AFIS data.
"Starting at the worst-case scenario, we saw a difference in yield of about 30 percent. This is a location with heavy damage, but we still made 1,300 pounds in the untreated. When we sprayed for stink bugs, however, we did quite better. Our intent was to minimize stink bug damage, so we sprayed 8 ounces of Bidrin once a week.
"The mean length of the fibers at node 10 in the untreated cotton was about 1 inch. In the treated, the mean length was about 1.05 inches. That doesn't seem like much difference, but .05 is about 1.5 to 2 staple lengths, and that's a big difference."
There's not a big difference, says Roberts, in the mean length between the treated and untreated on the bottom part of the plant, in nodes five through nine. In the top half of the plant, there is a significant difference in the length of the fibers.
"This mean fiber length information is not what you see on HVI. This is AFIS data, which is similar to HVI but is a very thorough investigation of the fiber. It is a mean length of all the fibers, while HVI is the upper half mean length."
In the second location, there was about a 16-percent yield response where researchers treated for stink bugs, and there was a significant difference in length up and down the plant.
"The difference in yield response was smaller at the third location because there wasn't as much stink bug damage. There was probably about a 50-pound response at this location to spraying when we sprayed with a pyrethroid for stink bugs. And, going from node eight to node 20, there's a tendency towards longer fiber length where we treated."
The final location was a small plot trial that showed only a 2-percent difference in yields between treated and untreated cotton. There wasn't a lot of difference in the mean fiber length, but there is a trend towards larger mean length in the treated cotton.
"The tests showed that the greatest improvement in fiber quality occurred where we had our greatest yield response or our most stink bug damage. And at just about every node, there's a tendency for the distance in length to always be positive where we treated for stink bugs."
Short fiber content has been a major fiber quality issue among textile mills, says Roberts. "Short fiber, in terms of our data, is fiber that is less than one half inch in length. You want to produce long fibers that are exactly the same length — that's what mills want. In Georgia, we tend to have a high percentage of short fibers compared to other states.
"Where we've treated for stink bugs, we decreased the percent of short fiber content, especially where we had our heaviest pressure.
You also want a more uniform fiber. Where we treated for stink bugs, we decreased the variation of fiber length."
Georgia data shows consistently that stink bug damage affects fiber quality, says Roberts. "However, we hand-picked all of this cotton, and we got it all, damaged or not. This also went through a table-top gin. We're looking at a worse-case scenario."
Cotton from one of the locations was run through a micro-gin recently, he says. The micro-gin simulates the commercial ginning process.
"The percent turnout where we treated for stink bugs was a lot higher than where we didn't treat. This is in the location where we saw a good difference in yield."