Late-season boll damage is puzzling

Stinkbugs were a consistent and widespread problem throughout the upper Southeast in 2006. Late season boll damage was another constant problem throughout the 2006 season, and some believe brown and green stinkbugs were the culprits.

At the recent North Carolina Agriculture Consultants Association meeting there was a consensus among members that stinkbugs were a big problem.

Heavy damage

Several consultants noted seeing heavy damage, attributed to stinkbugs, late in the growing season.

Ames Herbert, an entomologist and head of Virginia Tech's IPM program and long-time North Carolina State University Entomologist Jack Bacheler, conducted an informal clinic on cotton insect problems and shared some findings that mirrored the late season concerns of consultants.

“In some of the fields I manage, as I watched the boll as it got larger, it got past the point where it should be safe, and I found damage, particular in the crease or the bottom of the boll,” asked Stan Winslow, a consultant in Camden, N.C. “I'm not sure whether the damage occurred late, or I didn't see it earlier,” he continued.

Bacheler says it is difficult after a boll is a month or so old for a stinkbug to penetrate a boll wall. “We look at bolls all year and it's very difficult to differentiate stink bug damage from damage caused by many other internal and external factors. Sometimes we get mystery damage in year-end bolls in the form of a big brown spot at the base, but we don't think that is stinkbug damage,” he says.

Herbert says his research team found a number of basal punctures to cotton bolls in 2006. There is no evidence at all on the outside, but when you break open the boll, you see locks that are completely destroyed. There is a weak spot where the locks come together that seem to be susceptible to stinkbug penetration and feeding, he says.

Herbert contends that there may be a variety relationship to late season damage. Some varieties have thinner boll walls and can be susceptible to stinkbug damage longer than other varieties, he adds.

“If you put yourself in the head of a stinkbug, you are not going to an old boll to feed, if there are young bolls on the plant. If I'm a stinkbug, I am going to the quarter-size bolls that are easier to get into. I want to feed on the seed and the little bolls aren't as desirable because the seed haven't developed. The older bolls are more difficult to penetrate. So I am going after the quarter-size bolls that have tender seeds and the boll wall is easier to penetrate,” he explains.

Bacheler agrees that some varieties are more susceptible than others to stinkbug damage, but contends work at North Carolina State University has not been successful in determining which varieties are more or less susceptible.

“We see a lot of mature bolls that are spotted up and have a lot of external damage, but the boll wall is too tough for the stinkbug to penetrate. When you see a large number of bolls spotted up, chances are there will be heavier than normal stinkbug damage. However, sometimes you see the external spotting and find very little actual damage to the boll,” Bacheler says.

Unfortunately, the relationship between stinkbug damage to bolls and its impact on cotton yields appears to vary significantly from field to field and from year to year. In 2004 for example, in four “bug-protected” vs. untreated replicated tests, each 10 percent year-end boll damage resulted in yield losses ranging from 36 to 107 pounds of lint per acre.

Given that the damaged boll assessments were taken just prior to boll opening, the relationship between bug damage and yields would likely have been even more variable if these evaluations had taken place earlier when these bolls were quarter-sized. The mean yields loss of these four tests was approximately 60 pounds of lint per 10 percent boll damage, or 7 pounds of lint for every 1 percent damaged boll (the narrower range for bollworm damage to bolls varied from 8.9 to 19.3 pounds of lint per 1 percent boll damage in a series of 8 bollworm threshold tests).

“Looking back at our 2004 state-wide boll damage of 15 percent by bugs on Bt cotton, by any yardstick stinkbugs scored some significant boll damage,” Bacheler says.

“Stinkbugs damage growing cotton bolls by piercing the boll wall and feeding on the seeds and juices within the bolls. We conducted this research to determine whether this damage influences cotton textile mill performance. We found that fiber properties and yarn and fabric quality were not improved with insecticide applications to control this pest,” says Philip Bauer, with USDA.

Danny Pierce, a crop consultant in Princeton, N.C., says last year he saw more cumulative damage late in the season from stinkbugs and worms. “In one field, we sprayed a high rate of a pyrethroid and Bidrin when bolls were first susceptible. Then, we sprayed two more times with a high rate of pyrethroids,” he notes.

Despite the three applications on this Bollguard cotton, applied every two weeks, Pierce recorded 40 percent bollworm damage. Bacheler came back and checked the same field and rated the field at 30 percent damage. That is another problem, Pierce says, damage ratings are variable, and until the crop is harvested, there is little way to know what the real damage is to the crop.

Winslow says he seems to see more stinkbug damage on early planted cotton. Herbert says that stinkbugs tend to come into early planted fields, lay eggs, and stay as long as the food source is there. So, it's likely that early-planted cotton will get more damage because the crop is susceptible for a longer period of time.

“We are trying to quantify how much damage is actually done by stinkbugs to the internal boll. If you go out and collect bolls from a field, sometimes you are likely to find external damage, pop the boll open and find the kind of damage you expect. But, in other cases, you see the same amount of external feeding damage, pop the boll open and there is no damage.”

“We are in our second year of a test that we hope will provide some answers and some guidelines for determining the relationship between external feeding damage and actual damage to the boll.

To do this, we are marking two sets of bolls. We look at 100 bolls with external damage, pop them open and categorize internal damage. We mark a second 100 bolls, tag and number these, and leave them in the field. “For every number, we record the external punctures, and come back when bolls open and determine damage to the lint.

“We categorize the lint damage and have those bolls ginned and graded. Hopefully, we will know how much lint is lost and how much quality is affected. Our hope is to deliver back to growers a rating scale with a fairly high reliability of how to relate external damage to actual boll damage,” Herbert explains.

In cooperative tests with researchers in South Carolina and Georgia, North Carolina State scientists are seeking to determine what effect external damage has on yields. “On the basis of seven tests this season, in fields treated with insecticides, there does not seem to be significant yield damage to plant bugs,” Bacheler says.

In North Carolina, in some tests we saw only 10-15 percent damaged bolls and got a big yield difference. In other tests, we saw 30 percent external puncture damage, but no yield damage. Averaged across all the tests, in fields with heavy stinkbug populations, we saw little correlation between feeding damage and yield damage, he says.

“There is no doubt the damage is done early in the season to young cotton bolls, but by the end of the season, the plant is able to compensate for this early season damage. It is not clear whether the plant compensates from this early damage, or whether the damage is not yield-robbing. Cotton has tremendous ability to compensate for damage, which is affected by soil type, climate and many other factors, making damage evaluation very difficult.

“In Virginia, I believe we are over-estimating yield damage based on external punctures,” Herbert says. “The threshold may change from 50 percent the first week down to 30 percent the second week and 15 percent the third week,” Bacheler says. He adds that all these thresholds will be affected by weather and other factors.

Assessing damage to final cotton yields and quality based on stinkbug damage early in the season is difficult at best, Herbert concludes. Herbert and Bacheler agree there will be some guidelines, but these will be more starting points than absolute recommendations for evaluating damage.

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