Even though North Carolina cotton producers should now be aware that stink bugs and plant bugs are most often the primary insect management target in Bt cotton, much of the lesson that could have been learned in 2004 was probably overshadowed by record yields.
While we can hope that stink bug levels and damage will be less than last year's tsunami, producers can not afford to take this pest group lightly.
In its first nine years (1996 to 2004), Bollgard cotton acreage received an average of 0.95 late season treatments in North Carolina, varying from none to a high of about two applications. Most of these sprays have been targeted for bollworms.
During this period, however, stink bug damage to bolls averaged approximately 3 percent on Bollgard cotton, while bollworm damage to bolls on Bollgard cotton was on the order of 1 percent, based on extensive fall damaged boll surveys of producer-managed cotton fields.
During this time period, many producers probably under-appreciated the impact of the bug complex on late season boll damage. In 2004, a ”perfect storm” of poorly understood circumstances resulted in unprecedented brown and green stink bug populations, causing an average of 15 percent boll damage across the state.
To add insult to injury, this damage began early in 2004, at initial boll formation in some cases, and the proportion of harder-to-control brown stink bugs vs. green stink bugs was generally much higher than in the past.
Even on our conventional cotton in 2004, which averaged three applications, stink bug damage to bolls was a record-setting 5 percent, (higher than any previous year's boll damage to Bollgard cotton).
Stink bug damage to bolls vs. yield reductions:
Unfortunately, the relationship between stink bug 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 yield 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 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 stink bugs scored some significant boll damage.
Stink bug scouting:
Although stink bug thresholds vary somewhat across the Cotton Belt, generally ranging from 10 to 20 percent, virtually all entomologists agree that assessing internal damage to quarter-sized bolls is probably the best current indicator of recent stink bug feeding and potential yield loss.
Although data collected here and elsewhere suggest that bug damage to bolls during the first two weeks of flowering is less likely to impact yields than feeding during weeks three to six or seven, we nevertheless recommend that boll assessments begin within a week of blooming initiation.
The cutting open or crushing of approximately 50 bolls per cotton field should provide an adequate sample size to determine if an action threshold has been met.
In North Carolina, as in most other states, we suggest counting warts on the internal boll wall and stained lint toward the boll damage threshold.
Dynamic thresholds: We recommend an initial action threshold of 10 percent internal boll damage for approximately the first four weeks of blooming when all of the bolls are susceptible to stink bug damage (however, based on preliminary research, the threshold may be closer to 15 to 20 percent during the first two weeks of blooming due to the lower boll population).
After the first four weeks of blooming, the proportion of bolls no longer susceptible to bug damage increases steadily until cutout.
When developing bolls reach approximately 3.5 weeks old, in most cases they are no longer susceptible to stink bug damage. Also at this “bug-safe” time, bolls are often about 1.25 inches in diameter (although boll size shows some variability due to variety, moisture availability, fruit position and how far into the blooming period).
Therefore, damaged boll thresholds can be increased somewhat later in the bloom period. For example, if half of the bolls in a cotton field have bolls sizes over 1.25 inches and half less than this diameter, our 10 percent threshold could be raised accordingly — to 20 percent, and so on.
Making this assessment can be done by measuring or estimating boll diameters in a few feet of row in a representative part of the field. By employing this ”dynamic threshold” approach, producers should be able to avoid unnecessary late season application for stink bugs.
Further details may be found at the NCSU Cotton Insect Corner Web site, or in the 2005 Cotton Information booklet.