North Carolina's cotton producers have turned to Bollgard cotton to the tune of approximately 50 percent again this season. With an average up-front investment of $20 per acre in the Bollgard technology fee here, the bill for a significant portion of the insect control costs is due well before the first late-season bollworm or stink bug takes to a cotton field.
However, by carefully scouting Bollgard cotton for bollworms, stink bugs, plant bugs and occasionally fall armyworms, and by following established thresholds, producers can manage this new technology for maximum profit.
With our typically high mid to late July and August bollworm moth flights, Bollgard cotton is often exposed to treatable levels of bollworms in North Carolina.
In the five years of its commercial availability, Bollgard cotton acreage received an average of 0.78 late season treatments. Hidden within this average, however, is a range of cotton fields which required one, two, rarely three, or no insecticide application. This underscores the importance of good scouting with this technology.
As is the case with conventional cotton, most insecticide treatments are targeted for bollworms. However, stink bugs are often figured into Bollgard treatment decisions.
The recommended procedures for scouting Bt cotton for insects are similar to those used in conventional cotton. However, the relative status of our late season insects is different between Bollgard cotton and the more heavily-treated conventional cotton (no budworms and fewer European corn borers, and higher levels of stink bugs and plant bugs).
Although more late-season stink bug and plant bug boll damage is expected with Bt cotton than with conventional cotton again this year, the thresholds for both are the same as with conventional cotton. However, because of the way the Bollgard gene functions to kill caterpillars, thresholds for bollworm eggs and caterpillar size worms must be adjusted for Bt cotton.
Because hatching bollworms must feed on cotton to ingest a lethal amount of the toxin within Bollgard cotton plants, scouts should not use low numbers of eggs or first stage bollworms as a treatment trigger, but only as an indication of potential pressure to help gauge scouting frequency.
Scouts should direct their attention to detecting high egg levels, economic square or boll damage and second stage bollworms.
Thus, it is essential to both recognize the difference between first and second bollworm stages (a second stage bollworm will have molted once, and be approximately one eighth inch in length), and to identify the difference between insignificant, superficial square damage and damage which will cause the square to abort.
However, if egg levels are in the 75 to 100 range per 100 terminals, or eight to 10 per 100 fruit, an economically-damaging infestation of bollworms is likely and treatment is advised.
Since the introduction of Bollgard cotton here in 1995, most of the bollworms which become established on Bollgard cotton appear to do so on or within blooms, especially dried ‘bloom tags’ stuck to young bolls. The B.t. protein is not well expressed in the pollen anthers and petals of the flower.
Because our thresholds are quite low (protective) for Bollgard cotton, however, flowers must not be over-sampled.
Flowers and dried bloom tags should be sampled in proportion to their occurrence in the overall boll population. If a higher proportion of pink blooms and bloom tags is sampled, bollworm thresholds must be raised accordingly.
Check your Extension publications for actual suggested bollworm thresholds for Bollgard cotton in 2001.
Stink bugs, and to a lesser degree plant bugs, may also be present in damaging numbers in late-season Bollgard cotton, primarily due to its low spray environment. Stink bug boll damage assessments are best initially taken along with bollworm scouting, with a representative sample of all boll sizes inspected.
All thumb-sized bolls (up to 21 days old) from the sample are then cut open with a pocket knife or crushed open between the thumb and forefinger.
If five percent or more of the bolls show internal spotting or lint staining, especially on developing seed, treatment is advised.
Mostly, but not always, external boll spotting and a wart-like structure on the inner boll will point to stink bugs as the culprit.
Alternatively, if the original sample of 10 bolls of various sizes shows three percent or more boll damage, an additional sample of 50 to 100 small and medium bolls may be taken (up to 21 days old).
Because this sample contains exclusively stink bug-susceptible bolls, a higher threshold of 10 percent stink bug damaged bolls is recommended. Stink bug thresholds may differ somewhat between states.
Automatic ‘over-sprays’ are not recommended for Bollgard cotton in North Carolina. However, treating in response to the above protective thresholds for late-season bollworms and stink bugs often pays for itself many times over.