Whether you're fighting a single pest, a pest complex or multi-pest thresholds, weeks two through six of flowering are a critical time for insect management in cotton, according to University of Tennessee cotton IPM specialist Scott Stewart, speaking at Cotton Incorporated's Crop Management Seminar in Memphis.
Any decision you make on insecticides late in the season must first consider any technology in the field, Stewart says. “Think of Bt cotton as a way to filter out the pests you don't want to deal with. We know that when we grow Bt cotton, we're not going to have to deal with tobacco budworm. We also know that with some of these new Bt cottons like Bollgard II and WideStrike cotton, we're going to reduce the threat from other pests such as bollworms, beet armyworms and fall armyworms.”
Stewart says that very limited testing indicates “Bollgard II and WideStrike are doing what they are advertised to do. They are reducing boll damage from populations of bollworms. Another point is that they doesn't reduce it to zero.”
Stewart noted that in some west Tennessee fields in 2006, under significant bollworm populations, “it was not rare to have to spray some Bollgard II and WideStrike fields for bollworms. I did observe some damage in fields to both Bollgard II and WideStrike from bollworm infestations. So the technology is not necessarily bulletproof.”
If bollworm is the primary pest in the field, “in Bollgard, Bollgard II and WideStrike cotton, you still can use a standard pyrethroid for control. It may change, depending on any bollworm resistance you're encountering. Bollgard II and WideStrike will be a real alternative if you start seeing widespread pyrethroid resistance in the bollworm populations.
“In non-Bt cotton, we bring tobacco budworms back into the picture and we have to go to some more expensive products like Tracer, Steward and Denim if tobacco budworms are present.”
Stewart does not recommend using a pyrethroid alone in this situation, “unless we're convinced we don't have any tobacco budworms in the field. If we have moderate to high mixed populations of budworms and bollworms, we're going to have to tank mix the more expensive products with a pyrethroid.”
For fall armyworms in Bt or non-Bt cotton, “Diamond does a good job at a 4-ounce to 6-ounce rate. Intrepid, Steward, Orthene (acephate), even pyrethroids give you some control. It's common in Tennessee to recommend a pyrethroid plus one of the other products.”
For plant bugs and stink bugs, “you almost can't go wrong with Bidrin,” Stewart said. “I rarely see a trial where Bidrin doesn't win at some rate of 6 ounces to 8 ounces. Acephate usually does well, but every now and then you see a test where there are resistance issues.”
Other options include Trimax, Carbine and Diamond, noted Stewart. “Pyrethroids have a place, especially in areas where you're dealing mostly with green stink bugs and clouded plant bugs and not too many tarnished plant bugs.”
If there is a pest complex, be sure you know which pests make up the population, according to Stewart. Many times, the complex includes some combination of tarnished plant bugs, clouded plant bugs and stink bugs.
Stewart rarely recommends a pyrethroid application alone for west Tennessee cotton producers, unless there is exclusively a bollworm or bollworm/green stink bug infestation. “If I had a lot of everything, bollworm, stink bugs and plant bugs, I'd go with a three-way tank mix of a pyrethroid, Diamond and acephate.”
On non-Bt cotton, you add one more layer of complexity, “something to control tobacco budworm if you have them.”
Multi-pests at sub-threshold in the field can create a dilemma for growers, according to Stewart. “Sometimes growers don't have enough of anything to reach a threshold, but they do have enough of everything.”
But it's really not that complicated if you do more than visual sampling. A drop cloth could reveal that a pest is actually at threshold.
“A lot of times, it's only an issue for a short while because that pest population is on an upswing. In about a week it's going to resolve itself for you because you're going to break the threshold.”
If you have multiple pests below threshold in a field, there is a way to estimate a sort of collective threshold, according to Stewart. Count one stink bug as three tarnished plant bugs; one clouded plant bug as 1.5 tarnished plant bugs, and one fall armyworm as a third of a bollworm.
“Also ask yourself the tough question, ‘Am I using multi- sub-threshold pests as an excuse to treat?' Being at a quarter threshold for four weeks is not a threshold. Criteria for treatment is not how long it's been since your last treatment. Let's don't make this rocket science. But if you have three-quarters of the threshold on stink bugs and plant bugs and you have 3 percent bollworm, I'd recommend a spray.”
One standby that still works well is to terminate insect control at 5 nodes above white flower (NAWF 5) plus 350-400 DD60s. At that stage, bolls can tolerate more damage, and fields become relatively less attractive. “There are some times when it doesn't work, for example, on exceptionally late fields.
“Earliness still pays in insect control,” Stewart said. “You can terminate insecticides two weeks early just by choosing the right variety, which is very likely going to save you a couple of sprays. But it's only going to pay if that variety is going to yield competitively.”
Local knowledge and situation should also be considered in the insecticide decision, notes Stewart. For example, note any local resistance issues or traditional pest pressures in your area.