The future of cotton insect control will be centered on the “bug complex,” says Auburn University Extension Entomologist Ron Smith.
“It appears to me in recent years that we've been leaving some cotton on the table to the bug complex,” said Smith, speaking at the Wiregrass Cotton Expo held in Dothan, Ala. “The bug complex is very important, and it's not going away.”
At one time, says Smith, growers were adept at controlling boll weevils. “You knew how to check for them and how to control them.
Weevils were taken out of the system, and you got pretty good on worms. But we've moved on. With Bollgard and other genetically modified cotton varieties, this bug complex will only get worse.
“We're reducing the number of foliar sprays for worms. Our cotton fields basically are an insecticide-free environment for most of the season for this bug complex,” says Smith.
Plant bugs are an economic pest of cotton, he says.
“They're not as big a pest in the Wiregrass region of Alabama as in other parts of the state, and they're not the same every year. We conduct scouting schools each June in south Alabama. And almost every June, we go out and see cotton that's getting economic damage from plant bugs. They can be a problem, but it varies from year to year and from field to field,” says Smith.
Growers or scouts can get an idea of the amount of plant bug pressure in a field by looking at the pinhead squares, says Smith. “You can see if they're still green and viable, or if they're turning yellowish, brown or black before they drop off the plant and leave a square. We suggest that you need 80 to 85 percent of your little squares set in early season on your first fruiting node branches.
“In a really bad plant bug year, that could drop to 50 or 60 percent. Someone needs to know when that's happening and go in and control those plant bugs,” he says.
This is a situation where planting Bollgard cotton can be advantageous, says Smith. “In this part of the world, if you go in during June — at pre-bloom — and you have to spray for plant bugs, there's a good chance you'll get a worm infestation behind that spray. If so, it'll be tobacco budworms, and you'll need a $12 spray to clean that up with one of the new chemistries.
“That's an advantage of Bollgard cotton — you can spray plant bugs when you need to, without upsetting the system and getting into worm problems. That's the primary reason growers in the Tennessee Valley plant Bollgard cotton — so they can spray plant bugs when they need to.”
After cotton begins blooming, growers should no longer monitor pinhead squares for plant bug damage, says Smith. “By that time, immature plant bugs are beginning to hatch out, and they like to feed in blooms. You can spot them by walking across a field and looking in white blooms. If you really want to quantify the immature plant bugs, you can shake them off onto a drop cloth. But most people won't do that, so we have a better way of deciding when plant bugs are a problem after bloom.”
Smith advises growers to look for the damage being done by plant bugs. “This damage was done just before the bloom unfolded in the late square stage. They feed through, giving the bloom a ‘dirty’ appearance. This interferes with pollination and causes some ‘hawk-shaped’ bolls or poorly filled-out bolls.”
In deciding when to spray for plant bugs, growers want to look for about 10 to 20 percent dirty blooms and adult and immature plant bugs in the blooms, says Smith.
Another insect pest causing concern is the fleahopper, he says. It's pale green and more fragile than a plant bug, but it is seen most years in south Alabama, he adds.
Also seen last year was the clouded plant bug, he says. “They are a much bigger, more robust plant bug. You don't normally see them early in the season — they came in last year after the first of July. They've always had a reputation of being more of a boll feeder, primarily because they are seen more towards the end of the season when bolls are present.”
A more consistent problem in south Alabama's Wiregrass region, says Smith, is the stinkbug. The stinkbug, he says, has become more of a problem than some ever thought it would be.
“We knew it would be a problem when we took the weevil out of the system and took out worm sprays for Bollgard cotton, based on what we had seen in North Carolina. The mix of crops in the Southeast and the milder winters have made the stinkbug problem greater than we ever imagined.”
Most stinkbug damage, he says, is done by the late instar nymph or the adult. They prefer bolls that are about 10 days old or about the diameter of a quarter, up until the boll is about 21 days old.
“To our knowledge, the stinkbug does not damage squares. But part of the equation we fail to factor in is that stinkbugs have a long lifecycle. It may take about 28 days to go through the immature stages. The adults will live for about a month. If you have a few stinkbugs, they're not going away, and the situation won't get better.“
You won't detect damaging levels of stinkbugs, says Smith, by looking only for the adults. Stinkbugs are very conscious of noise, he explains, and they will move away from you in a field.
“We've found that a better way to survey or scout is to look at these quarter-size diameter bolls that stinkbugs prefer and slice or look internally to see if there's any damage. Sometimes, you'll see external damage, but there will be do damage inside. Other times, a boll may look normal on the outside but it's already damaged on the inside.
“We suggest that you slice these bolls. You're looking for brown tissue on the inside or a place where you can see where the mouthparts came through the boll wall. Other times, you'll see a callused area on the inside where the boll wall is trying to heal itself. Yellow-stained lint also is a good sign that you have stinkbug damage.”
Researchers started out by slicing the bolls to detect stinkbug damage, says Smith. But slicing a boll, he adds, gives you only one cross-section look.
“It's actually better to crush these bolls, when they're about the size of a quarter and still soft. You can use a set of pliers, putting one side on the top and one on the bottom of the boll, and you can try and get a quick look at the entire boll.”
Some states use a threshold of 20 percent damage, says Smith, but that may be too high. “People in the field are telling me that they're not using that threshold. That's real high, especially considering how long stinkbugs stay in the field. I would suggest something in the neighborhood of 10 damaged bolls per hundred. I wouldn't argue with going even lower than that.”
Stinkbug control can be a problem, says Smith, especially if you have migration from other crops.
“In peanut country, and if you have corn drying down or maturing, you can get migration from these other crops. You'll get about two weeks of control unless you have this migration. In a bad stinkbug year, it'll take multiple sprays to protect a crop against stinkbugs. In south Alabama, July 15-20 is the ideal time to start concentrating on stinkbug damage.”
All stinkbugs, says Smith, are not equal. In most years, growers deal primarily with the Southern green stinkbug. This species is relatively easy to control, particularly with phosphate chemistry and some of the pyrethroids, he says.
“But there's a brown species, and in years like last year, it can dominate during some parts of the season. The brown stinkbug is much more difficult to control. The pyrethroids, with the exception of Karate and Capture, don't do a very good job on the brown species. And even those two materials won't do a clean-up job. If you have brown stinkbugs, you need to go to the phosphate chemistry — something like Orthene, Bidrin, methyl-parathion or malathion.”
Another plant bug being seen in Alabama cotton fields is the leaf-footed plant bug, says Smith. “They come in late in the season, and they are becoming more common each year.”
A couple of other insect pests also are becoming a problem for cotton producers, says Smith. “In the central part of Alabama, we've had a lot of problems in our conservation-tillage cotton with grasshoppers. Grasshoppers are a pest only from the time cotton emerges until it gets up to about the five or six true-leaf stage, because they cut down the stem.”
Grasshoppers aren't big feeders on cotton, he says, but they will cut down the stem. “And we don't have a threshold for that. If you're losing too much stand, you need to control them. After cotton gets up to some size, grasshoppers will try and feed on the leaves, but that'll be of no consequence.”
Another pest growers have noticed in recent years has been the three-cornered alfalfa hopper, notes Smith.
“This is an old soybean pest that girdles the stem of young cotton. They're a pest just in the seedling stage. They girdle like a beaver and leave swollen internodes around the plants. A couple of years ago, we had calls within 48 hours from Jay, Fla., to Talladega, Ala., and it was three-cornered alfalfa damage.”
The three-cornered alfalfa hopper is another sporadic pest, says Smith, that will be worse in conservation-tillage fields. Plants damaged by the pest will turn a reddish color and wilt in the row.