Since the arrival of Bt technology and boll weevil eradication, there has been a marked shift in the spectrum of pests seen in cotton fields, say Extension entomologists in both Louisiana and Arkansas.
“It's kind of hard to separate the amount of impact of Bt and boll weevil eradication since they've come on the scene at essentially the same time,” says Ralph Bagwell, LSU AgCenter entomologist. “Where we are today is a function of both, I think.”
In the past, Louisiana's primary cotton pests were boll weevils and tobacco budworms. Now, with eradication eliminating boll weevils and Bt technology taking care of tobacco budworms and, to an extent, bollworms, other cotton insect pests are rearing their ugly heads.
“Louisiana has been planting about 80 percent to 85 percent of the total cotton crop is in a Bt variety,” says Bagwell. “So these pests that we used to not be as worried about have come to the forefront.”
One such pest, surprisingly, is the bollworm. Through some of the difficulties in finding the insects and adjusting thresholds, “we've had to put a bit more emphasis on timing and new scouting procedures,” says Bagwell.
“Our thresholds are a bit different than in the past. Before, we suggested concentrating on the terminal area of the plant. Now, what we recommend in scouting pre-bloom Bt cotton, is sampling squares and treating when you get around 5 percent damaged squares with live larva present. I've never actually seen that occur in Bt cotton.”
More often than not, says Bagwell, scouts see an infestation that takes place after first bloom. At that point, the Extension Service recommends a sampling procedure that's based on evaluating multiples of fruiting forms — be it squares, blooms or bolls on a plant.
“Or we recommend treating if there is 2 or 3 percent live larva found on fruiting forms. My preference in scouting is to actually go back and look at white blooms. If I find enough larvae inside the blooms, it's time to treat.
“Others prefer to concentrate on looking under stuck bloom tags. Either way, scouting well is important.”
For treatments, Bagwell says primary products used will be pyrethroids.
“Pyrethroids remain the most effective means of controlling bollworms regardless of whether the cotton is Bt or otherwise. Pyrethroids are very efficacious, are fast acting and growers are confident using them.
“Almost all pyrethroids are very effective on bollworms.”
When you remove a pest from the scene, something will take its place.
That doesn't mean what shows up is as bad as its predecessor, but Bagwell has seen tarnished plant bugs move up the “major pest list.”
Another pest Louisiana growers are seeing more of is stinkbugs.
“There has been a really big stinkbug increase. Prior to eradication and Bt cotton, stinkbug infestations were hard to find in cotton fields. Now, a stinkbug infestation is fairly common.” Fall armyworms are also showing more vigor.
“These are all insects that have populations that tend to flourish in lower spray environments like we have today,” says Bagwell. “In the past, the spectrum of activity with the insecticides we used took care of a lot of primary pests but they also knocked back the secondary pests. Stinkbugs and plant bugs are excellent examples of this.
“In the 1980s and early 1990s, the multiple shots of pyrethroids put out for bollworm and tobacco budworm were giving us excellent control of fall armyworms and other pests. We just didn't recognize that fact at the time.”
On beet armyworms, newer labeled products are the ones most used, says Bagwell. Tracer is probably first in line followed by Intrepid, Denim — “assuming it's available for use (through a Section 18 or Section 3 registration)” — or Steward.
For fall armyworms, there's not a big advantage of these newer materials over just a regular pyrethroid, says Bagwell. Generally, by the time scouts find an infestation of fall armyworms, it's going to be rather hard to knock them back.
“It's hard to gain control of fall armyworms. More often than not, most insecticides will only control 50 percent of the population. Lately, producers have moved towards tank-mixes to control fall armyworms — something like a pyrethroid and Intrepid.”
On stinkbugs, in Louisiana producers see a mix of green, southern greens and browns. It could be a mix as balanced as 50 percent browns and 50 percent greens.
“There are a lot of brown stinkbugs down here. Unfortunately, because browns are so hard to control, that really limits the insecticides we're able to use for stinkbug control. To deal with browns, we usually employ methyl parathion, Orthene or Bidrin,” says Bagwell.
Arkansas is also seeing an increase in plant bugs and stinkbugs “that we didn't have big problems with before,” says Glen Studebaker, Arkansas Extension entomologist. “I think that's not just due to Bt cotton, but also Roundup Ready crops.”
“Because often producers wait a bit longer to control weeds and that allows pest populations to build up more than normal,” says Studebaker. “I've seen situations where the pests — especially plant bugs — have come off weeds onto cotton after the Roundup applications.
“Also with the Bt cotton and boll weevil eradication, we're seeing these secondary pests moving into primary status. These pests have always been around but since we're not spraying early like we used to, their numbers are able to rise unmolested. Stinkbugs are particularly problematic lately. We used to rarely see stinkbug troubles.”
On plant bugs, Studebaker says Arkansas producers often use Centric, Trimax and Intruder. Producers “really lean” towards those products if there are also aphids in the pest mix.
“But we also still see pretty good control using Orthene and Bidrin.”
Last year, Studebaker saw mostly green stinkbugs and the year before, “we had mostly browns. It seems to vary yearly probably due to what the weed host is early and how populations build. With brown stinkbugs, we usually go with Orthene and Bidrin.
“Organophosphates seem to do a better job with browns. With green stinkbugs, pyrethroids do a good job.”
Arkansas had a fairly good run of both bollworms and tobacco budworms last year — particularly in the northeast part of the state. Studebaker wonders if there will be a repeat this year.
“We've had some traps out catching fairly high numbers of budworms. It's hard to know if that will translate to problems later in the season. I am concerned about the trap catches being high, though.
“And not everyone is planting Bt cotton around (the northeast). There are plenty of growers going all conventional cotton. Since we had a bad situation last year, a lot of producers were geared up to plant Bt last fall. But then we got a cold winter and there was a lot of thinking that the cold dropped budworm numbers. So some producers went away from their Bt plans. I don't think the winter affected budworm numbers too much because of the trap numbers we're seeing even though there's not a lot of cotton out yet.”
Any changes to the Arkansas pest scouting guidelines or thresholds?
“The main changes have been in the Bt/Bollgard cotton,” says Studebaker. “We still recommend scouting but we don't treat on eggs. We have tweaked guidelines trying to get scouts — particularly when flights come through — to not just scout terminals but to look at bolls. We had a little of that last year. After a few years of this new technology, scouts are doing better now — looking at blooms and lower on the plant.”
Scouting stinkbugs has also been refined. “We like to see a good number of bolls sampled — 25 to 50 at least. And we'll not just look on the outside but we'll cut the boll open and see if there's any damage to the seed.
“We treat on 20 percent damage or one per 6 row feet.”
How about resistant management strategies? “We still recommend staying away from early season pyrethroid sprays on conventional cotton,” says Studebaker. “We also lean towards the new chemistries — Tracer and Denim and others — for bollworms. Tracer is a little weaker on bollworms.”