Plant bug numbers remain extremely high in parts of the Mid-South and, with the cotton canopy about to close, farmers are scrambling to gain control over the pest.
“We've had higher-than-usual plant bug numbers throughout (Mississippi's) Delta region for almost a month,” says Angus Catchot, Mississippi Extension entomologist. “But there are four counties that are seeing extremely high numbers: Washington, Sunflower, Bolivar and Coahoma, with numbers steadily increasing in other Delta counties.”
Plant bugs are especially bad in cotton next to corn. And the problem isn't confined to field edges.
“In some fields, the numbers are incredibly high even 250 yards away from corn. Some producers have been trimming the edges between corn and cotton, a tactic used successfully in the past. But one or two passes won't get it done this year.”
In Arkansas, Gus Lorenz says the plant bug situation is the worst he's seen at this time of the season.
“Over the last couple of years, we've seen large numbers of plant bugs moving into the field once cotton begins to bloom,” says the Arkansas Extension entomologist. “But this year, they're hitting pre-bloom cotton at extremely high levels — sometimes six times threshold.”
In such situations, the cotton is “overwhelmed and it's extremely difficult for growers to gain control. Many producers have sprayed three or four times in hopes of keeping square retention up. Too many aren't having any luck.”
Producers are quickly running out of options because of the limited insecticides available. “Folks are trying to avoid some organophosphates due to fears of flaring secondary pests like aphids or spider mites.”
And there are use rate issues. For example, Lorenz points to the label on Bidrin being changed to prevent application pre-bloom.
“That limits us to mostly neonicotinoids like Trimax and Centric. But there's a 5-ounce use rate on Centric. It doesn't take very long to run out of options.”
As a general rule, the closer a Louisiana field is to the Mississippi River levee, the higher the plant bug density.
“That's true up and down the levee,” says Ralph Bagwell, LSU AgCenter entomologist. “We've had lots of plant bugs for the last three or four weeks.”
One of the hardest-hit fields Bagwell checked on June 28 had one to two nymphs per row foot. “That's the first time we've run across a significant number of nymphs. Mostly, we've seen transient adults moving in and out of cotton fields.
“(On June 27), though, we began picking up nymphs. The nymphs are in early stages, but they're there. Obviously, a lot of the plant bugs are beginning to reproduce in cotton.”
Back in Mississippi, Catchot has Washington County research plots in the thick of the plant bug problem. Intended to monitor early-season plant bugs, Catchot and colleagues began checking the plots three weeks ago at pinhead square. At the time, plant bug numbers were low.
However, a week later, plant bug numbers were at 31 per 100 sweeps with square retention at 69 percent. Catchot made an insecticide application and seven days later, in the same field, plant bug numbers were at 33 per 100 sweeps and square retention had dropped to 59 percent. Because of rains, the second pesticide application was about two days late.
On June 25, three days after the second application, the plots were checked again. Plant bugs were at 48 per 100 sweeps and square retention had dropped to 35 percent.
“Those plots are a prime example of why, in the face of such plant bug populations, you can't stay on a seven- to 10-day spraying interval under this pressure. Plant bug numbers are steadily rising while square retention is steadily dropping.
“We're also beginning to pick up a lot of nymphs in cotton. When we hit threshold, we're encouraging producers to add Diamond to the mix. That will help stay on top of the nymphs.”
On June 28, Lorenz spoke with a consultant in east Arkansas' Phillips County. “He'd just done 25 sweeps in a field that had been treated a week earlier and caught 24 plant bugs. The threshold is 2 plant bugs per 25 sweeps. In such situations, even if you get 90 percent control it won't be below threshold.”
A week earlier, near Altheimer, Ark., Lorenz checked a cotton field five days after an insecticide application. The count was 16 plant bugs per 25 sweeps.
“We put a test there to try some different treatments. When we checked the field this week, all the treatments were still above threshold and no treatment had a square retention value above 80 percent.”
Lorenz says with the fast fruiting, early-maturing cotton varieties many producers have adopted, “there's a month window to get fruit on the plant. You've got to maintain square retention at 80 to 85 percent. But with the current plant bug numbers, keeping that percentage is almost impossible. In the hardest-hit fields, square retention values of 40 percent aren't uncommon. Fifty percent is very common.”
Looking at this historically, “we know once the crop's canopy closes and begins to lap, if the plant bugs aren't controlled they never will be. That's my biggest concern: that the nymphs will get into the canopy and we can't get an insecticide to them. I'm afraid if we don't get control prior to canopy closure, it'll be game over. We're probably, at most, two weeks from the canopy closing in the bulk of the crop.”
When scouting for plant bugs, all interviewed suggest employing both a shake sheet and sweep net.
“A shake sheet is superior for nymphs but the sweep nets are unbeatable for adults,” says Lorenz. “I was just in a field with a consultant who worked a shake sheet. He counted and said ‘well, this doesn't look too bad.’
“While he was using the shake sheet, I'd been watching adult plant bugs flitting around. So, I got the sweep net out and made 25 sweeps. When I opened it up, the plant bugs boiled out — looked like fire ants pouring out of a mound.”
What's really worrying, says Catchot, is the worst plant bug problems usually begin mid-July. “That's when we usually begin picking up a lot of nymphs in cotton.
“With the numbers we're dealing with and a steady migration, these plant bugs are also laying eggs. In mid-July, when it's time to fight nymphs, the cotton canopy will be closed. That means it will be even harder to gain control.”
Why does Catchot believe plant bug populations are so high?
The year has been unusual, he says. With three weeks of unseasonably warm weather, March was “definitely atypical.” During that period, USDA entomologists at Stoneville, Miss., collected “much higher than normal levels of plant bugs on wild hosts with a lot more reproduction for that time of year going on.
“Then, we got into a drought and wild hosts began drying up moving these high populations into irrigated corn and flowering soybeans. Our counts in corn have been averaging 35 percent to 50 percent. With 950,000 acres of corn in Mississippi, we're looking at tough times ahead.”