Cotton producers in Alabama – like those in the MidSouth and in other parts of the Southeast – found pockets of thrips resistance with their at-plant treatments in 2014.
“We’re definitely in the middle of some changes with thrips resistance,” says Ron Smith, Auburn University Extension entomologist. “Some of these changes are part of a trend, and some of them might be different this year with weather conditions and other factors.”
Smith says there are several questions that need to be answered about thrips resistance in cotton.
“What do we know about thrips resistance, and how to we manage the seed treatments for the longest term possible? And, what are we doing to do if we get even greater resistance?”
With Temik no longer on the market, farmers have relied on two neonicotinoid seed treatments, Cruiser (thiamethoxam) and Gaucho (imidacloprid), to control insects that infest fields as cotton emerges.
Smith’s colleague, Extension Entomologist Tim Reed, says thrips resistance has been confirmed to the neonicotinoid seed treatments. Thirty-two thrips populations from 10 states, including Alabama, were analyzed by North Carolina State University, he explains.
“Those findings concluded there was a level of resistance to Cruiser – about 11 times greater than the non-susceptible population, and the level of resistance to Gaucho was about 4.9 times greater than the non-susceptible population.
“So we had resistance to both Cruiser and Guacho, but for the populations submitted, resistance was greater to Cruiser. Populations came from wheat, cotton and weeds, and resistant thrips were found in all three scenarios. It didn’t matter if they came from late-season cotton or early season weeds, they still saw the resistance issue,” says Reed.
In the three populations submitted from Alabama, he says, the level of resistance was low to moderate. Samples were taken from Belle Mina, Prattville and Headland, representing the northern, central and southern portions of the state, respectively.
“This thrips resistance issue is spotty,” says Reed. “We looked at in-furrow treatments last year, and in my tests at Belle Mina, we saw a little more damage to cotton that was treated with Cruiser than with Gaucho. In the Prattville tests, the same level of damage occurred in both treatments. It’s not a real bright picture now for the neonicotinoids as a seed treatment on cotton, and the long-term outlook is not positive at this point as far as efficacy.”
And, says Smith, there’s not a backup plan at the moment. “The seed treatments are the only thrips control option at planting that we’re aware of,” he says.
The problem appears to be more severe in the MidSouth, according to Smith. “Tennessee, Arkansas, Louisiana and Mississippi have such widespread resistance that they’re considering taking Cruiser out of their recommendations in those states.”
Other factors affecting efficacy
Smith says he rates thrips based on visible damage to the plant, on a one to five scale, with three being the point to where you’d want to go in with a foliar spray.
“Cruiser was always about one-half point higher, which means more damage than with Gaucho, particularly at the Headland site. But there are some other factors at work here too. Cruiser is a little bit more water soluble than Guacho, and we had a number of rainfall events during that window when we were planting and cotton was emerging. That could have been a factor in making Cruiser look a little less effective than Gaucho.”
There were no significant yield reductions in the three Alabama sites, even when comparing the untreated seed to the insecticide-treated seed, he says.
“Too much happens between seed-treatment time and yield time on cotton for you to always see a direct comparison on yield decreases. The most common thing you see with poor thrips control is delayed maturity, and that’s really difficult to see in a field. In small plots, we can go in at first bloom and first open boll and count the number of plots, and you can see the difference in maturity from these treatments. But by the time you get to yield, cotton compensates and it equals out.”
There’s also another factor involved, says Smith.
“For the past two years, we’ve had relatively abundant rainfall in the spring. Thrips are hanging longer on the wild host plants, and they’re coming out in their peak numbers and getting on seedling cotton, probably at least three weeks late. So instead of having April-planted cotton catching some of the highest thrips damage, it has been May-planted cotton. That’s because this peak of thrips has been later, which would also work against seed treatments, so there are a lot of things going on there.”
From a long-term point of view, overspraying is an option, he says. “Orthene is a phosphate insecticide that’s a real good fit there. That’s probably where that particular chemistry fits best. Pyrethroids are not an ideal class of chemistry to put in that early window. Bidrin works on thrips, but I prefer to use it later for plant bugs and stink bugs. And you’re limited to 1.2 pounds active ingredient per acre per year. I wouldn’t waste Bidrin on thrips when I could use Orthene, which I won’t need later in the season.”
Another neonicotinoid – Admire Pro – also is getting a look, says Smith. “But we haven’t seen enough of a bump to justify using it. They have, however, had success with it in North Carolina.”
Heavy crop residue does appear to be effective in battling early season thrips, he says.
“The more crop residue you have over the soil when cotton is in the seedling stage, the less thrips injury you’ll have. For some reason, the more residue, the fewer number of thrips that’ll find the cotton. They love bare ground, but they don’t like solid residue at all.”
Spraying at the first sign of visible damage may be too late to do you much good, says Smith.
“If you wait until you see the visible damage, like three, four and five true-leaves, you’ll get a minimal amount of benefit from it. The best benefit you get from a foliar spray on top of a seed treatment is when the first true leaf is beginning to bud out. We’re trying to get it to about the fifth true-leaf stage, and then it’s beyond where thrips will do economic damage. Anything you can do to make your cotton grow faster, and shorten that interval from emergence to the five true-leaf stage, is a benefit.”