January 19, 2015
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.