Research efforts between the Lower Southeast land-grant colleges of the University of Georgia, Clemson University and Auburn University continue to focus on thrips injury on early season cotton and how to avoid it.
“From an entomologist’s standpoint, thrips are one of the easiest insects we work with,” says Philip Roberts, University of Georgia Extension entomologist. “Infestations are usually very uniform, and thrips are very predictable pests if you plant your trials at the appropriate time. It’s easy to get data for thrips.”
Obviously, slow seedling growth results in increased injury from thrips feeding on the plant, says Roberts. Preventive treatments used at planting could result in very consistent yield responses, whether with Temik or seed treatments, he says.
“If you’re looking for injury from thrips, it’s important to look at that newly expanded leaf. Thrips is a pest we almost take for granted. But if we didn’t have such good tools to control thrips, we’d have a battle trying to manage them. When we have excessive thrips injury, we see an obvious stunting of plants,” says Roberts.
And wherever you see the above-ground stunting of cotton plants, you can be assured there’s a similar response below the ground, he says.
“In trials, we have dug plants in the field and cut and weighed the roots to get dry weights of the roots. We potted seedlings and came up with treatments where we could generate three levels of thrips damage — low, moderate and severe.
“At 14, 21 and 28 days after planting, we washed the roots and took dry weights of above-ground plant parts and below-ground parts. Where we had low thrips injury, we had much more root growth than with the moderate or severe thrips injury,” he reports.
Rapid root growth also is a very important consideration in a situation where there is nematode pressure, he adds.
“When we talk with our county Extension agents about thrips, I like for them to think in terms of risks. With the loss of Temik and the anticipated shortage of aldicarb this year, this will be an even more important topic for growers in the Southeast.”
Earliest planted cotton is where growers tend to see the highest thrips populations, says Roberts.
“Typically, we get to a point in the year to where thrips numbers crash based on planting date. Also, when we plant early, we usually have cooler temperatures.
“Anytime we have a slow-developing seedling, whether from cool temperatures, a low-vigor variety, herbicide injury or any plant stress that will delay seedling growth, that’s when thrips can punish us in terms of yield loss.”
Conservation-tillage seems to help
The other factor in thrips management is tillage, as more damage is seen in conventional-tillage systems. Growers in the Lower Southeast who use reduced-tillage or strip-tillage systems tend to see lower thrips populations.
In one trial, researchers planted cotton every two weeks beginning in mid-April, in a germination test, going through July. Then, they counted the number of thrips per plant two weeks after planting.
“Once we got past the middle of May, we had very low numbers of thrips, and this is on untreated cotton seed. This emphasizes the importance of planting date, and when we can expect larger numbers.”
Comparing 2009 to 2010, says Roberts, it took six additional days to reach the four-leaf stage in 2009, and that’s important because once a plant reaches that stage, it’s out of the thrips susceptibility window. The quicker a grower can get to that safe zone, the better, he says.
When is the plant most susceptible to thrips attack in terms of yield? In a study conducted in 2009 and 2010 with locations in Georgia and South Carolina, researchers planted untreated seed — seed treated with fungicide only.
“We sprayed at zero days after emergence and seven days after emergence. We also sprayed at seven, 14, 21 and 28 days old, making multiple Orthene sprays. We were trying to determine which growth stage is most susceptible to thrips injury.”
Dry weights of the plants were measured at 42 days after planting. “Where we delayed sprays until the cotton was seven days old, we saw a reduction in growth. This tells me that it is very important that you control on the day cotton emerges.”
Georgia data showed that thrips were infesting cotton on the day it emerges, says Roberts. “That’s one of the reasons we see such a consistent response from at-plant treatments. We saw this in 20 trials with Temik and seed treatments.”
Commercial seed treatments are growers’ primary option in the lower Southeast, with the loss of Temik, he says.
“Many of our growers in Georgia are still learning about seed treatments, since they have been using Temik for so long. Seed treatments provide control for about three weeks. So they’ll work fine where we have rapid seedling growth, but not in all situations.”
As for foliar sprays, most states in the Lower Southeast recommend treating cotton at the four-leaf stage or younger, says Roberts.
“Our objective is to get to the four-leaf stage and to be growing rapidly, to get us out of that thrips window. In Alabama, they discuss Western flower thrips and how they’re more difficult to control, requiring higher rates of insecticides.
“Georgia and South Carolina recommendations mention the presence of immature thrips, which is a good thing to consider whenever making a decision about over-sprays. If you have immature thrips infesting seedling cotton, that means your at-plant treatment has failed or is failing.”
Protection very early in a cotton plant’s life is most important, says Roberts, but you still want to scout thrips closely and treat according to the local threshold. “When you’re looking for injury, pay special attention to those newly expanded leaves and the presence of immature thrips.”
In summary, he says, growers should use an at-plant insecticide for managing thrips, no matter when they plant, not only for thrips but also for aphids. “Aphids can build up early without these at-plant treatments. As we enter an era with no Temik, we need to think about risks, and design programs that address and manage these risks.”
(Growers in the Upper Southeast are also preparing for a battle with thrips. For that story, see Upper Southeast cotton growers brace for thrips invasion).