Thrips damaging to Virginia cotton

"We see a lot of pressure from thrips and we suffer a lot of damage," says Ames Herbert, Virginia Tech Extension entomologist. "Anything that happens early in the season usually translates into significant yield loss at the end of the season."

Thrips pressure on cotton in Virginia can be characterized as "very high," says Herbert. "We’ve seen high pressure for the past two to three years, mostly from tobacco thrips. We’ve seen only once case where Western flower thrips was the dominant species, and that was in a cotton field that already had been sprayed with Orthene and Karate," said Herbert at the recent Southeast Cotton Conference in Rocky Mount, N.C.

Researchers in Virginia have conducted trials for several years measuring thrips injury to cotton plants, he says. The trials have looked at the effectiveness of several materials and a broad range of treatments, foliar banded and broadcast.

"As we increase the rate of the broadcast treatment or foliar band, we get better control of thrips, and we’ve been seeing this for a few years now. We wanted to see if Temik responds to an additional foliar band of Orthene. At the 3.5 and 5-pound rate, we definitely saw less damage when it was followed by a spray application of Orthene. In our tests, Gaucho alone always has more damage than Temik alone.

"Due to the variability in the tests from one year to the next, it’s difficult to get a reading on exactly what we should be doing for thrips control. We’ve often seen that the 3.5 and 5-pound Temik rates are the same in terms of thrips control. Sometimes we see a response to a foliar treatment, and at other times we don’t see a response," says Herbert.

Researchers also have looked at how these treatments translate into increased cotton yields, he adds. The best Gaucho treatments were with higher rates — either the 4-ounce band or the 4 to 6-ounce broadcast. The lower rates didn’t result in as good a yield as the higher rates, he says.

"With Temik, we didn’t see a big difference in yield between rates. We saw a big difference in plant injury but not in yield. Our take on this study is that if you’re going out with a Gaucho seed treatment, you need to go over the top with it. Our yields basically are equal with Temik. Temik sometimes does and sometimes doesn’t respond to that extra Orthene treatment."

Researchers also have looked at Cruiser in comparison to Temik and Gaucho for thrips control, says the entomologist. "We’ve gone in both early and late with Karate and Orthene 97, early being the first true-leaf stage and late being at two to three true leaves. We found a general trend in Virginia towards better control and better yields with the early treatment compared to the late treatment.

"In general, Cruiser treatments have a little more injury. Gaucho has a little less injury and is not too different from what we see with Temik."

In measuring injury from Cruiser, Gaucho and Temik alone, researchers in Virginia have seen more injury from the Cruiser alone treatment, notes Herbert. Cruiser definitely responded to the foliar applications, he says, and Cruiser combined with foliar sprays offers a reasonable alternative treatment.

Since 1998, Virginia has been involved in a 12-state research project sponsored by Cotton Incorporated looking at the survivorship of corn earworm moths that have been exposed to pyrethroid insecticides. All states are using the same systems to collect data, says Herbert.

"Over the years, we’re seeing a gradual increase in survivorship. This means a gradual resistance or tolerance is being developed by bollworms to pyrethroid insecticides."

In 2000, researchers trapped 843 moths in Virginia. This was followed by 928 moth captures in 2001 and 1,147 this past year. These moths were trapped at two locations. "We’ve increased our number of moths, but 1,147 still is a tiny sample, and I’m not quite sure such a sample is meaningful."

The moths were tested in 5 and 10 micrograms of insecticide, he says. In 2000, the survival rate was 12 percent at the low rate and 6 percent at the high rate. In 2001, survival dropped to 3.5 percent at the low rate and no survival at the high rate, and the rates stayed low in 2002, says Herbert.

"However, we’ve found situations in Virginia where — for the first time — soybeans were treated a second time in September for corn earworms, and there were a lot of survivors being treated with pyrethroids. Our concern is whether we are doing enough trapping and getting enough adults to really get a feel of what’s going on.

"We want to add sample sights this summer in our northern soybean-growing region and also add sample sights in our cotton region so we can get a better handle on this." e-mail:e-mail: [email protected]

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