Cotton insects an annual puzzle

This past year was a very light one for cotton insects, but you never know what to expect, with each growing season bringing new surprises, says Ron Smith, Auburn University Extension entomologist.

One unpleasant surprise for many growers this past year was the prevalence of and the damage done by thrips. “Our tests in Prattville, Ala., this past year showed that weather has a large impact on the consistency of your at-planting cotton insecticide. The bottom line is if you want consistency, Temik is still the best choice. If you want economics and convenience, you have several choices,” says Smith.

In replicated tests, with treatments made side by side, researchers looked at Gaucho Grande, Cruiser, Avicta Complete Pak, Aeris – a new product from Bayer — and Temik at 5 pounds versus a non-treated plot. Another treatment was a combination of Aeris plus another product still being evaluated.

“We evaluate these products in several different ways. The first thing we do is to count the thrips,” says Smith. “As usual, we were looking at the numbers of thrips per row foot, over several dates. All treatments looked distinctly better than the untreated.”

There’s no correlation, he says, between thrips numbers and how cotton appears in the field. “We looked at the damage rate because that’s what you do whenever you go to the field. Damage was rated on a 1 to 5 scale, with 5 meaning that every plant is damaged by thrips and 1 being that there are no plants with thrips damage. Everything looked better in the treated versus the non-treated,” he says.

This past year, all treatments had damage ratings of about 3 to 3.5, says Smith. “That’s exactly what most growers saw last year. The thrips materials didn’t look very good because of the extended cool period and the slow grow-off of cotton,” he says.

Another way to evaluate it is to look at plant height, says Smith. “All treatments were much similar and much better than the untreated. I like to measure maturity because that’s one thing you normally get from thrips control on early season cotton — an earlier maturing crop.”

Maturity, he adds, can be evaluated in a number of different ways. “At the earliest bloom, we can go in and measure the number of blooms per 30 row feet. In our test, on July 5, every treatment had more blooms than the untreated. We also can measure maturity in the early open-boll period by going in and measuring the number of opened bolls per 30 row feet. Again, everything looked better than the untreated plot. Cruiser and Temik were fairly similar in looking at maturity, but there wasn’t much difference in any of the treatments,” says Smith.

Yield also was measured in the thrips test, he says. “So much can happen before the crop matures, that thrips usually even out. The untreated yields were about as much as the others last year.”

It’s also important this year that cotton producers consider their sampling methods for bugs, says Smith, particularly plant bugs.

“I feel as though our knowledge base of plant bugs is still growing, and I think the adoption of some of our recommendations has been a little slow in some areas from a growers’ standpoint.

You can’t correlate the numbers of stinkbugs to damage because the pests are very difficult to find, says Smith. “So walking into a field and looking for stinkbugs is not very accurate.”

There is a lot of variability from year to year in the number of stinkbugs, he says. While this past year was a very light one for stinkbugs, growers saw heavy pressure in 2005.

“This business of deciding you’ll automatically have stinkbugs won’t work at all. Where we conducted one test in 2005, as many as five stinkbug applications were not enough. But we never started the test last year because the cotton never required one application. If you had been making three applications in both years, you would have been very under one year and very over the other year.”

Growers need to look for plant bugs according to damage, says Smith. “We didn’t look for boll weevils back when we had them — we pulled squares and looked for the damage. We’ll need to do the same thing with stinkbugs. We’re not going to look for stinkbugs — we’re going to sample bolls because that’s what they go for instead of squares. They go for half-grown bolls or those that are about the diameter of a quarter.”

Over the years, he says, Bidrin has been a good bug material, and the company that manufactures the product has been in negotiation with the EPA for the past two to three years over a new label.

“They are getting down to the final stages. They do have the amount you can use up to 1 pound per year. But in future years — it may not go into effect this year — you won’t be able to use Bidrin before first bloom. That means you won’t have Bidrin for thrips or plant bugs unless it’s July plant bugs. We’ll have to begin looking for some alternatives for that early season window.”

In a test conducted at Headland in 2005, researchers looked at a stinkbug test where eight rows of cotton were planted through a peanut field. “We know that peanuts feed stinkbugs into cotton throughout the season. We needed a lot of stinkbugs for this test so we could work on thresholds and see if we could get a feel for how many it takes to do damage.”

The test consisted of three treatments, he says, including one untreated plot.

“We treated one plot just as we recommend that growers treat their cotton — based on 20-percent boll damage. Then, in the third plot, we tried to keep out all stinkbugs. We went in each week and measured two things — the percent of damaged bolls and the number of stinkbugs per 12 row feet. We sampled 12 row feet with a drop cloth.

“Where we didn’t spray, we were getting 80 to 95 percent boll damage each week throughout the season. And we were finding tons of stinkbugs in the untreated plot, up to a high of 60 per 12 row feet, with an average of about 30 throughout the summer.”

Where grower-recommended sprays were used, the stinkbug numbers were knocked down pretty well with five sprays, says Smith. However, the boll damage still was running from 20 to 50 percent during different weeks of the growing season.

“In the plots where we wanted to keep out all stinkbugs, we sprayed seven times with Bidrin plus Karate. But we still were getting some level of stinkbug injury, running at 10 to 20 percent or more. We were not picking up stinkbugs, but we still were getting damage.”

Growers can’t go in their fields looking for stinkbugs and they can’t even go in with a drop cloth, says Smith. “You have to sample the bolls to see what’s happening with stinkbug damage.”

Researchers harvested the plots and measured yield, he says. The untreated plot yielded 834 pounds of seed cotton per acre with a 32-percent turnout and 272 pounds of lint per acre. Where five sprays were made, the yield jumped to 3,600 pounds of seed cotton with 35.5 percent turnout and 1,283 pounds of lint per acre. Where cotton was sprayed seven times, the yield jumped by 400 pounds of seed cotton, about the same turnout, and an increase of about 100 pounds of lint per acre.

“You’ll have to treat the borders of cotton fields differently whenever they are adjacent to peanuts, and you won’t be able to afford to treat your entire field in this manner.”

Researchers also are looking at threshold treatment levels for stinkbugs, says Smith. During the first couple of weeks of bloom, there aren’t a lot of bolls on the plant susceptible to stinkbug damage, he says.

“We want to use a 20-percent threshold. This is the latest thinking of the multi-state research project. In weeks three, four and five of bloom, we have to drop that to 10 percent if we want to make optimum yields. We think we can go back to 20 percent as the bolls thin out. Then, if cotton matures out, we can go up to 30 percent. We don’t have as many bolls at risk as the cotton gets towards the end.”

When surveying for boll damage, growers should look for any kind of discoloration or markings on the outside showing that it has been pierced, says Smith. Normally, if the stinkbug penetrates the boll wall, one or more locks will rot, he says.

Growers also should keep in mind, he adds, that stinkbugs have a long life cycle. Immature bugs last about four weeks while adults last four or more weeks, he says. If a stinkbug hatches in a cotton field in July, it’ll be there when the cotton matures.

“A boll is safe when it is about 25 days old. If it’s hard, you don’t need to sample it. If it doesn’t feel soft to the touch, you don’t need to pull it. You’ll have to take about 25 bolls to get much of a sample. You need to be concerned about stinkbugs for as long as there are soft bolls.”

Turning to other cotton insect issues, Smith says that in the future, there probably won’t be any single-gene Bollgard cotton available to producers. “We’ve had Bollgard since 1996, and we’ll probably have it through 2009. They’ve re-registered it once, but they do not plan to re-register it again. That means there probably won’t be any single-gene Bollgard cotton after 2009 unless they allow the sale of the seed grown in 2009 for 2010 planting. Everything will be stacked-gene because of resistance management. So we need to be looking at our options for the next few years.”

Another issue of concern to growers is bollworm resistance to pyrethroids, he says. It has been a standard practice to over-spray Bollgard cotton if there were bollworm escapes, and that has worked since 1996.

“There are some areas of the United States — one in Georgia and one in Louisiana — where it no longer works to over-spray Bollgard with pyrethroids. It’s fairly widespread, in New Jersey sweet corn, Minnesota and even in Canada, there are pockets where you can’t control bollworms or corn earworms with pyrethroids. These pockets eventually will spread. If you get into a heavy bollworm year, you might start seeing this.”

Every stalk of corn grown will grow a corn earworm, says Smith. “Between 25,000 and 30,000 corn earworms per acre will come out of corn and go into cotton the next generation. We could see a heavier bollworm year. The alternatives for pyrethroids are very expensive, and there aren’t too many available.”

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