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Guidelines for...: Controlling late-season insects

IT'S THAT time of year when cotton producers must make the difficult decision whether to stop spraying the insects in their fields or hit them with one more shot of insecticide.

It's also the time of year when your crop may already be made and insect control becomes questionable because the late-season insects making their home in your fields are likely becoming tolerant to insecticide treatments.

Louisiana entomologist Roger Leonard at the LSU AgCenter's Northeast Research Station in Winnsboro, La., believes a cotton crop has reached its highest value right before defoliation. "Squares don't have any value but harvestable bolls do," he says. "Anyway you look at insect control, it's expensive and it's not going to get any cheaper as the season progresses. Late-season insect control can, however, protect what you've got in the field."

Among the late-season insect pests growers may be debating whether to treat with insecticide sprays are bollworms, budworms, boll weevils, armyworms, loopers, plantbugs, whiteflies, aphids and spider mites.

"When fall armyworms set up camp they are very difficult to control. At this point in the season, there is no insecticide that you can put out that's going to kill more than 50 percent of these mature pests," Leonard says.

Similarly, Leonard says, growers should be on the lookout for damage caused by plantbugs and stinkbugs. Stinkbugs, which cause damage similar to that caused by fall armyworms in late-season cotton, may be difficult to control. Green stinkbugs are easily controlled by pyrethroids, but both brown species of the insect require other, more costly, insecticides for effective control, he says.

"When can you stop spraying for insects?" Leonard asks. "When you get within a couple of weeks of defoliation you are wasting your money if you are treating insects with an insecticide."

In Louisiana, Leonard recommends growers begin terminating their insect control methods when their cotton crops are past the cut-out stage.

In a normally developing crop, cut-out is defined as beginning when terminal growth declines to the point that there are only five nodes above the first position white flower (NAWF5). During this period the crop has set and is maturing the last population of bolls that will effectively contribute to yield, according to Mississippi State University guidelines.

When cotton reaches NAWF5 and has accumulated an additional 350 to 450 heat units, also known as DD60s, any damage by tobacco budworms, boll weevils, bollworms and plantbugs, isn't expected to adversely affect crop yield. However, a cotton crop at this stage can sustain economic damage from insects such as beet armyworms, loopers, fall armyworms and aphids.

"As you move up the plant, the boll value decreases," Leonard says. "We find out very quickly that the most important part of your crop is in the middle of the plant closest to the stem."

Leonard says it's time for Louisiana growers to quit spraying for insects when their cotton crop has reached NAWF4 or NAWF5 and has accumulated an additional 450 heat units.

In Mississippi, Delta Research and Extension Center entomologist Aubrey Harris recommends growers terminate the control of bollworms, tarnished plantbugs and boll weevils when their cotton reaches NAWF5 plus 350 to 400 heat units.

Another way to determine when to terminate insect control is to use the weather-oriented rule. Based on 30 years of weather data, this method dictates that insect control be terminated as soon as a cotton crop accumulates 450 heat units after Aug. 17. "At that point, the majority of your cotton crop is safe from insect injury," Leonard says. "I've never had the weather-oriented rule fail."

"When you get big caterpillars in your cotton field, however, the rules change because they can continue to damage the crop," he says.

Harris says cotton growers trying to control late-season leaf-feeders should continue spraying until NAWF5 plus 650 to 700 heat units, or eight to 10 days before defoliation. "We haven't done any research on this, but leaf-feeding insect populations will require additional late-season control in order to protect your cotton crop."

For late-season salvage treatments for the control of fall armyworms, beet armyworms, cabbage loopers, or soybean loopers, a grower's best bet may be a pyrethroid treatment, Leonard says. "It's a good opportunity not to waste your money because no matter what you do, you're not going to kill more than 50 percent of the population in your field that late in the season," he says.

"The best thing you can do for insect control is to harvest your crop and mow it down the next day," Leonard adds.

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