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Unchecked spider mites spell trouble

What has the potential to increase from one to several million in just a few days? If you answered spider mites, you know all too well the reproductive capability of the pest that plagues many Delta cotton growers.

Effects of the pests can range from stippling of leaves to dead plants, or with heavy infestations, completely dead spots in a field.

“We'll spray for spider mites somewhere in the state every year,” says Angus Catchot, entomologist for the Mississippi State University Extension Service. “But last year was a bad one, particularly in the south Delta. They hit really early in the season, and then when the summer heat and drought set in, they exploded. We sprayed 300,000 acres, which is almost unheard-of.”

At grower meetings in northwest Mississippi recently, he said Delta growers averaged 1.5 spray applications for spider mites last year, those in the hills averaged one.

“In some cases, we were seeing as many eggs on plants as adults; there were so many, they were even on tops of the leaves. By early July, they had spread across the state. The true miticides and acaracides were the treatments of choice — moreso than the knockdown materials — and demand was so heavy that companies were running out of products weekly.”

Mississippi applied for and got a 24c emergency use label for abmectin (Zephyr), “which got us out of a jam,” Catchot says.

Spider mite reproduction is heavily influenced by temperature, he notes — at 60 degrees, a female spider mite may lay only 20 eggs; at 80 degrees, the number increases dramatically. The pest can go from egg to adult in five days or less, and a lone female has the potential to become millions over a period of several days.

“We found last year that we needed to revise our threshold level,” he says. This year, the recommendation will be that when 40 percent to 50 percent of the plants in a field are infested, and the weather is hot and dry, treatment is warranted.

The problem with infestations like those in 2005, Catchot says, is that “growers don't want to spend that much money, $9 to $16 per acre, that early in the season. But in every case last year, those who made the decision not to treat made the wrong decision.”

When weather conditions are conducive for rapid population increases, he says, “these early-season spider mites can cause serious problems, so don't delay in making a treatment.

“In such situations, the true miticides provide the best control. Good coverage is essential, so increase the spray volume and stay away from low-drift sprayer tips. With some products, two applications may be necessary.”


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