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5 lessons for spider mites

Gene Johnston 05/08/2013 @ 10:58am On the scene at the 2012 Cattle Convention, Nashville

There’s a reason spider mites are causing more grief in soybean and cornfields now: We’re helping them thrive.

Solve one problem, create another. As we better control weeds, insects, and plant diseases, we may be opening a door for the mites.

Ed Bynum, a Texas A&M Extension entomologist, has seen the front line of the spider mite battle in Texas corn.

The mites have been a scourge there for a long time, slicing yields by 23% or more if unchecked. They’re also a pest in Midwestern fields.

Bynum says the two most common spider mites that damage crops are the banks grass mite and the two-spotted mite. The former mite infests only grassytype plants like corn, wheat, grain sorghum, and grassy weeds. The two-spotted

mite infests grass plants and soybeans or broadleaf weeds. “There’s not much difference in their feeding patterns and damage,” says Bynum.

Here is his checklist of five points to remember about spider mites.

  1. Know that they emerge slowly in early season. Early mite infestations originate from field margins, where mites overwinter in grassy areas, CRP fields, or neighboring grass crops like wheat. As temperatures warm up in the spring, the mites blow across crop fields. In hot and droughty conditions, numbers can explode in midseason. Early infestations may or may not damage yields later, since predators and a mite fungal disease (neozygites) may check their numbers.

  2. Be on the lookout at tasseling. Mites damage corn most from the tassel to dent stage. Partly due to the period’s heat and dryness, the mites can reproduce at about four times their normal rate. They tend to start feeding low on the plant and move up.

    “You can easily see damage from the road in severe cases,” says Bynum. “That early browning of corn plants that you see sometimes could be due to spider mites. They can kill corn.”

    Grain yield loss up to 23% usually occurs between the tasseling stage and the dent stage. “In heavy infestations, the plants are more susceptible to stalk rot, and you might see lodging on half the plants that is directly related to the mites,” he says.

  3. Be aware that closing one door may open another. Some insecticides may kill insects that control spider mites. That lets the mites flourish. Predator insects include six-spotted thrips, minute pirate bugs, predatory mites, spider mite destroyer beetles, and syrphid fly larvae. Studies show each of these bugs can consume two or three spider mites per hour.

    Several organophosphate and pyrethroid insecticides are especially effective at killing beneficial insects. “In a normal year, we only treat about 20% of our cornfields for spider mites. In 2012, it was 90%,” says Bynum. “That was due to a combination of hot, droughty conditions during the grain-filling stages and a lack of predators.”

    Some applicators who control mites mites per hour.

    Several organophosphate and pyrethroid insecticides are especially effective at killing beneficial insects. Also, says Bynum, continually using the same insecticides for mites and other crop pests will quickly lead to resistance.

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