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5 lessons for spider mites

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.

    “The mites in our area are resistant to the organophosphorus and pyrethroid insecticides, even Bifenthrin,” he says. “We have had to switch to different and newer miticides like Comite II, Oberon, Onager, and Zeal. Fortunately, these products do not get the predators.”

    Another factor that may lead to more mites is the use of foliar fungicides to control plant diseases. Some fungi are known to be lethal to mites, so when you spray corn or beans with a fungicide, you may also be inviting more mites, he says.

    Mites also like some grassy and broadleaf weeds that can emerge prior to planting. If your herbicide programs kill those weeds, this too may push the mites to move where you don’t want them.

  4. Watch them move up and be ready to act. A threshold formula for spider mite treatment based on crop value and control costs was formerly used in Texas. “This worked well for us in Texas for years when we had effective and fast-acting miticides,” says Bynum. “The current ones are slower acting, so we don’t use the threshold anymore.”

    Rather, he now recommends treating for spider mites whenever the mites move up corn plants and get close to the ear leaf, especially under increasing populations.

    “Then you are losing yield,” he says. “If you know you have beneficial insects in the field, then you might want to wait a couple of days and check again to see if they control the mites.”

    Current control cost has been $25 to $30 per acre or a little more. The best approach is to spray spider mites with a miticide that doesn’t impact the beneficial insects, and then let them clean up any remaining mites. In many cases, that prevents the need for a second miticide application. Miticides are not systemic, and if the mite population rebuilds, you will have to control them again later.

  5. Aim low. Good miticide coverage on corn and soybeans on lower plant parts is key, since that’s where the mites establish themselves. Since mites proliferate when the plants are in full foliage, the canopy can act like an umbrella and keep the chemical from reaching infestations, says Bynum. Some insecticides are a chemical repellent and cause mites to spread out to avoid contact. That can actually stimulate more reproduction.

    “Do anything you can to get better coverage,” says Bynum. In some tests they’ve done, only 17% of the chemical mixture gets to the bottom third of corn plants, and it’s a similar story in full-height soybeans. It may mean you need to put on higher volumes of the chemical mixture, up to 2.5 gallons per acre.

To deter miticide and insecticide resistance, avoid use of what Bynum calls “hard” chemicals (miticides and insecticides that tend to destroy the beneficial insects). “Rotate the chemistry on the mites,” he says. “If you have to treat twice in a season, never use the same chemistry the second time.”

He adds that mixing insecticides doesn’t always improve control and may lead to quicker resistance to both products.

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