Spider mite control

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Unfortunately, with the dry conditions mites are more common than aphids. Many are spraying in the southern tier of counties, and I’ve had a bunch of reports of mite populations returning after treatment. Several things may be happening:

  • Egg hatch: Mites lay eggs on the plant surface. (The eggs are quite large and easily seen with a hand lens. Here is a picture: http://extension.missouri.edu/explore/images/ipm1025art18.jpg
  • Insecticides kill adults and nymphs, but do not kill eggs. Since Dimethoate and Lorsban have short residual, the newly hatched nymphs survive and repopulate the plants.
  • Rebound or flaring of mites: Insecticides kill beneficial insects, but likely don’t kill 100 percent of the mites. The mites reproduce in the absence of predators, potentially leading to a rapid increase, or flaring, of the population. This is one of the reasons we recommend scouting and spraying only when mites have reached a threshold, avoiding insurance applications of insecticide.
  • Resistance: Spider mites are notorious for becoming resistant to insecticides. This problem increases with the number of applications. This is another reason we recommend scouting and spraying only when mites have reached a threshold.

If you do plan to treat, check fields before you spray to make sure mites are still present. We have had rain this week, plus the mornings have been “dewy.” Rain itself reduces plant stress and replaces water lost to pest feeding. But more importantly, high humidity is critical for promoting the growth of fungi that naturally infest and kill mites. Humidity must be elevated for an extended time, 48 hours or more, before naturally occurring fungi are active. Mite populations can crash in a matter of days once fungal pathogens become active.

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