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Spruce Spider Mite – Facts at a Glance

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SSM (Olygonychus ununguis)

PDF version of SSM – Facts at a Glance (including pictures)

Where from: Native to US

Host plants: Firs, spruces, hemlocks, other conifers

Symptoms: Yellow spots on otherwise green needle. Damage starts at needle base and moves upward towards tip with continued feeding.

Life cycle:

  • How overwinters: Eggs on shoots that hatch in spring when weather warms.
  • Number of generations per year: Multiple generations per year.
  • Eggs hatch to 6-legged larva followed by two, 8-legged nymphal stages, and adults (male or female). As it molts, the mite goes into a resting stage which doesn’t move and may appear dead.
  • Weather conditions: The spruce spider mite is considered a cool-season mite and is well suited to conditions in western North Carolina. Spider mites are move active in dry weather and low humidity because of effects on spider mite hatching and on predatory mites.

Important cultural practices:

  • Site selection – spider mites are worse on windy ridges, fields with southern and western exposure, and elevations lower than 3,000 feet.
  • Dust from gravel roads that drift onto needles create more problems with spider mites.
  • Maintain flowering ground covers. Allow weeds with stems such as briars or stick weeks in field borders as these are habitat for predator mites.

Scouting: In trees nearing harvest, scout for spider mites in the spring and during periods of dry weather. Look for mites and eggs on small shoots of most current growth – lower half of tree. Beating foliage will dislodge mites if they are at high numbers. Keep an eye out for predatory mites.

Biological control: Predatory mites are small, lighter in color, and move faster than spider mites

Control considerations:

  • Don’t be too quick to treat for spider mites – allow predators a chance to do their job.
  • Some miticides control mite eggs – other don’t. Check the label and info on-line.
  • Rotate chemicals from year to year – mites can develop resistance!
  • Some insecticides make spider mites worse if they are already present in the field – including imidacloprid (Merit), carbaryl (Sevin), and esfenvalerate (Asana).