Spruce Spider Mite

Spruce Spider Mite Oligonychus ununguis (Jacobi) is considered one of the most destructive spider mites in the United States. It injures the foliage of spruce, arborvitae, juniper, hemlock, pine, Douglas-fir, and occasionally other conifers. Dwarf Alberta spruce, Picea glauca ‘Conica’, is one of this pest’s preferred host plants.

The spruce spider mite is considered one of the most destructive spider mites in the United States. It injures the foliage of spruce, arborvitae, juniper, hemlock, pine, Douglas-fir, and occasionally other conifers. Dwarf Alberta spruce, Picea glauca ‘Conica’, is one of this pest’s preferred host plants.

The spruce spider mite is a cool weather mite. Hot dry weather causes this “insect relative” an adult mite has eight legs so it’s not a true insect which has six legs to cease feeding and to disappear till fall. Spruce spider mite can cause severe feeding damage to evergreens when they build to a high population. Spider mites are a piercing sucking type insect relative. They feed on chlorophyll in leaf cells. The feeding empties the cell of the green chlorophyll. Thus a tiny cream to yellowish spot occurs. The feeding damage is called stippling. Severe stippling/feeding damage can cause the needle or scale leaves of the evergreen to die. Spruce spider mites often produce a single silk thread similar to a regular spider.

Life Cycle : Winter Red eggs found on the bark of small branches. Spring Egg hatch occurs as early as mid-March, but most eggs hatch by mid-April. Young mites feed on the previous years foliage and do not attack the current season’s growth until it hardens off in summer. Spider mites thrive when daytime temperatures are in the 60’s and 70’s.

Summer When the daily temperatures consistently exceed the the mid 80’s, adults become inactive and populations decline due to an increase in predatory mites and insects. If temperatures remain over 90 F for an extended period, the adults become dormant and lay tan or salmon colored eggs that only hatch when cooler temperatures return in late summer/early fall.

Fall Adult mite feeding activity resumes and continues until late fall. Adult females lay eggs on the bark of small branches, starting in early September and continuing until a hard frost occurs.

Damage : This species damages host plants by sucking plant fluid from needles as they feed. Infested trees at first have a speckled, yellowish appearance, and lack rich green color. After prolonged feeding, needles turn rusty colored and may drop prematurely. Mites usually attack older needles located in the lower and inner parts of the plant. Damage may spread as the season progresses. This species also produces silken webs on the needles.

The spruce spider mite injures the needles by inserting its slender mouthparts and sucking out the sap. The puncture causes a stippled, bleached discoloration. In severe infestations the needles become dingy yellow or brownish and often needles dry and drop off. Damage is usually first noticed on the lower inside branches but spreads upward and outward as the infestation progresses. The mites spin fine silken webbing throughout the twigs and needles.

The webbing is most noticeable on the underside of the branches where dust particles, dead needles and dead mites adhere. In spruce plantings, infestations can persist for years resulting in reduced tree vigor and eventually death. Recently planted trees, nursery seedlings, or trees under stress are especially susceptible to serious injury. To check for mite infestations, vigorously shake a branch over a piece of white paper, then rub your hand across the paper or press the paper in half. If red smears appear, mites are probably present. Trees should be checked regularly since mite populations can increase rapidly.

Control: Frequent monitoring for spider mites early in the season is suggested to determine if controls should be started. The easiest method available to check if SSM are present is to hold a white object, such as a piece of paper or paper plate, under a branch. Tap the branch with a pencil or similar object to get a consistent force, which will dislodge the mites. If mites are present, they will look like small specks of soil or pepper moving around the paper. Spruce spider mites will leave a red mark when squished. Their presence should be verified by using a small hand-lens. Sample 3-5 branches per plant suspected to be infested and check each suspected plant in the landscape. Also look for the white, fast moving Phytoseiid predatory mites. If the predaceous mites are in abundance, they will effectively suppress pest mite populations. However, if you see more than 10 spruce mites per tap and no predators, treatment may be required. Use a lower per tap threshold on Alberta spruce.

Horticultural oil may be used as a dormant spray in April before new growth begins. After new growth begins in mid- to late May and in late August through mid-September, treat with acephate, horticultural oil, insecticidal soap potassium salts of fatty acids, lambda-cyhalothrin, malathion, neem oil, or Mite-X plant extracts. Treat undersides of leaves as well as upper sides.

Check for mites by holding a piece of white paper under a branch and tapping the branch against the paper. If small dust/ like particles are seen moving around, check the moving speed or their color when squashed. If only a small number of spruce mites are found, you may not need to do anything. If, however, a high population of only spruce spider mites is found, a miticide control is usually recommended. If there are numerous beneficial mites present at the same time as the spruce mites, a chemical control is not suggested.

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