Spider mites on juniper

Junipers are among the most commonly planted conifers in North American landscapes and are disproportionately affected by the spruce spider mite. The mite attacks in spring and fall when temperatures are cool -- the opposite of most garden pest problems. Gardeners who expect a summer pest and delay.

Junipers are among the most commonly planted conifers in North American landscapes and are disproportionately affected by the spruce spider mite. The mite attacks in spring and fall when temperatures are cool — the opposite of most garden pest problems. Gardeners who expect a summer pest and delay inspection until July find the spring damage already done and the mite population temporarily dormant.

I don't grow junipers at my Long Island property, so this guide is sourced from Penn State Extension, Cornell Cooperative Extension, and NC State Extension research.

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The pest

The spruce spider mite (Oligonychus ununguis) is the primary spider mite of junipers and other conifers in the northeastern and mid-Atlantic US. Per Penn State Extension, it is a cool-season species with very different seasonal behavior than the two-spotted spider mite that dominates warm-season ornamental and vegetable pests:

• Active periods: April–May (spring) and September–October (fall)
• Dormant in summer heat (above approximately 85°F, 29°C) — eggs remain viable but active mites are absent
• Overwinters as reddish eggs on juniper foliage and bark
• Each generation takes 10–14 days to complete in cool conditions

Per Cornell Cooperative Extension, a second mite, the juniper mite or Panonychus spider mite, may occasionally infest junipers in some regions, but O. ununguis is the dominant species.