Biology
Eriophyid mites are very tiny, white or yellowish, worm-like, and spindle-shaped. Their bodies have definite annulations or rings, and only two pairs of legs directly behind the mouthparts.
Most trees and shrubs have species that attack them and almost always create predictable damage like galling, etc. On many broadleaved plants, these mites can cause pale green, yellow, orange, or even reddish blisters to appear on the upper leaf surfaces. The mites feed on the lower leaf surfaces, where blisters turn white, yellow, or brownish. Sometimes these blisters can contain masses of enlarged leaf hairs (erineum). The mites overwinter under outer bud scales. They emerge and feed on the leaf surfaces from May to July, before migrating back to the outer bud scales. Highly susceptible plants include maple, mountain ash, fuchsia, walnut, pear, and grape. On apricot and peach trees, eriophyid mites facilitate “silvering” of foliage; heavy infestations can lead to defoliation and death.
Eriophyid mites on conifers can be found on needles and buds, and may cause multiple bud formation (rosetting) and severe yellowing, stunting, and curling of needles. One common species affecting conifers is a sheath mite on pine. Sheath mites overwinter on pine, and migrate in spring to the needle sheath where they feed and multiply. Sheath mite infestation will produce yellowing of the needles. Affected needles later turn brown, die, and drop off, leaving naked branch tips. Two other species are leaf or needle vagrants which wander about and feed on needle surfaces of Douglas fir and true fir. Needle vagrants overwinter as eggs, hatch in April, and begin feeding on needles. As many as six generations may occur in one season. Needle vagrants disperse by wind, as immature mites stand on their heads and blow off the foliage to other trees. Foliage affected by leaf vagrants turns olive green or bronze and may drop prematurely. Symptoms of eriophyid mite damage on conifers may be confused with damage caused by environmental stress, chemicals, and nutrient imbalances.
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