Pest description and damage Adults of the pearleaf blister mite are very tiny and can be seen only under magnification. These mites are light to amber yellow, cylindrical, tapered at the posterior end, with two pairs of short legs at the front of the body. The overall appearance is that of a small worm. Nymphs are similar in appearance to the adult but are smaller. Pearleaf blister mite feeding on leaves causes reddish to yellowish-green blisters; blisters turn brown or black as the tissue dies later in the season. Leaves may drop prematurely. Loss of foliage weakens trees and reduces shoot growth and flowering.
Biology and life history Blister mites overwinter as mature females under outer bud scales. As buds swell in the spring, the mites burrow inside to feed. As leaves unfurl, blisters are already evident. At petal fall, they move to more leaves or fruit, causing the characteristic blisters. Several generations per year may develop within the blisters. As they become more crowded, mites move to growing terminals, where their feeding produces more blisters. Mites move from tree to tree by wind or carried on birds or insects.
Pest monitoring Scouting for pearleaf blister mite is not effective during the current season. By the time blisters are noticed, the mites are safely inside and the damage is done. Plan to take action the following fall or winter if damage is noted. Sufficient control is achieved by midsummer by biological agents.
Phytoseiid predator mites almost always keep mites under control if broad-spectrum insecticide applications are avoided. Heavy rain and cold weather also suppress mite numbers.
Suppression of broadleaf weeds such as mallow, bindweed, white clover, and knotweed with cultivation or grasses may reduce mite numbers. Wash mites from the tree with a strong stream of water. Water trees properly, as drought-stressed trees are more susceptible. Avoid excessive nitrogen applications, as this encourages mites.
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