NC State Extension Publications

Description and Biology

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The willow bladdergall mite, Aculops tetanothrix, is an eriophyid mite that causes tiny, hollow galls on willow. These mites are virtually microscopic (0.2 mm long) and have two pairs of legs at the front. Reddish willow bladdergall mites overwinter on the bark and under bud scales of willow and in spring crawl to the new, tender leaves just as the new growth emerges from the buds. (After the leaves have fully expanded, the mites can no longer induce gall formation.) They feed on the lower surface and cause leaf cells on which they feed to become swollen and to bulge up into a tiny balloon-like structure. The gall protects the mites from the environment and predaceous insects to some extent. Females lay eggs that develop into white bladdergall mites that feed and induce the plant to maintain the bladder galls in which the mites grow and reproduce. Eventually, the gall "matures" and is no longer suitable for the mites. By that time the mites have given rise to a generation of reddish mites that are able to survive outside the galls. The rest of the growing season, fall and winter are spent on the bark, in bark crevices, and wedged into bud scales. The galls are not likely to cause any real injury to pussy willow, and these mites will not infest any other kind of ornamental plant that is not a willow.

Life cycle illustration of willow balddergall mites

Willow bladdergall mites are barely visible with a 10 X hand lens.

Photo of willow bladdergall mites

Willow bladdergall mites can cause considerable aesthetic injury to willow.

Photo of young willow bladder galls on leaf

These are young, fresh willow bladder galls.

Host Plants

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Apparently several of the 400 species of willow (Salix) are suitable hosts for the willow bladdergall mite including black willow, pussy willow, and Ward's (Carolina) willow.

Residential Recommendations

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In general, eriophyid mites are sensitive to Sevin insecticide. If there is real interest in preventing gall formation next year, control measures can be directed against the overwintering mites. Spraying this season while the bladder galls are fresh probably won't kill the mites inside, and spraying won't make the galls disappear, either. Infested trees can be sprayed with Sevin at the rate for caterpillar control any time after the galls change to brown or black. Other insecticides available in big box stores that are labeled for residential landscape use should control the mites as well.

Author

Professor Emeritus
Entomology & Plant Pathology

Publication date: July 10, 2020
Revised: July 10, 2020

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