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Description and Biology

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The wool sower gall is a distinct and unusual plant growth induced by the secretions of the grubs of a tiny gall wasp, Callirhytis seminator. These wasps are about 1/8 inch long, dark brown, and the abdomen is noticeably flattened from side to side. The grubs are translucent to white, plump, and legless. Their heads are more or less shapeless blobs. The wool sower gall is a “leaf gall” specific to white oak and only occurs in the spring. When the gall is pulled apart, inside are small seed-like structures inside of which the gall wasp grubs develop (the wool sower gall is also called the oak seed gall). Gall wasps have alternation of generations in which one generation develops in one type of gall (leaf gall) and their offspring develop into another type of gall (stem gall). Wasps of each alternate generation are slightly different in size and the galls of each generation are enormously different from the parent's. The wool sower gall may be the leaf gall of this species because of its transient nature. We don't know what the stem gall looks like. If a fresh wool sower gall is held in a plastic bag out of the sun (so it won't get too hot) within a week or three, tiny gall wasps will emerge. These wasps are harmless to people. Fortunately, wool sower galls are hardly ever abundant enough to cause real harm to white oaks.

Wool sower gall

Wool sower galls are fluffy white, then white with pink spots, then brownish.

The wool sower gall is sometimes called the oak seed gall.

The wool sower gall is sometimes called the oak seed gall.

The "seeds" are actually hard plant material that protects the w

The "seeds" are actually hard plant material that protects the wasp grub.

Wool sower gall wasp grub

Wool sower gall wasp grubs are plump, legless, and white.

A typical female gall wasp.

A typical female gall wasp.

Host Plant

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White oak is the only host for the wool sower gall wasp.

Residential Recommendations

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I've never seen this happen, but if the galls are so abundant as to actually damage a tree, the best time to try to control them is in mid-winter when the wasps are laying their eggs or in spring just as the buds are breaking. The eggs hatch just as the new growth emerges in spring. Orthene or some other contact insecticide might give adequate suppression. However, by the time the galls are noticed, it is way too late to control the gall wasps. The grubs are well protected by the amazing gall tissues.

References

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For assistance with a specific problem, contact your local Cooperative Extension center.

This factsheet has not been peer reviewed.

Author

Professor Emeritus
Entomology & Plant Pathology

Publication date: May 10, 2017
Revised: Oct. 23, 2019

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