Tree Notes is about trees -- especially native trees, trees for wildlife, and trees in history.

Thursday, March 8, 2007

Galls on a Young Pin Oak Tree

Abnormal growths caused by insects


Galls on a young pin oak

This young pin oak tree in Hopkinsville, KY, is sadly afflicted with galls. Galls are spots of abnormal growth caused by an insect. A gall can develop in any part of a plant that an insect can burrow into, and in oak twig galls, the insect is usually a small wasp.

The various university extensions are a great source of information about tree problems like this. Here's a summary of the life-stages of gall wasps from North Carolina State University Extension.

Many gall wasps develop for 2 or 3 years in woody galls on the twigs of oaks. Adults then emerge from the twig galls during the winter. They lay eggs in the buds and die. When these eggs hatch, and new growth resumes on the oak, salivary secretions of the gall wasp grub act as powerful plant growth regulators and force the tree to form the gall. Gall wasp galls typically have an outer wall, a spongy fiber layer and a hard, seedlike structure inside of which the gall wasp grub develops. Although gall wasp grubs have chewing mouthparts, they do not seem to chew plant tissue. Evidently the gall secretes nutrients which the grubs lap up.

Source: Galls on Oaks, by James R. Baker and S. B. Bambara, Extension Entomologists at North Carolina State University.

Branches with galls should be removed and destroyed before the tree's infestation becomes serious. Spraying with insecticides is rarely effective because the wasp spends most of its life as a larva, protected inside the gall. There's a small window of opportunity when it emerges to lay eggs.

A University of Kentucky publication about oak galls states that the weight of the galls can cause stress on infected branches, and the deformation can girdle (thus killing, I assume) a branch.

While researching for this post, I learned some sad news (it was news to me!) about oak trees and galls:

Oaks can have numerous types of galls. Out of the over 800 species of gall making wasps in North America, 731 of them attack oaks. Oak deformities are of various sizes, shapes, and colors on leaves, twigs, flowers, acorns and buds. Galls are so commonly found on oaks that many people think the galls are typical parts of the plants.

Source: "Trees With a Lot of Gall (growths called galls on trees)" by Sandra Mason of the University of Illinois Extension.

I have only seen one other oak around here as badly deformed with galls as the little tree in the photo is. The other tree is a mature pin oak, and it is a sad sight indeed. I'm going to try to watch our oaks here at the house carefully. Hopefully we can "nip the problem in the bud" if it occurs.

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Enrich your life with the study of trees.

"The power to recognize trees at a glance without examining their leaves or flowers or fruit as they are seen, for example, from the car-window during a railroad journey, can only be acquired by studying them as they grow under all possible conditions over wide areas of territory. Such an attainment may not have much practical value, but once acquired it gives to the possessor a good deal of pleasure which is denied to less fortunate travelers."

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Print references I frequently consult

Benvie, Sam. Encyclopedia of North American Trees. Buffalo, NY: Firefly, 2000.

Brockman, C. Frank. Trees of North America: A Guide to Field Identification. Ed. Herbert S. Zim. New York: Golden, 1986.

Cliburn, Jerry, and Ginny Clomps. A Key to Missouri Trees in Winter: An Identification Guide. Conservation Commission of the State of Missouri, 1980.

Collingwood, G. H., Warren David Brush, and Devereux Butcher. Knowing Your Trees. Washington: American Forestry Association, 1978.

Dirr, Michael. Dirr's Hardy Trees and Shrubs: an Illustrated Encyclopedia. Portland, Or.: Timber, 1997.

Elias, Thomas S. The Complete Trees of North America; Field Guide and Natural History. New York: Book Division, Times Mirror Magazines, 1980.

Grimm, William Carey. The Book of Trees;. Harrisburg, PA: Stackpole, 1962.

Hightshoe, Gary L. Native Trees, Shrubs, and Vines for Urban and Rural America: a Planting Design Manual for Environmental Designers. New York: Van Nostrand Reinhold, 1988.

Little, Elbert L. National Audubon Society Field Guide to North American Trees. New York: Chanticleer, 1996.

Martin, Alexander C., Herbert S. Zim, and Arnold L. Nelson. American Wildlife and Plants. New York: McGraw Hill, 1951.

Mitchell, Alan F., and David More. The Trees of North America. New York, NY: Facts On File Publications, 1987.

Randall, Charles E. Enjoying Our Trees. Washington: American Forestry Association, 1969.

Settergren, Carl D., and R. E. McDermott. Trees of Missouri. Columbia: University Extension, 1995.

Sternberg, Guy, and James W. Wilson. Native Trees for North American Landscapes: from the Atlantic to the Rockies. Portland: Timber, 2004.

Wharton, Mary E., and Roger W. Barbour. Trees and Shrubs of Kentucky. Lexington: University of Kentucky, 1973.

Wyman, Donald. Trees for American Gardens. New York: Macmillan, 1965.

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