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Conservation-Plant Galls By: Kevin Williams, Grundy County Conservation Director

September 27, 2018
Reinbeck Courier
It is a weird but wonderful place out there in the landscape if we pause long enough to notice. Take galls, for example. Galls are deformities found on the leaves, stems and flowers of trees, shrubs and other plants. Plant galls come in a fascinating variety of strange forms, textures and colors. Some are irregular, bumpy or warty; others are smooth and spherical. Some galls sport thick growths of fuzz, hair or spines. What causes galls? Galls are abnormal, vegetative growths that are  usually formed as a response by plants to the action of fungus, mites, or insects such as wasps, aphids, or true bugs. Galls can be formed in the leaves,  twigs,  buds,  trunks, or on the roots. Gall formation is an equally bizarre biological puzzle. Galls result from an interaction between two living organisms. Gall-makers - the organisms that initiate the process - are tiny, highly specialized wasps, flies, mites, aphids or microbes. Their attack on a host plant unleashes a unique response. The mystery of gall formation was not understood until recent times, even though galls have been written about since the days of the Roman Empire. Plant growth-regulating chemicals produced by the gall-maker interact with the plant’s own hormones to produce galls in young, newly developing tissue. The plant abruptly changes the course of normal growth and modifies growing tissue into a special swelling that surrounds the tiny insects and mites. On a recent walk, I encountered two examples of plant galls. The first was a large gall on the trunk of a Burr Oak tree. On large tree trunks, galls may reach a diameter two to three times that of the tree at point of occurrence and at times encircle the stem. Like a true burl, a gall is a product of excessive division and enlargement of cells from abnormal cambial activity stimulated by bacteria or fungi; the wood is characterized by wildly contorted grain. Many galls contain small knots with pith centers, ingrown bark, and concentrations of stain. The second was on a much smaller plant. The Goldenrod Gall Fly (Eurosta solidaginis)  lays its eggs in  the stem of the goldenrod. The plant reacts to the larva’s chewing  at the affected site by swelling into a ball.  This phenomenon  creates a “goldenrod gall.”    Notice the small hole near the base of the gall.  This is the hole from which the fly exits after maturing. Fishermen for years have harvested the “worm” by cutting open the gall and removing the larva. They soon learn that if a small hole exists on the gall, the larva has exited the temporary home to become the adult fly.

Article Photos

Plant galls can be quite large like the one on this maple tree. and much smaller as on this goldenrod.



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