While exploring along the Oregon coast on the last day of March, I noticed that the willow bushes which grew on the lower edge of the hillside were just starting to bare their catkins. One bush in particular caught my eye. Almost every branch had a large deformed mass about halfway between the base and the tip.
This willow, possibly Hooker’s willow (Salix hookeriana), was infected with galls. Stem galls are unusual growths that are formed by the feeding or egg laying activities of insects or other small organisms. Willows in general seem to be susceptible to galls, and this genera has one of the highest number of infecting species. This particular gall is probably the result of a developing fly or wasp that has bored into the branch and has secreted a chemical which causes swelling as a result of the irritation. In some cases, the chemical is injected by the adult female as she lays eggs within the stem. As the gall grows, it provides protection from predators, food and shelter for the parasite.
Although galls may look bad, they usually don’t threaten the health of the plants they occupy. Besides stem galls, there are also galls that develop on leaves, buds and roots. In many cases, experts are able to tell the species within the gall just by its shape. There are over 1,500 species that produce galls.
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Galls are interesting. I just found a bunch of nipple or pouch galls on cherry leaves. I’m looking for more oak galls and maybe a goldenrod stem gall or two.
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Happy gall hunting. I am going to see what kind of galls I can find in my woods. I’ve never actually done a search for them.
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Rick, I’ve seen many galls on galls on willows and other tree species, but oaks are host to more than 200 species of gall wasps (Family: Cynipidae). These insects g cause galls (abnormal structures) on the leaves, flowers, buds, acorns, and twigs of oaks. Although produced by the host plant, these galls are initiated by gall wasps. Galls are stimulated to grow when female gall wasps lay their eggs on their preferred host when the plant is in a specific stage of development. This may be one of the reasons why population levels are variable. Individual larvae develop within a cavity or multiple cavities inside each gall, feeding on the materials lining the cavity. Formation of galls may be within or on the affected plant part. Galls may contain one to many larvae. At maturity, the larvae pupate and chew their way out. Each species of gall wasp produces a gall of a particular size, shape, and color. In some species, alternating generations form different galls on different plant parts. For more information see Field Guide to Plant Galls of California and Other Western States (Russo 2006).
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I was curious about your interest in galls from the standpoint of an arborist. My understanding from what I have read is that a majority of the trees and plants are not harmed by galls, except for maybe the slight amount of extra energy they must expend to provide for the parasites. If this is the case, would your focus be to minimize galls for the sake of aesthetic beauty?
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Fascinating stuff, Rick. I find galls on willows, oaks and other species here in the UK too. I wonder how the wasp stimulates production of a structure that feeds its young, it’s a cunning piece of evolution.
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Thanks, Finn! I wish I had a little more information about the specific organism that inhabits this gall, so I could have provided a more detailed description of this particular gall.
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Interesting information!
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