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Weekly “What is it?”: Leaf galls

As an ecologist and often the token “outdoorsy” person in any given crowd, people often point to a plant or animal or describe something they’ve seen and ask me, “What is it?”  I relish these small opportunities to teach my family and friends about the fascinating things they can find in nature.  To that end, I decided to share the answers to some of those questions in a weekly blog post.  The “Weekly What Is It?” will include a new photo and explanation every Wednesday.  This week, we’ll discuss a fascinating little phenomenon that I’m often asked about during trail hikes.

These spherical galls seen on live oak leaves show where three gall wasp eggs were laid.  The adult wasp's exit hole is visible on the right.

The spherical galls seen on live oak leaves show where three gall wasp eggs were laid. The adult wasp’s exit hole is visible on the right.

Insect galls are hard, three-dimensional growths on the leaves or stems of plants. While galls can form on many species of plants, typically (60% of the time) they are found on oak trees and created by gall wasps that have laid eggs into the plant tissue.   Much like human skin reacts to a splinter or insect bite by swelling around it, plant cells respond to this foreign tissue by growing a gall around the expanding egg sac to protect the plant.  Insect larvae eventually feed on the gall tissue until they’ve reached adulthood.  Many galls have small pinholes, a sign that the adult insect has emerged and the gall is empty. Each gall-laying insect has its own characteristic shape. The gall wasp creates one (see photo) approximately a half inch long and spherical.

Another commonly seen gall is that of the swamp bay (Persea palustris), a tree that I’ve never seen without many green, oblong galls wrapped into the curled leaves.While strange looking, galls rarely cause problems for the plants and are an example of commensalism, a relationship in which one organism benefits while the other is neither harmed nor helped. The word “gall” means “bitter”, a reference to the taste of these tannin-filled growths.If you have a mysterious “something” that you’ve always wondered about, email me a photo or description at, and hopefully it will be featured in a future post.

One Comment on “Weekly “What is it?”: Leaf galls

  1. Pretty sure my live oak had stem galls; what treatment, if any, is available?