Insect + Tree = Something Weird

Lately I’ve been seeing insect galls on the ground that were blown down from the strong winds of spring thunderstorms. A gall is often an odd-shaped structure on a plant stem or leaf. They can resemble a nut or fruit, but they won’t look like any you’ve ever seen before. Some are brightly colored such as a white sphere with scattered red dots. Others resemble an inflated green or brown beach ball with protruding short points. On leaves, some galls look like numerous short pins that stick up from the leaf surface. To someone familiar with plants, the common response to seeing a gall is to think, “That doesn’t look right!”

The strange-looking galls are formed by the growth of the plant tissue, often following an insect laying its eggs in the plant’s twigs or leaves. The gall serves to wall-off the invader from the rest of the plant. The gall itself is usually composed of spongy tissue and encapsulates the eggs and eventually the larvae of the insect that laid the eggs. If you break open or cut open the gall, you may find eggs or squirming young larvae resembling maggots. As the insect matures, it will eventually burrow out of the gall and fly free.

Some of the common galls that might be seen now are on oaks, ashes, maples and other Missouri trees. A number of small wasps as well as midges will lay their eggs in Missouri trees and cause gall formation. Members of the rose family, willows and goldenrods also commonly contain galls. Galls can also result from the fungal or bacterial infection of the plant.

Galls don’t usually have an appreciable effect on the health of the plant. It is not unusual for a healthy tree to provide the nursery space for the next generation of the local insects. Trees occupy a lot of space and it shouldn’t be surprising that other species in the ecosystem have their own ideas about how to use all of that plant tissue.


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