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Nature provides fascinating 'galls' on plants' leaves, branches, stems

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Nature in fall

Insects can cause interesting galls on various plant species.

Any time of the year and especially in the cool days of fall, people may notice some strange tumor-like growths on some leaves, twigs, or flower stems as they walk around their yards or stroll through a park.

Surprisingly, although these growths are part of the plants, they’re not actually made by the plants themselves. They’re called galls and they’re a little-known component of our fascinating outdoors.

Although many people become concerned when they see these growths, galls are usually harmless to the plant. Most begin to grow in the spring, but they become more noticeable later in the year, once the plants lose their leaves. Throughout history, people have known about and even used galls. A mixture of gall juice and iron was once widely used as a source of ink. It was used by Leonardo DaVinci and J.S. Bach as well as Rembrandt and VanGogh. Even our nation's Declaration of Independence and Constitution were originally written using a mixture of iron and juice from oak galls.

Altogether, there are over 2,000 different kinds of galls. Each one is found on a specific plant and produced by specific organisms, including beetles, moths, aphids, flies, wasps, mites, nematodes, fungi, and bacteria. Oak trees host the most different kinds of galls (800), so oaks are good places to look for them. Galls are also commonly seen on willows, poplars, aspens, blackberries, raspberries, goldenrods, and asters.

If you see a fuzzy bump on an oak leaf, you’re seeing a gall. But, galls on goldenrods appear as large swellings on the stems, while other galls appear to look very much like large growths on a twig.

Although many organisms can cause the formation of a gall, galls usually form when an insect or a mite lays its eggs either in or on a plant leaf, twig, or stem. This usually happens in the spring, as the plant is actively growing.

Along with the eggs, the female also injects a chemical into the plant. In some cases, this chemical directly stimulates the growth of new plant cells. In other cases, the chemical actually turns some of the plant’s genes on or off, causing the plant to convert some of its stored starch into sugar. This produces food that stimulates the plant’s cells to grow, producing the gall that eventually envelopes the eggs. When the eggs hatch, the larvae are then enclosed in a warm, protective shelter, surrounded by an abundance of food. For many insects, this is how they survive the winter - as larvae enveloped in a protective gall.

Once the larvae mature, and become adults, they will chew their way out of their home and leave its protection. They will then go on to mate and lay eggs, producing galls for their own offspring, and continuing the life cycle.

Although galls are made for the larvae housed inside, occasionally other tiny critters will move in and use the gall, as well. This occurs either while the larvae are maturing or after they have grown and left. Predators such as birds, squirrels, and mice, will also often tear into a gall to get to the tasty grubs inside. Some people even use the tiny, wormlike larvae inside goldenrod galls for fish bait.

This fall, look carefully at the fallen leaves, the bare twigs, and the now-visible flower stems. If you see something that doesn’t look normal, it could be a gall. If it is, you’re seeing a unique and complicated, yet amazing, interaction between a plant and another organism. It’s just another example of how fascinating our natural world can be.


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