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Word of the Week: Gall
February 19, 2021 by Haya Aldoori

Welcome to Word of the Week! Stay tuned for a new word each Friday to amp up your nature vocabulary!

Gall [GAWL] (noun): An unusual, localized outgrowth or swelling of plant tissue caused by infection from bacteria, fungi, viruses, and nematodes or irritation by insects and mites.

Many of the galls that we notice in High Park develop on plants, courtesy of insects! Maybe you’ve noticed that too in your more recent winter moments outside?

Insect galls often start when the time of year is warm and a female insect lays an egg in some actively growing part of a plant. Something in the developing young or the female’s saliva/other fluids causes the plant tissue to grow into a gall. The young insect will live inside the gall, acting both as a form of shelter and a source of food, throughout the colder months. Once the young insect is big enough (usually when it becomes an adult), and the outdoor environment is more welcoming, it will simply exit the gall.

The details on how exactly those fluids trigger the formation of galls is still a big nature mystery. However, the best evidence suggests that the insect fluids are somehow affecting the host plant’s hormones, which then influences the gene expression that forces the plant tissue to grow and form a gall.

The insects that set in motion the formation of galls are very particular about where they lay their eggs. Sometimes, they’ll choose only a single species of plant, and then just a single location on that plant (e.g. leaf, stem, bud). In addition, the galls themselves have incredibly unique shapes and appearances, which can be used as an ID tool to figure out the insect species inside.

It is also important to note that a single plant can host a variety of different galls. For example, a single oak tree can produce dozens of different types of galls, all triggered by different insect species!

Keep an eye out for insect galls in High Park! Here are a few examples of insect galls that you might see:

Elliptical Goldenrod Gall

Goldenrod Ball Gall

Hedgehog Gall

…and so much more!

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