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The Gall of Insects: Why Some Plants Have Growths

You see a tree with its leaves covered in spots. Is it sick? Should you be worried? You find a stem that looks really swollen in one spot. How did that happen? Chances are if you see one of these signs, you’re looking at a gall.

Oak leaves covered in galls.
Oak leaves covered in galls. (Photo via Shutterstock)

Galls are a really cool way some species are able to protect and feed their larvae, or babies, until they mature enough to survive on their own — at least until a predator catches them!

How does it work?

There are more than 2,000 species that make galls, including certain wasps, flies, beetles, moths, aphids, fleas, midges, mites and even some worms! When one of those females is ready to lay eggs, she chooses a plant. She uses her ovipositor, or egg-laying organ, to cut a tiny opening into a soft part of a plant, like a stem or leaf. She lays an egg or eggs (depending on the species), then seals it with a special chemical that helps the gall grow around the egg.

Do galls hurt the plant?

Nope. Galls are mostly harmless to plants. They don’t look pretty and are often mistaken for disease, but the plants carry on as usual. In fact, galls can actually benefit the plant by containing the damage to a small space. Since galls are made of plant tissue, the larvae and plant are able to share resources and both survive.

Why galls?

Galls have two important purposes. They protect the eggs and larvae from predators as they grow, and they are a food source. Galls are the perfect place to grow up — shelter and food in one spot. What more could a growing larva want?

Just like the parent has a special chemical to start the gall, larvae also release chemicals so the gall will grow with the larva. The bigger the larva, the bigger the gall.

A safe place — or is it?

Larvae still have predators — birds, mice, squirrels and even humans! Sometimes the shelter of the gall is not protection enough. Birds can find galls and poke their beaks inside, or squirrels and mice can crack them open to eat the gooey center, like a piece of Gushers candy.


Words to know

Inquiline: An animal that exploits the living space of another species.

Larva: The immature form of an insect or other animals that undergo metamorphosis.

Lure: To tempt with a reward or something desirable.


People aren’t usually looking for galls to eat them, but to lure (hint, hint!) their own prey. Some people who fish look for goldenrod galls. They break them open and use the larvae as bait for the fish they hope to catch and eat.

Inquilines, or species that live in the homes of other species, can also cause problems. Some inquiline insects may lay their eggs in a gall that already has another larva in it. While trying to share the food supply provided by the gall, the invader species may overeat, starving the original larva.

How do they get out?

A closeup look at a gall with a hole in it.
A gall with a hole in it. (Photo by Heather Van Zyl)

When the larva inside the gall has matured enough to survive on its own, it leaves the gall. Creatures with chewing mouthparts can chew their way out. Those with sucking mouthparts have to wait for the gall to open on its own. Luckily, nature knows what it’s doing, so these galls do open at just the right time. How do you think galls know when to open?

Second chances

Even empty galls have an important role to play. Whether empty because a critter emerged or it became a meal, galls can provide winter shelter for other insects. Certain wasps use empty goldenrod galls as nurseries. The female lays eggs in the hollow gall, then seals the opening with mud.

Do all galls look the same?

Galls form in all different shapes and sizes. They can be round, oval or egg-shaped or look like columns, buttons or tubes. Some even look like they are covered with spines! They are all different colors too. Find any red, green, yellow, black or pink spots on a plant that look like they don’t belong? They could be galls!

Red galls on a linden leaf.
Red galls on a linden leaf. (Photo via Shutterstock)

Do you think the size, shape and color of the gall depends on the plant, the egg-laying species, or both?

It’s the egg layer! Usually a species will lay all its eggs in the same type of plant, but not always. If one species lays its eggs in different plants, the galls will all look the same. But if different species lay their eggs in the same plant, the galls will all look different! For example, an oak tree can host 500 different species on their leaves and twigs. The galls on that oak tree will look different because different species made them.

Where are they?

Spring is a great time to look for galls on stems before new plants start to emerge and cover them up. You might even be able to find one with a hole. Get up close to leafy trees through the summer to look for spots that could be galls. How many different kinds can you find?


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