The Curious World of Galls — November - December 2000

By Larry Weber

"Imagine yourself living in a globelike room with greenish walls bulging outward and upward and then arching in to meet above your head," the naturalist Edwin Way Teale wrote. "Imagine such a room constructed of succulent, edible material, forming a house that at once provides food and shelter, plenty and protection. That is what you would find if you traded places with one of those gall insects that now live in the globular swellings on the stem of my hillside goldenrods.

We have all seen these curious swellings on goldenrod stems, though usually we do not imagine them as Teale did.

An Introduction to Galls

Surprises Inside and Surprising Outsides

Galls are unique growths on plants, started in reaction to the egg laying or feeding of gall-making insects and mites. The physical irritation or chemical secretion of these insect activities results in a distinctive enlargement or swelling of the plant, ranging from simple to highly complex forms and shapes. Each kind of gall maker can usually cause only one kind of gall and prefers to use specific plants.

Midges and other flies, gall wasps, and mites make up the largest number of gall makers. Galls are also caused by aphids, by sawflies, and less often by a few kinds of moths and beetles, nematodes, psyllids, and true bugs.

About 2,000 kinds of galls are known to exist in North America on half the families of plants. They may be on any part of a plant, depending on the gall maker's specialty: stems, twigs, leaves, buds, flower heads, fruits, and roots. They resemble seeds, cones, burrs, flowers, balls, swellings, fur, and fingers.

Usually a particular insect forms a gall of a distinctive shape on a single species of plant, such as the Canada goldenrod. Both woody and nonwoody plants have galls. Some are green, others red or purple, but most change color with the seasons. Some are seen only in summer; some are visible all year.

The Goldenrod Ball Gall

Protected Year-Round Especially From Freezing

The goldenrod ball gall is a good choice to study because scientists know about the life cycle of the goldenrod gall fly, the insect that causes it. In Minnesota this gall grows on two species of goldenrod -- late and Canada goldenrods -- which are plentiful and easy to find in almost any prairie, unmowed field, or weedy place.


In late spring the female goldenrod gall fly, only one-third of an inch long, lays her eggs. She uses her ovipositor, the pointed tube on the end of her abdomen, to deposit eggs on each tender goldenrod shoot. Special cells on her feet help her know this is the right kind of plant. Only one gall usually appears on each stem.


Ten days later, the egg hatches and the larva burrows into the stem. As the plant grows, the gall begins to thicken in response to the chewing and secretions of the larva. As it eats the nutritious gall tissue, it passes through two larval stages.


The full-grown galls of fall are about 1 inch in diameter. Inside, the fat larva is now full-sized too, and it is forming a tunnel that stops just short of going all the way outside. It will use this tunnel in spring to escape. In the meantime, it will overwinter in the gall.


Triggered by cold weather, the larva makes a type of antifreeze call glycerol. By keeping the liquid inside the cells from freezing, the glycerol protects the larva's delicate cell membranes from being destroyed by ice crystals. The rest of the larva can freeze solid (See Gall Projects for winter projects)

After many months of being frozen and inactive, the larva finally turns into a pupa as the warm spring weather arrives. Two weeks later the adult fly emerges, scoots through the tunnel, and bursts through the thin gall wall. Then the cycle begins again.

The Center of a Busy Community

Plants are at the first level of energy users. They collect energy directly from sunlight and use it to make sugars. For this reason they are known as primary producers.

When a goldenrod produces a gall, it provides specialized nutrition with its inner gall tissue and shelter with its tough outer layer. (In addition to supporting gall makers, goldenrod feeds many other animals that eat everything from the pollen to the roots.)

Gall makers are plant eaters, the second level of energy users. Plant eaters are called the primary consumers.The goldenrod gall fly larva feeds on the goldenrod gall's plant tissue. The goldenrod gains nothing from the larva inside it, while the larva gains food and shelter. This one-sided relationship is called parasitic.

Predators make up the third level of energy users in this community. They prey on the gall-making larva.

One invader, the tumbling flower beetle, lays an egg in or near the gall. Once hatched, the larva lives side by side with the fly larva and will sometimes eat it. Animals that live in nests they do not build are called inquilines.

Other parasitic invaders, called parasitoids, lay eggs right inside the gall maker's eggs or larvae. One kind of parasitoid wasp hatches inside the host larva, quickly eats it, then feeds on the gall. Another kind of parasitoid wasp hatches but stays inactive in the host larva through summer. In fall, it forces the host larva to pupate early, then takes over the gall to overwinter and finish growing.

Other predators include downy woodpeckers, chickadees, and possibly gray squirrels. They peck and chew their way into galls to devour the larva inside. Even ice anglers sometimes open galls to use the larvae for bait.

More Minnesota Galls

Large and Very Small

Oak Apple Galls

Oaks appear to be the most popular choice for the gall makers of North America. Most oak galls are caused by small wasps called cynipids or gall wasps. About 80 percent of the more than 700 different species of gall wasps in the United States produce galls specifically on oak trees. Most noticeable of the oak galls produced by the gall wasp are the golf-ball-sized oak apple galls.

Oak apple galls form on leaf stems or veins that run down the center of the leaf. Springtime oak apple galls look like little green apples, thus accounting for their name. The green galls blend with the leaves, but those that turn brown in summer are easy to see.

Blackberry Knot Galls

Perhaps the largest of Minnesota's galls are those found on the blackberry plant. Among the thorns on this brambly bush is a solid, knotty, furrowed swelling up to 6 inches long. Green at first, the gall gets darker with age. A group of larvae of the tiny knot gall wasp causes these huge growths. This wasp is related to those that form oak apple galls. On blackberries they live in a colony.

Jumping Oak Galls

Would you be surprised if you saw thousands of tiny yellow balls falling like rain from oak leaves, then jumping around on the ground?

These pinhead-sized galls and the jumping are caused by jumping oak gall wasp larvae. The larvae's saliva contains chemicals that stimulate the oak leaf tissue to form yellow blister-shaped galls.

As the galls mature, they detach from the leaf and rain down onto the ground. The galls jump around until they become lodged in the soil, where the wasps overwinter. At times these galls can become so numerous that they discolor the oak leaves and even cause the leaves to fall early. Yet the trees stay healthy.

Jumping oak galls are found in any oak tree for only a year or two, usually during times of plentiful rain. So be sure to enjoy these marvels when you find them.

In All Shapes and Textures

Cherry Pouch Galls

A mite forms cherry pouch galls, fingerlike projections on top of cherry tree leaves. These numerous galls usually grow in spring as the leaves grow. Their presence is a sign of mites feeding and breeding. Anyone looking at cherry leaves during the summer might notice them. They don't occur on all cherry trees, but they are abundant when present. They drop with the leaves, and the tree appears to have never been visited by these gall makers.

Willow-Cone Galls

Even though these galls grow on willow shrubs, overlapping scales make them look like pine cones. Sometimes a willow has so many, it seems there's a gall at the end of every branch.

During the spring and early summer, a midge (a tiny gnatlike insect) lays eggs on new willow growth tips. The gall, stimulated by the hatched larva, grows up to 11/2 inches long and about 1 inch wide. Both the larva and the gall develop during the warm months. The larva becomes dormant in winter and completes its growth the following spring.

Many insects take advantage of the napping larva's large nest and move in. Some of the uninvited guests live as inquilines, others as parasitoids. In one study, researchers found 23 galls contained 564 insects, only 15 of which were the original gall makers.

Gall Projects

Now with the leaves off the trees, we can see many of these galls and again get a sense of the marvelous adaptability and diversity of insects. Before the snow gets too deep, look for galls. Cut open the different kinds of galls you collect and try to find the gall makers. Certain kinds will contain a larva inside; others will be empty.

Collect goldenrod galls in late fall or early winter to observe the antifreeze effects of glycerol on goldenrod gall fly larvae. Store harvested gall stems outside, protected from rodents and mildew by a cloth sack in a large, airy container.

When you are ready to begin, have a grown-up help you crack open a frozen gall by twisting the tip of a knife into it. You might find a larva so hard it will bounce. Put it into a dish and observe it as it warms. It will change from a frozen, white lump to a creamy, live wriggler. You can place the dish in a freezer for an hour or two to refreeze the larva. Then watch it warm up all over again. When you are finished with this demonstration, you might be able to get the larva to pupate. As a larva, it must undergo obligate dormancy. So keep it in the freezer until late spring. Then bring it to room temperature. It should pupate within 24 hours, if ready.

Release the adult fly where you found the galls.

Here are a few gall web sites to explore:

Larry Weber is a teacher at The Marshall School in Duluth. He lives on an old farm in Carlton County, a great place to watch critters.

A complete copy of the article can be found in the November - December 2000 issue of Minnesota Conservation Volunteer, available at Minnesota public libraries.