Harmful insects

Leaf Galls: Surprising, But Rarely Harmful

By Larry Hodgson

As a gardener, or even just an observant hiker, you’ll often come across plants bearing abnormal growths. They’re usually found on the leaf blade, but sometimes on the petiole. Or you may see them on the plant’s stem or at its tip. These growths are called galls and have various origins. Most are caused by insects (aphids, wasps or flies) or mites. More rarely, they can be caused by fungi, nematodes or bacteria. For purposes of this text, we’ll mostly look at galls caused by insects or mites which, for simplicity’s sake, I’ll simply call gall insects.

Female gall wasp
Female gall wasps pretty much go sight unseen. Photo: Insects Unlocked

Typically, a gall is formed when the female gall insect pierces a leaf or stem early in the growing season and lays one or more eggs inside. She also injects (or the larvae give off) a substance that causes abnormal growth in the surrounding tissue, a tumor called a gall. This gall serves as both shelter and food to the young gall insects: they are protected from wind, rain, and predators and, at the same time, use some of the plant’s cells as food. When the young gall insects mature, they pierce a hole in the gall and leave its confines. What they do after depends on the species: some drop to the ground, others fly away, and others stay on the plant. However, the following spring, the cycle is repeated, on the same or some other plant.

Treatment Rarely Necessary

maple leaf galls
These tiny maple leaf galls really aren’t worth worrying about. Photo: Ronald S. Kelley, Vermont Department of Forests, Parks and Recreation, Bugwood.org

What is important to understand about gall insects is they are generally fairly innocuous. Even when they seem very numerous (and sometimes they seem to practically cover the leaves!), the small amount of tissue they actually consume really doesn’t do the plant any real harm. And they rarely reduce the leaf’s photosynthetic cells enough to be of harm. So, normally, you can simply ignore them or at least just learn to put up with them, especially when it comes to leaf galls, the least harmful of all. 

This is a clear case where you should apply the 15-pace rule: If you can’t see the problem at 15 steps, there is no need to treat it.

In fact, this is also one of those cases where the treatment can be worse than the infestation, because the only logical way of getting rid of leaf galls is to pull off all the leaves that carry them and that can practically defoliate the plant when galls are numerous. Even if you were able to get to all the galls (on trees, they can be out of reach), such a defoliation would be more harmful to the plant than the leaf galls ever were.

Pesticides are rarely worth trying on gall insects, either, at least not once the galls are visible. By that stage, the young insects are living inside the plant’s tissue where your spraying is unlikely to reach them.

Apple oak gall
Apple oak galls are huge and impressive, but not harmful to any degree. They can be dried and used as ornaments. Photo: Grand-Duc, Wikimedia Commons

Of course, if you apply an insecticide spray at just the right time, early in the spring as the leaves form, just when the female gall insect lays her eggs, you might be able to nip the infestation in the bud. But you rarely know if the plant will be infested that year (gall insects are notoriously sporadic in their habits), and secondly, most female gall insects are small enough to be barely visible: it would take a very perceptive gardener to notice them in time to react. And all that spraying with what might well be a pesticide harmful to beneficial insects is not that environmentally friendly!

Some species do overwinter on their host plants and do come back year after year (the spruce gall midge is an example). If so, a dormant oil treatment in the spring is worth trying and could seriously reduce their numbers.

Stem and Tip Galls

willow pine cone gall
The willow pine cone gall changes the plant’s stem tip into what appears to be a big flower bud. It is rarely abundant enough to do more than provoke curiosity. Photo: Ryan Hodnett, Wikimedia Commons

The galls that form on stems and stem tips, such willow pine cone gall, are more harmful to the plant, because they prevent that the stem from developing normally. Fortunately, they are rarely numerous: usually only one or two per plant. If their presence bothers you, just prune them off with garden shears when you see them.

To Each Plant Its Gall

Walnut erineum gall
Walnut erineum gall on a walnut leaflet. Photo: Mathieu Turgeon

Galls insects are usually very specific: there are literally thousands of species and each one has only one host plant (oaks, roses, willows, etc.). That’s good news for gardeners: you don’t have to worry that galls will spread to all the plants in your garden. 

Oaks seem to have the widest range of galls: each year can bring a different species!

rose gall
The rose gall is actually quite pretty! And is only found on roses! Photo: Bob Harvey, geograph.org.uk

And galls come in a wide variety of forms, sizes, and colors. There are oak galls that look like colored bonbons, the surprisingly frothy-looking rose gall, the tiny green bumps that turn red of the various maple leaf galls … and the list goes on and on.

In Short

Leaf and stem galls are usually more a curiosity than a problem. Most of the time, you just have to close your eyes to their presence! Or, better yet, open your eyes and learn to enjoy one of the marvels of nature!

Article originally published on June 25, 2016. 

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

5 comments on “Leaf Galls: Surprising, But Rarely Harmful

  1. Valley oak gets the biggest galls of any oak here. They are known as oak apples. Most trees do not get many. I have never seen them on any of the valley oaks here, except for one, and it is severely loaded with them. The tree is healthy, but looks ridiculous. Oak apples had been used to make ink.

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