What are Plant Galls?

Plant galls are caused by irritation and/or stimulation of plant cells due to feeding or egg-laying by insects like aphids, midges, wasps, or mites

You’re buzzing around your garden, watering the beautiful flowers, and picking unsightly weeds. Then you see a bunch of ugly bumps on the leaves on your favorite tree’s leaves and wonder ‘what in the world?’

Yep, you’ve got plant galls, and they’re not pretty. But what are they? Plant galls “are abnormal growths that occur on leaves, twigs, roots, or flowers of many plants.”

Plant galls are caused by irritation and/or stimulation of plant cells due to feeding or egg-laying by insects like aphids, midges, wasps, or mites, according to the same website.

Additionally, some plant galls are infections created from bacteria, fungi, or nematodes and can be hard to decipher from insect-caused galls. Observing the insect or its eggs in the area might help you figure out if it’s an insect gall caused by other organisms.

Do these odd-looking bumps have a function? The Morton Arboretum says, “Galls provide a home for the insect, where it can feed, lay eggs, and develop. Each type of gall-producer is specific to a particular kind of plant.”

While typically they look like yellow balls or bumps they can also appear as knobs, lumps, or even warts, depending on the causal organism.

Types of Galls

Leaf galls are the most common plant galls, formed on leaf blades or petioles. They appear as leaf curls, blisters, nipples, or perineum (hairy, felt-like growths) and on the upper or lower leaf surface. 

Stem and twig galls appear as deformed growths on stems and twigs. They can range from slight swelling to large knot-like growths.

Bud or flower galls are also deformed in size and shape and appear so on buds and flowers.

Plant galls also can be seen in various colors: red, green, yellow, or black. Depending on the weather, plant susceptibility, and pest populations the occurrence of galls on plants can differ year to year.

Is it Serious

If you notice plant galls, don’t go digging up your garden as these nuisances are more of an aesthetic problem and are not serious to plants, flowers, trees, or the like.

For example, if your tree is affected by galls, there’s little injury to them, but younger trees can become deformed which ultimately may make them appear ugly as they mature.

How Galls Emerge

Plant galls usually come to light during the increased growth period of new leaves, shoots, and flowers in late springtime.

Then, insects or mites damage plants by gnawing on them and their spit can result in plants increasing production of normal plant growth hormones. Higher hormone production means an increased cell size or cell numbers, resulting in more plant galls.

According to this article, “mature plant tissues are usually not affected by gall-inducing organisms, and the gall continues growing as the gall-making insect feeds and grows inside the gall.” Note, if galls begin developing they continue to develop even after the insects perish.

Most plant galls remain on plants for more than one season since they become noticeable only after they are fully formed.

And sorry if you have large Oak trees on your property as these are one of the most susceptible for plant galls. They serve as a host to more than 500 different wasps, aphids, mites, and midges all cause galls on leaves and twigs.

Protecting Your Garden from Galls

Chemical applications won’t rid plant galls in your garden because the timing of sprays is crucial. To be effective, apply pesticide before gall formation begins, but when insects and mites are out and about.

Once plant galls start to form, it’s too late for treatment, as the plant galls protect the insects or mites.

For insects or mites that spend the winter on the host plant, apply horticultural oil before insect activity begins in the spring.

For the most part plant galls are harmless, and you needn’t worry too much about them as you continue your daily gardening practices.


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