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Galls, those mysterious garden shapeshifters

May 11, 2019
Marty Nelson

The appearance of unusual lumps, bumps or swellings on the leaves, stems, branches or roots of garden plants can be alarming. These abnormal growths of plant tissue are often identified as galls. They are caused by a wide variety of insects, mites, nematodes and microorganisms. However, galls rarely cause severe damage to garden plants. Galls produced by insects and mites are the result of the following fascinating trick of nature.

Gall formation begins with the entrance of a gall maker. This could be an insect, such as the cynipid wasp that likes to lay eggs on oaks. Gall makers typically arrive in spring when plants are growing rapidly and more nutrients are available. Once the eggs are laid on susceptible plant tissue, the gall forming process gets underway. Plant tissue begins to form around a single egg or cluster of eggs and accelerates rapidly once the larvae hatch and begin feeding. The larval secretions thus stimulate the construction of a snug little home, tricking the plant into providing food and protection for the developing insect.

Once mature, the insect emerges through a small hole leaving an empty gall behind. Occasionally, however, there is a dramatic turn of events. A parasite invades the gall, kills the larvae and takes over. Then the insect that emerges is not the same as the original gall maker.

Although galls aren’t often in the limelight, they are numerous, widespread and come in an amazing variety of shapes and sizes. They may be barely visible or as large as 4 feet in diameter. Some are smooth and round and others are spiny or hairy.  Galls can be colorful and they may be shaped like cups, saucers, donuts, or even sea urchins. More than 700 types of galls occur on oaks alone.

Preferred host plant

Each of the thousands of insects and other gall-causing organisms has a preferred species of host plant and forms its own distinctive type of gall. Galls can also be developed in response to fungal and bacterial invasions sometimes triggered by physical injury to a plant. Like the insects and mites, fungi and bacteria have the ability to steal resources from the plant to nourish and protect them. Cecidology, the area of biology focused on the study of galls, is an evolving science as many types of galls and species of gall makers are yet to be identified and studied.

Most gall invasions are limited and the plant is seldom seriously harmed.  When infestations are heavy or appearance is a concern, the best method is to prune out and destroy affected plant parts. Once gall formation has begun on a plant, the process is irreversible. The organism inside is protected from any surface applied or systemic insecticide. Effective insecticidal control would need to be timed to coincide with periods when susceptible gall makers are laying their eggs. However, insecticides can be harmful to the natural predators of the gall makers and are therefore not usually advocated as controls for galls.

Practical uses

Galls also have practical uses and cultural significance. The tannic acid and resins contained in oak galls have long been used in making ink, producing dyes, and in tanning. Some have been valued traditionally for their medicinal properties. In parts of England, Oak Apple Day is celebrated on May 29 in remembrance of the crowning of King Charles II, who reportedly hid in an oak tree following the battle of Worcester.

Oak apple galls are common in our native Marin oaks and so are the gall-makers that produce them and the parasites that invade them. Look for them and other types of galls on your next garden walk. Those weird-looking plant formations can be admired as examples of nature’s resourcefulness.

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