Imagine an insect capable of changing a living cell from what it already is to what it wants it to become, a gall.

There are such bugs. They are called gall-inducers because their larvae exude hormones that can convert the relatively differentiated cells of the host plant upon which they’re feeding into “meristematic tissue” which in turn divides and forms new cells that are of greater nutritional value to the larva. In other words, they tell the host plant to make a space for them to feed and grow and provide a protective shell in which to do it.

That is the nature of galls. They are larval chambers.

Some gall inducers are aphids, some are midges, some are moths, some are mites, but most are wasps. They’re not the sort of wasps that make paper nests nor the kind that have a stinger the size of a harpoon. They’re called cynipid wasps and one of the larger of their species is about a quarter inch long.

That wasp has a name to match its mass – Andricus quercuscalifornicus. It induces the galls on valley oak trees that look like red or green apples in the spring or as cream-colored balls in late summer. Oak apple galls can be as large as four and a half inches long and three inches in diameter.

A much smaller but brightly colored turban-shaped gall one might spot on a valley oak leaf is induced by the red cone gall wasp, A. kingi. On the same tree on the underside and midleaf one might see brain-like clusters induced by the convoluted gall wasp, A. confectus.

Oak trees are a favorite target of gall inducers with some 95 species creating varied larval chambers on oaks. Fifteen species prefer willow trees as a host plant while 14 or so favor cottonwoods, aspens, and poplars.

One insect, the spiny leaf gall wasp, Diplolepsis polita, induces little red colored galls with scattered spines on their surface on the leaves of the wild rose.

Galls are as diverse in size, color, and shape as are the plants that host them, but 16 plants are visited most by gall inducers with 227 insects choosing them on which to lay eggs. Many of those insects like the round gall wasp, the fuzzy gall wasp, lipped needle gall midge and the California jumping gall wasp (which induces tiny brown galls on the underside of valley oak leaves) are named for the galls they induce.

Gall inducers are so selective of host plants that the insect inducing the gall can be determined by looking at the gall, the gall can be described by looking at the insect and both the insect and the gall can be determined by a rough description of the gall and the tree or shrub on which it was found.

The study of galls is called cecidology, which brings in proximity entomology, botany and parasitology not to mention chemistry.

A cecidologist walks into an arborists’ convention. The convention’s chairperson says to him, “You got a lot of gall coming in here.”

Look for a gall in the trees, shrubs and bushes near your home or in any of the nearby public wildlands.

The Merced National Wildlife Refuge (located at 7430 West Sandy Mush Road), the San Luis National Wildlife Refuge and the San Joaquin River National Wildlife Refuge comprise the San Luis National Wildlife Refuge Complex. Its headquarters and visitor center are located just north of Los Banos off Highway 165 at 7376 South Wolfsen Road.

The refuges are open to visitors daily from one half-hour before sunrise to one half-hour after sunset. The visitor center is open Monday through Friday except federal holidays from 8 a.m. to 4:30 p.m.

For more information call 209 826-3508 ext. 127.

A. Rentner