Galls of Bidwell Park: Of Oaks and Apples

By Jon Aull

Chico is known as the city of trees, and no trees epitomize that more than the majestic Valley Oaks of Bidwell Park.

The oaks, some of the biggest oaks in the world, host a great diversity of wildlife, from acrobatic gray squirrels to combative Acorn Woodpeckers. But one species creates a sensation without ever being seen by most people. Even casual visitors to the park notice the golf to tennis ball sized growths that adorn certain Valley Oak branches and litter the forest floor. In the spring, these are fresh and green, often with a blush of red, which earns them the nickname “oak apples.” But you wouldn’t want to bite into these apples, as you would find a worm inside (or worse, half a worm).

ca-gall-wasp-5_newThese apples are actually galls, and those worms are larvae, juvenile forms of an insect called the California Gall Wasp.

It is a tiny wasp, only 4-5 millimeters long, and not the stinging kind with which we are so familiar. The galls are mini habitats for one or more of the insect larvae who parasitize their particular plant host. In the case of the California Gall Wasp, an adult female lays a dozen or so eggs in a tree branch with her ovipositing stinger in the fall. The eggs lay dormant in the tree branch until the tree starts to leaf out in the spring. As they hatch, the larvae release a chemical that stimulates the tree to form a gall to isolate the disturbance, like forming a tumor. Inside the gall, the larvae have food to eat and shelter from predators. As the galls develop, they redirect nutrients from the tree to the gall, creating a “nutrient sink,” that has higher concentrations of certain nutrients than other parts of the tree. After its metamorphosis, the adult wasp chews its way out of the gall and flies off to another branch or a neighboring tree to lay its eggs. These adults live only about a week or so, and don’t even eat as adults. Their only purpose is to disperse and lay their eggs. All of the members of this species that have been observed are female, causing scientists to assume that this is a parthenogenic species, which lays fertile eggs with out the benefits of mating with a male.

saucer-galls-4_newIt may seem like a great system for the wasp, but things are never that simple in nature.

The larvae’s nutrient rich habitat is attractive to other species of wasps. Some of the invading larvae simply feed on the gall tissues (as inquilines). Some parasitize the gall inducers, feeding on the larvae (as parasites). Yet others are “hyperparasites,” feeding on the parasites. Some even form galls within galls. But the gall inducers are not such easy targets. They do have another trick. They induce the tree to coat the galls with a sticky sweet substance. This substance attracts ants, bees and yellowjackets, who then are enlisted as security against the would-be invaders. The relationships in this community of insects can get very complex, with one study finding 90 species of insect associated with the galls of a single species of wasp. These relationships represent the culmination of millions of years of evolution, as the history of insect galls dates back over 200 million years. The sheer number and variety of insect species is astounding, no less in gall forming insects, which range from mites to beetles to flies to wasps. Worldwide, there are 13,000 known species of gall inducing arthropods. In North America alone there are 800 species of Cynipid Gall Wasps, a group that includes our California Gall Wasp.

While the California Gall Wasps induce the largest and most conspicuous kinds of galls locally, there are hundreds of different kinds of galls, with some two hundred kinds on California oaks alone.

Willows, alders, grapes, pines, walnuts, and even poison oak are other popular hosts for gall insects in the Park, with the oaks hosting the greatest number and variety of species. The diversity is such that Ron Russo, the author of the excellent and definitive Field Guide to Plant Galls of California and Other Western States, writes that each time he explores a new area, he finds new species of gall insects. He describes on one occasion finding 30 species on a single Blue Oak. The Blue and Valley Oaks of California (and Bidwell Park) host the most diverse assortment of galls, in color, shape, and size, recalling Hershey’s kisses, sea urchins, cups, and brains.

coral-gall-wasp-gall_newMany galls form on leaves, like those of the Red Cone Gall Wasp, the ones that look like miniature red Hershey’s kisses.

One tiny fat white wasp larva resides in the base of each of these. This species exhibits alternation of generations, a rare phenomenon in the animal world. An all female generation emerges from the cones in February, reproducing clones of itself, which stimulate a different type of leaf gall, from which the bisexual generation of males and females emerges in May.

Another conspicuous leaf gall is induced by the California Jumping Gall Wasp.

Although they are tiny (1 mm in diameter), they are noticed as they drop to the ground in early autumn by the thousands. Once they fall to the ground, the larvae thrash around inside, causing the galls to jump around. It is believed that they do this to work themselves into cracks in the ground or sidewalk or under the leaf litter to avoid predation and better insulate themselves against temperature changes. (Insect larvae are also the culprits in those “Mexican jumping beans.”)

Although insects are perhaps the most interesting group of gall inducers, some bacteria and fungi can also induce gall formation.

While gall inducing insects draw their sustenance from their hosts and give nothing in return, some gall inducing bacteria actually help their hosts. Nitrogen fixing bacteria induce galls on the roots of alders, lupines, and other plants, taking nitrogen out of the air and converting it into a form of nitrogen that the host plants can use. When those plants die, the nitrogen becomes available to other plants, enriching poor soil like that found in Upper Bidwell Park.


So the next time you’re walking in the park, stop to take a look at some of these galls, notice the exit holes of different sizes for the inquilines, parasites and hyperparasites, or notice the ants eating the honeydew from fresh galls.

But above all, take time to enjoy some of the intricate structures that nature has created, and appreciate the complexity of the relationships that exist in the ecosystems that surround us, the complexity and beauty that are always there for those who take the time to look closely and observe the wonders of nature.

“In all things of nature there is something of the marvelous.” – Aristotle