When you look at the bare branches of some oak trees at this time of year, you can see ball-shaped growths there, which almost look like nature’s Christmas ornaments.
These are galls.
Bile is an abnormal growth that is produced by a plant under the influence of another organism. Most galls lay their eggs in the host plant after insects (small cyanid wasps).
Each gall-forming insect produces gall of a particular size, shape and color; No species makes its gills like any other. While the commonly seen oak shells are spherical and woody, the shells of some oak leaves are horn-shaped, star-shaped, or have small jumpers.
During the winter in Butte County, oak groves connected by twigs are probably the most commonly seen gall. These oak galls (commonly referred to as oak apple galls) form smooth round balls one to two inches in diameter and range in color from cream and light green to pink, brown and black. They have a sponge-like interior and sit in groups on the twigs and trunks of oak trees. These gills are formed when a small gall wasp lays its eggs in the tissue of oak flower buds in spring. This wasp is one of hundreds of species of gall wasp (family Cynipidae) active in the United States. The tiny wasp also collects fluid that alters the cell’s multiplication process, resulting in gall. Wasp larvae develop inside the gall until they are fully adults, at which time they release the gall through an exit hole.
Right now, oak shells can be seen in large numbers on some small trees and less frequently on healthy mature oaks. Other smaller oaks are directly adjacent to a heavily-infested one that will have practically no galls. This seedling is the result of variation in genetic susceptibility to gall wasps among oak populations. Chico’s Bidwell Park is the perfect place to see the many-gulled oaks.
There are other insects that invade or inhabit the gall during or after the initial gall-maker’s residence. Some are parasites of gall larvae; There are other insects that live harmlessly within the gall (these secondary inhabitants are called interrogators). Birds feed on the larvae growing inside the gall. And galls can be attacked by Phoma gallorum fungus, resulting in dark brown or black galls.
In summer, an unusual gall develops to be seen on the leaves of mature valley oak trees. It is a small gall produced by the jumping oak gall wasp Neuropterus saltatonus. The wasp lays its eggs on the undersides of oak leaves, causing spots to appear on the upper edges of the leaves. Galls as small as the head of a pin and similar to mustard seeds develop.
Eventually these tiny gills fall off the leaves. Inside each gall is a small active wasp larva. Once on the ground, they can be seen speeding up and leaping an inch or more, which is quite an astonishing sight!
Another invading parasite that can be seen in bare branches of trees at this time of year is the mistletoe (family Viscaceae). The sticky mistletoe seed is spread by birds and basically germinates and develops independently of its host. However, as it matures, it forms a root-like organ called a haustorium that penetrates the tissue of the host tree, enabling it to rob water and nutrients from the host. The green mistletoe triggers photosynthesis, which makes the plant its own food.
American mistletoe is bushy-stemmed, with smooth-edged oval evergreen leaves that are borne in pairs along the stems. The ornamental white waxy berries of the mistletoe, formed in groups of two and six, are poisonous and contain sticky seeds that are carried from branch to branch and from tree to tree, spreading the mistletoe.
The American mistletoe species is native to California and can be found up to an altitude of 8,500 feet. The European mistletoe has been introduced to California and is often seen at Christmas.
Decorations and wreaths.
Butte County’s UC Master Gardeners are part of the University of California Cooperative Extension System, serving our community in a variety of ways, including 4-H, agricultural advisory, and nutrition and physical activity programs. To learn more about UCCE Butte County Master Gardeners, and help with gardening in our area, visit https://ucanr.edu/sites/bcmg/. If you have any gardening questions or issues, call the hotline at 538-7201 or email [email protected]