Plant galls are abnormal growths
(tumors) of plant cells formed as a response to a parasite. Galls
occur on a wide variety of woody plants. Insects are the primary
cause of galls. Gall-forming insects include aphids, gall gnats,
and cynipid wasps (gall wasps). The gall wasps are the most important
insects that induce plant galls. When an insect lays its eggs
in the bud, the plant cells grow abnormally producing this tumor.
Along your Sweeney Ridge walk, a look at
the terminal buds of coyote brush (left figure) often reveals
galls. A midge (fly) larva is developing in the gall. Cylindrical
galls on coyote brush stems are due to the presence of moth larvae.
And irregular galls on the stem are due to a fungal infection.
Look for red galls on willow leaves (right
figure) along the Sneath Lane trail.
These galls are caused by a sawfly (wasp).