Monterey Peninsula Wildflowers, Shrubs & Trees 

A photographic guide by Michael Mitchell & Rod M Yeager, MD


A number of trees and plants exhibit small bumps and protuberances which seem not to be part of the normal plant (or, in the case of the Chinquapin, may easily be thought to be the plant's fruit). These are often caused by one of the moths, midges, gall wasps, saw flies (or similar species) that lay their eggs either on or directly into the tissue of the plant, often in buds or young leaves.  The larvae feed on the appropriate part of the plant and the growth of the gall is the plant's response to the chemical or mechanical stimuli brought about by the larvae's activity.

The USA has about 2000 species of gall-inducing insects of which about half are gall wasps and about 40% are gall-inducing midges.  It seems that individual species of insect are attracted to very specific types of plant species.  The identity of the insect can therefore often be deduced directly by identifying the plant on which the gall appears.  

Some 70% of known gall wasps live in different species of Oak trees, but galls are found in a number of other species such as the Chinquapin, Coyote Brush, Willows, Artemisia and Roses.  The largest number of galls are those found on leaves.

The galls are shown by reference to the species on which they appear.

Note:  The study of galls is a very specialized activity and this page is intended merely to illustrate some of the galls that we have chanced to photograph from time to time.  More galls will be added from time to time but there is no intention to make this page in any sense comprehensive.

An excellent introduction to Plant Galls is Ron Russo's Field Guide to Plant Galls of California and other Western States published by the University of California Press.  Some fine photographs of a wide range of Galls will be found at Joyce Gross' website.


Coast Live Oak (Quercus agrifolia) - Gouty Stem Gall

These are caused by the Gouty Stem Gall Wasp (Callirhytis quercussuttoni).  The galls start to develop in summer and continue to grow until the following spring.  Larval development ceases at this point and the larvae begin to pupate in the fall with the adults emerging from Midwinter until the following March, nearly 2 years after the gall began to form.

Coast Live Oak (Quercus agrifolia) - Live Oak Apple

These are caused by the Live Oak Apple Gall Wasp (Callirhytis quercuspomiformis). This wasp is unusual in having two separate generations which induce two quite different looking galls.  The first generation comprise parthenogenetic females which emerge from the previous year's galls in early spring and lay their eggs in unopened leaf buds.  These galls are mushroom shaped, usually on the underside of leaves. The adults, both males and females, emerge quickly, by early summer.   The Live Oak Apple is induced by the females who lay their eggs in the stem buds, resulting in the growth of  large more or less spherical galls covered with spines or very small projections, about 2 mm long.  The Apples on Coast Live Oaks tend to be smoother than those on Interior Live Oaks.

There are similar but totally smooth Apples formed on White Oaks (such as the Valley Oak); these are induced by a different wasp, the California Gall Wasp (][).

Arroyo Willow (Salix lasiolepis) - Willow Apple Galls

These are caused by the Willow Apple Gall Sawfly (Pontania californica) and found on the Arroyo Willow.  The galls protrude on both surfaces of the leaf and  range from green to bright red.  There may be as many as 12 or more on a single leaf.

Chinquapin (Chrysolepis chrysophllya)

These are caused by the Chinquapin Flower Gall Wasp (Dryocosmos castanopsidis).  The galls are deep red with a golden bloom which rubs off.  There may be as many galls as fruits on an individual Chinquapin; the fruits being easily distinguishable by their very spiny coverings (as in the third picture from the left).

Coyote Brush (Baccharis pilularis) - Terminal Bud Gall

These are caused by a midge, the Coyote Brush Terminal Bud Gall Midge (Rhopalomyia californica).  The galls often have leaves protruding from them.  The larvae are fully developed in 30-70 days after which they burrow out of the gall, develop their white cocoons and pupate.  Galls develop and adult midges emerge throughout the year though with a noticeable burst of activity in the spring.

Coyote Brush (Baccharis pilularis) - Integral Stem Gall

Unlike the Terminal Bud Galls, these are caused by a moth, the Baccharis Stem Gall Moth (Gnorimoschema baccharisella).  Developing near the tip of the stem these are more than twice as long as they are wide (up to 35 x 15 mm) and are green and smooth in appearance, some tapering gradually, some swelling more abruptly.  Gall development begins on the spring and the larvae cut their way out in mid- to late summer and rop to the ground to pupate.  The adult moths lay their eggs on the outer branches later in the fall.

Wood Rose (Rosa gymnocarpa) - Spiny Leaf Gall Wasp

Sometimes mistaken for Bedeguars or Robin's Pincushion galls (which are bristly to hairy in appearance), these are caused by the Spiny Leaf Gall Wasp (Diplolepis polita).  These are pea-sized galls found on the underside of the leaves of the California Wild Rose or the Wood Rose.  They may be bright red, green or yellow-green in color and have a number of spines scattered over the top and sides.

Website Builder