By Doug Moore
Galls are abnormal, sometimes tumor-like, plant growths induced by manual and/or chemical stimulation from an invading organism. While no part of a plant is immune, galls most frequently occur in areas with actively growing cells such as leaves, new stems, or young roots.
A wide range of organisms produce galls, including insects, mites and fungi. The most striking and best known galls are made by insects. Usually the insect lays its eggs in the tissue of the plant. Depending upon the type of insect, gall formation results from the irritation caused by chemicals introduced from the parent or larvae and/or stimulation by the growth and chewing of the larvae itself. Acting to isolate the intrusion, the surrounding plant cells become enlarged and more numerous, providing the invader with food and shelter. The larvae eventually pupate and leave the gall as adults. A few insects, such as some species of thrips, produce “gall homes” that they use throughout their life cycles.
Over 2,000 species of insects, including types of beetles, moths, aphids, flies, and wasps, make galls. The galls caused by each species of insect are very characteristic in shape and color, as well as usually specific to a particular species of plant- some are so specific that the host plant can be identified by the type of gall and a gall-producing insect can be identified by knowing the host plant.
Many insects have alternating generations that produce radically different galls. One example of this “heterogeny” would be an insect that lays both male and female eggs that produce a specific gall in the spring. These spring insects breed and produce only female eggs that stimulate a completely different-looking gall later in the summer. This female generation then produces eggs parthenogenically (viable eggs that develop without fertilization) that overwinter to hatch as males and females and produce galls again in the spring. Some gall insects reproduce primarily through parthenogenesis.
Galls come in widely divergent array of size, shape, and color. Some galls are as small as the head of a pin, but an insect as small as a fruit fly may produce galls as big as volleyballs. Shapes can be as bizarre as to resemble sea urchins, crab spiders, fuzz balls, and Hershey’s Kisses. Colors range from reds, oranges, browns and yellows to purples, greens and beige; stripes, polka dots, and other patterns are common. Cynipid wasps produce the most highly colored and bizarrely shaped galls. Most oak galls are caused by these generally tiny wasps, including the round galls that we find on and under the oaks in Madera Canyon.
Woodpeckers extract larvae from galls; mice and squirrels eat the whole thing. Quite a few insect species bore into and prey on gall-insects eggs and larvae.
Other common galls on Proctor Loop are the conspicuous bulges at the base of hackberry leaves, white fuzzy galls on hackberry twigs, and hot-pink fuzzy galls on oak leaves.