You've Got Some Gall!

Red cone galls on valley oak (Quercus lobata)

Galls on Quercus rugosa in Arizona

Have you ever been strolling around outside and happened upon a bizarre, unexpected plant growth?  Freakishly hairy, spiky, huge, or colorful, like something out of science fiction or a medical manual, these plant “tumors” are the product of one of the most fascinating and intimate relationships directly observable in the wild— galls!

Galls are grown by plants in response to chemical or mechanical stimuli by invading organisms- generally insects, mites, fungi, or bacteria. In other words, an outside organism will rasp, pierce, exchange fluid with, and often lay eggs on a host plant. As a result, the plant grows something totally unique. 

These “gall-inducers”generally use plants as hosts for reproductive purposes—to release reproductive products and agents for fungi and bacteria, and to feed and produce offspring for mites and insects. In the case of insects and mites, there is the added benefit of creating a protective home for the larvae. The resulting growth can take on bizarre shapes, colors, and textures—sometimes mimicking existing features on the plant and sometimes creating something new. 

The "oak apple" is a common gall caused by the California gall wasp

This very intimate relationship is thought to be intricately connected by species. Gall-inducers require not only a specific host plant species, but also specific microconditions-- temperature, humidity and exposure. They can also be sensitive to the particular physiological condition and chemical suitability of the host plant. This minutely connected, mini-magic is wildly complex, as you might imagine, but here is a tiny glimpse into some gall-world generalities. To narrow things down a bit, let’s discuss insect-induced galls.

Galls from golden cup oak (Quercus chrysolepis)

Beaked twig gall wasp gall

  Galls induced by insects generally develop in spring and summer when metabolic activity in host plants is highest.  They can occur on any part of a growing plant, but leaf galls are most common, due to having higher metabolic activity over a short amount of time. Common gall manifestations include leaf and stem swellings, detachable outgrowths, flattened fan-shapes at the end of stems, rolling or folded leaf  edges, hair-lined depressions on leaves, root nodules, and dense collections of small branches and shoots. Often, this is a very consistent relationship between plant and animal resulting in specific size, shape, and color galls- a relationship that researchers have speculated may be due to evolutionary genetic programming. They are so specific, in fact, that often one can identify the species of gall-inducer based on the host plant species and the formation of the gall, without ever observing the species itself.

Fuzzy gall wasp on valley oak (Quercus lobata)

Why are some plants conspicuously covered in galls while their neighbors are not? This is probably due to the environmental factors mentioned above including favorable microclimate conditions, physiological condition of the host plant, and the dispersal capabilities of the gall inducer. For instance, the bacteria that causes crown gall (Agrobactrium tumefaciens) can only enter its host plant through an open wound. Gall-inducing cynipid wasps on the other hand, can fly and probably have access to more distant hosts than the bacteria. Generally, the health and vigor of host plants is not significantly affected by the seasonal production of galls, although some localized damage or stunted growth may occur. 

Rosette (midge) gall on arroyo willow Salix lasiolepis

Although there is still much to learn, plant galls have been documented on every continent except Antarctica, and evidence of galls from 300 million years ago has been observed. Worldwide there are currently 13,000 known species of gall-inducing arthropods. Over 2,000 species have been documented in the US with nearly 1,000 cynipid wasps and 800 gall midges. Gall-inducer dominance varies by continent- while cynipid wasps are our most common gall-inducers in North America, mites and midges dominate the gall world in Asia.

Hairy bud gall on rabbitbrush (Chrysothamnus sp.)

If you are intrigued by these amazing relationships, you may want to consider becoming a gall researcher. Cecidology is the study of plant galls and the insects that induce them. 

REFERENCES

Russo, Ron. 2006. Field guide to plant galls of California and other Western states. University of California Press, Berkeley and Los Angeles, California.






About the Author:

 Laura has worked as a field biologist in California, Arizona, and abroad since 2001, specializing in endangered species and ecological research and mitigation.  She has a B.A. in Environmental Studies from the University of California at Santa Cruz, and a M.S. in Natural Resources from the University of Arizona.  She joined the Conservancy in January 2014.