Bug Report: Galls Y'all

By: Zach Phillips, Ph.D., Terrestrial Invertebrate Conservation Ecologist

Some visitors to Santa Barbara Botanic Garden don’t need a membership or reservation to get in. They enter as they please, and once inside ignore requests to “not touch the plants.” They touch them, crawl on them, and if all goes well inject their children into them. These are the gall-forming insects — supreme plant manipulators. They induce plants to develop specialized growths (galls) that they exploit for food, shelter, and protection. In general, galls develop as a plant's response to insect stimuli such as feeding and saliva, but exactly how most gall-forming insects run this grift remains a mystery.

Many wasps, aphids, flies, and other bugs have evolved gall-forming relationships with California’s native plants. As such, some plants in the Garden are naturally ornamented with galls that belong to local insects. Below, we take a short tour of a few galls observed around the Garden, and one currently being tested as a biocontrol agent for an invasive plant. Hopefully, this will manipulate ... ahem ... inspire you to learn more about galls, to appreciate their uncanny biology, and to make your own field observations. Wasps

Wasps

Oak trees (Quercus spp.) are gall hotspots, and cynipid wasps (Cynipidae) are by far the most diverse group of oak-galling insects, with over 200 species occurring on the oaks in California alone. These miniature wasps form relatively large and elaborate galls on leaves (Photo 5a), stems (Photo 5b) and other plant tissues. Adults deposit eggs, galls develop, and larvae remain inside the galls for months or even years. The adults don’t eat, reproducing and dying too quickly to need food, and the larvae don’t poop, maintaining a clean gall during their extended stay. Hollywood, pay attention — this is good material for a cynipid-themed remake of “Freaky Friday.”

Despite their intrusive behavior, cynipids tend to do little or no harm to oaks, acting more like commensals (i.e., symbionts that have a negligible effect on their host) than parasites (i.e., symbionts that harm their host). Cynipids themselves are vulnerable to intruders. Galls are valuable real estate, and many non-galling creatures occupy or usurp them, including other wasps. By inducing galls, cynipids therefore serve as “ecosystem engineers,” creating homes and microhabitats for other organisms, and enriching animal communities associated with oaks.

5a: An old cynipid gall on a coast live oak, the woody legacy of a bygone bug

5b: A still-developing cynipid gall on a coast live oak

6a: A gall of Parafreutreta regalis on Cape-Ivy

4a: A Euura sawfly gall on a willow leaf

4b: A Euura sawfly larva poking its head out of a dissected gall

4c: Euura sawfly larvae: a tiny one and a large one

Flies

In some cases, galls cause significant damage to their host and can be harnessed as biocontrol agents for invasive plants. In California, the fly Parafreutreta regalis has been identified as a potential biocontrol agent for Cape-Ivy (Delairea odorata), an invasive plant run amok across the West Coast.

The fly forms swellings on Cape-Ivy stems (Photo 6A) that may harm the plants, but the effect of the galls on Cape-Ivy populations remains unclear. In order to test the fly’s effectiveness as a biocontrol agent, University of California, Santa Barbara (UCSB), researchers have released populations at Cape-Ivy invaded sites. At the sites, some of the galls have been eaten by animals, possibly rodents. Discovering the culprits has practical implications for control efforts, and would also address a broader ecological question: How do galls contribute directly to vertebrate food webs? Stuart and Callan Halewood, two local naturalists, are working with UCSB Professor Tom Dudley, Ph.D. and the Garden to set up camera traps to monitor the galls and whatever might be eating them.

Sawflies

Sawflies have a name that is both misleading and on the nose/abdomen. They are not true flies (order Diptera); they are in fact close relatives of wasps, bees, and ants (order Hymenoptera). True to their name, however, they do possess saws. Adults have saw-like ovipositors which they use to cut into plants and lay eggs. These eggs develop into larvae that look like caterpillars wearing full-face motorcycle helmets (Photos 4b, 4c). In Santa Barbara, the pearly red galls of Euura sawflies (Photo 4a) are common on willow leaves (Salix spp.).

Aphids

The primary hosts for Pemphigus aphids are poplars (Populus spp.). At the Garden, an unidentified species of Pemphigus forms galls on black cottonwoods (Populus trichocarpa) (Photos 2a, 2b). Like other aphids, they produce a liquid waste called “honeydew,” which, if not dealt with, can contaminate or drown them. Wax secretions (Photo 2c - the white material covering the large Pemphigus mother) can help aphids package, transport, isolate, and remove honeydew. For example, some Pemphigus aphids use wax to shape honeydew into “marbles” and roll the marbles out of holes in their galls.

2a: A Pemphigus aphid gall on a black cottonwood leaf

2b: A Pemphigus aphid gall on a black cottonwood leaf

2c: A Pemphigus aphid in a dissected gall, covered in white wax and surrounded by her babies

1a: Multiple Tamalia coweni aphid galls on a manzanita

1b: The narrow opening of a Tamalia coweni aphid gall

1c: A nymphal Tamalia coweni aphid

1d: An adult Tamlia coweni aphid

Another gall-forming aphid found at the Garden, Tamalia coweni, lives on manzanitas (Arctostaphylos spp.). The female reproductive, called a “stem mother,” builds a tiny manzanita home to share with her kids. She induces a leaf to fold over on itself, enveloping her and her brood (Photos 1a, 1c, 1d).

The leaf’s curling edge stops short of the blade, and the narrow opening that remains gives the gall a sea shell-like appearance (Photo 1b).

Interestingly, if you hold one of these galls against your ear, you can hear the sounds of the ocean. THIS IS NOT TRUE…DO NOT DO THIS…YOU COULD GET APHIDS IN YOUR EAR.

Ripe for Discovery

On a recent Garden collecting trip to Sonoma County, a midge — a small, elegant type of fly — was collected from Yerba Santa (Eriodictyon californicum) flower galls. A pupa was dissected out from one gall, and an adult midge emerged on its own from another. Based on photos of both the pupa and adult, Raymond J. Gagné, an authority on gall midge biology, identified the midge as an undescribed species in the genus Asphondylia, and likely the first record of a gallforming midge collected from Eriodictyon plants.

Finding undescribed species of gall-forming insects is not uncommon, and in California many remain to be discovered and studied. Plant Galls of the Western United States (Russo, 2021) is the best field guide to our region, and the “Galls of California” project on iNaturalist (inaturalist.org/projects/galls-of-california) is a great online resource for exploring gall biodiversity and sharing images that biologists, naturalists, and other gall-curious folks can help ID.

A Gall to Action

In their own way, galls are beautiful. Go admire some. Appreciate the roles they play in healthy ecosystems. Pretend you’re a miniature wasp. Make miniature wasp noises, move as a miniature wasp moves, dream as a miniature wasp dreams (of galls). A good place to start is under the shade of an oak tree.

Yerba Santa Gall Midge

Yerba Santa Gall Midge Larvea

Contact

If you find galls around the Garden, you can email photos and locations to the author (zphillips@sbbg. org), who is very gall-curious but not very gallknowledgeable, and the Living Collections Curator (cvarnava@sbbg.org), who tries to maintain an omniscient gaze on all things plants. O

Acknowledgements: Thank you to Christina Varnava, José Flores, Stephanie Ranes, and Helen Noroian for their help spotting and collecting galls. References: Russo, R. 2021. Plant galls of the Western United States. Princeton University Press. All photos by Zachary Phillips.

References: Russo, R. 2021. Plant galls of the Western United States. Princeton University Press.

All photos by Zachary Phillips.