Galls, Y'all By: Zach Phillips, Ph.D., Terrestrial Invertebrate Conservation Ecologist
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ome 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.
5a: An old cynipid gall on a coast live oak, the woody legacy of a bygone bug
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Ironwood
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
5b: A still-developing cynipid gall on a coast live oak
6a: A gall of Parafreutreta regalis on Cape-Ivy
