By Leslie Mertz
Apples on oak trees, pine cones on willows, finger-like projections on leaves — what’s going on? These are some of the amazing growths called galls that develop on plants, courtesy of insects. Exactly how the insects orchestrate gall formation, however, remains one of the great mysteries of nature.
Galls start when a female insect lays an egg in some actively growing part of a plant, and something in the saliva or other fluids of that female or in her developing offspring prompts the plant tissue to grow into a gall. The young insect lives inside the gall, which can provide both shelter and a food source, and when it gets big enough — usually when it becomes an adult — the insect makes its way out.
“How those fluids trigger the gall is unknown. It’s just one of those things that science hasn’t figured out yet,” said John Tooker, associate professor in the Department of Entomology at Penn State University, and a Penn State extension specialist for field and forage crops. “The best evidence suggests that the insect fluids are somehow influencing plant hormones, which are then influencing the gene expression that forces the gall tissue to grow, but there is a lot of hand-waving there.”
What is known is that the insects that spark gall formation are very particular about where they lay their eggs, sometimes selecting just one species of plant, and then just one location on the plant, whether that be a leaf, a stem, a bud, or some other site. In addition, the galls themselves have such a distinctive appearance that they can be used to identify the insect species inside. At the same time, the plant itself may harbor a variety of different galls. A single oak tree, for instance, can produce dozens of different types of galls, all triggered by different insect species.
“Insect galls are the epitome of plant manipulation,” said Paul Nabity, an assistant professor at Washington State University who is studying the evolution of galls. “Galls are a constant reminder of how complex nature is and that there are so many things to continue to observe and question.”
Nabity, Tooker, and other gall experts have their favorites when it comes to galls. They include:
Grapevine phylloxera leaf gall. An aphid-like insect (Daktulosphaira vitifoliae) causes this gall, which rose to infamy in the mid-1800s when it nearly decimated grape production. The insect is “phenomenal,” Nabity said, because it not only induces galls, but also reconfigures vital plant organs, known as stomata, which allow plants to “breathe” or assimilate carbon dioxide. This unique capability makes stomata where the plant normally lacks them, ultimately providing a direct benefit to both plant and insect.
Jewel oak gall. When asked about the most interesting galls, entomologist Richard Grantham of Oklahoma State University remarked, “I think the Cynipidae (gall wasp family) have to win hands down. There is such a tremendous variation in size, color and pattern, weird shapes, gall arrangements, and just the way-cool factor!” The larva of the wasp Acraspis macrocarpae grows inside the jewel oak gall, which is found on the leaves of bur oak trees.
Beaked twig gall. Found on scrub (Quercus berberidifolia), leather (Q. durata) and blue oaks (Q. douglasii), the beaked twig gall’s shape and incredible color pattern are distinctive, according to Ron Russo, author of A Field Guide to Plant Galls of California and Other Western States. A wasp called Disholcaspis plumbella induces this gall.
Crystalline tube gall. The wasp Trichoteras tubifaciens directs the Oregon oaks (Q. garryana) to make these galls. Dozens of these galls often sit side by side. Russo remarked, “When seen in nature, they are stunning under a hand lens.”
Stellar gall. Induced by the wasp Andricus stellulus, these galls are generally found in isolated, small clusters of scrub oak (Q. dumosa) and shrub live oak (Q. turbinella) in the Mojave Desert in California. “These tiny galls consist of thin, hair-like stalks about 6-7 mm long, atop of which are toothed cups about 3 mm in diameter,” as Russo describes them. “While they can be easily overlooked, once seen, their shape and survival under such hostile extremes in the desert is intriguing to say the least.”
Creosote gall. Induced by some 15 species of flies (Asphondylia spp.), galls like this one all occur on a single creosote species (Larrea tridentata), according to Nabity.
Crystalline gall. With an appearance like bristly red or pink caterpillars, these galls can cover the entire leaf surfaces on several white oaks, especially blue oak and valley oak (Q. lobata), according to Russo. “I have found nearly 100 percent of the leaves on an individual tree covered with these galls.”
Urchin gall. Induced by the wasp Antron quercusechinus, this gall is most common on blue oaks, but is also seen on other white oaks. Russo commented, “I have seen individual trees with thousands of these 1.5 cm in diameter galls. They are, in my opinion, the most striking galls that I have ever seen, and I think they are among the most bizarre galls on oaks in the world.”
Tooker added one last gall — a personal favorite — to the list. While studying galls of the tall-grass prairie plant in the genus Silphium during his doctoral work at the University of Illinois, he discovered two new species of gall-inducing wasps. He named one Antistrophus meganae for his girlfriend Megan, and revealed the honor as part of her Valentine’s Day gift. How did that go over? He laughs, “We did go on to get married, so I guess it worked out OK.”
Leslie Mertz, Ph.D., teaches summer field-biology courses, writes about science, and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.