Back in 1969, while I was working as a naturalist for the East Bay Regional Park District in Oakland, California, my boss, Chris Nelson, dropped a bunch of purple spiny things on my desk. “What are they?” he wanted to know. Well, I might have said they were miniature purple sea urchins, except that those strange objects were attached to the leaves of a blue oak, Quercus douglasii. Was he playing some sort of botanical prank on me? At that nascent stage in my career I aspired to pursue research on a grand scale, involving, say, entire forest ecosystems. But with his simple (and innocent) action, Nelson had sparked my curiosity about something small and wonderful, helping to define the course of my life.

The search for a better understanding of those natural “plant urchins” quickly led me to the late Sara S. Rosenthal, then a young wasp biologist from the University of California’s Essig Museum of Entomology at Berkeley. Together we visited an old blue oak in Briones Regional Park, in the hills above Martinez, California. In the space of an hour or so, she pointed out twenty-one structures of every imaginable shape and color on the leaves and stems of that single tree. They were galls, casings built by the tree in response to the manipulation of insects—in this instance, all species of wasps belonging to the family Cynipidae. Each cynipid species had made a distinct gall (you can identify the species of wasp by the shape, size, and color of the gall). One type of gall matched the purple bumps I had seen earlier: it was the product of the urchin gall wasp, Antron quercusechinus. I was nearly speechless. Living inside all those different kinds of galls were insects, some barely visible to the naked eye—but their homes were now so obvious! In all the time I had spent in woodland and forest habitat, how could I not have noticed them before?

Plant galls are tumorlike swellings initiated mostly by insects, but also by mistletoe and by some bacteria, fungi, and mites. With around a million species of insects known worldwide, their inventive feeding choices know no bounds: carrion, dung, pollen, fungi, leaves, nectar, sap, and blood, for starters. But only 13,000 species have evolved the ability to induce galls in specific host plants, thus co-opting the plants’ resources. Overshadowed by more dramatic species—ants, honeybees, disease-bearing mosquitoes, butterflies, and crop-damaging caterpillars—gall insects exist in our midst with little fanfare or publicity. They rarely have any economic or agricultural impact, despite their large numbers and widespread ranges. But they leave behind clues to their existence as distinctive as mountain lion tracks imprinted in the mud or an eagle feather fallen by a lakeshore. The diversity of those gall insects and their bizarre biology have fascinated me for forty years.

Midges—those in the family Cecidomyiidae—make the most galls, accounting for about 20 
percent of all species of gall inducers worldwide. Next come a variety of wasps: tenthredinids (sawflies), tanaostigmatids, pteromalids, eurytomids, agoanids (fig wasps), and cynipids. Other inducers include some aphids, psyllids, thrips, moths, beetles, and fruit flies. For many such insects, a gall serves as a larval nursery, a protective, nourishing lodge for their young. A gall may house one larva or many, depending on the species of insect.

I have focused on cynipid wasps. They provided my introduction to galls, and they also have an interesting claim to fame: Alfred C. Kinsey (prior to embarking on his controversial work on human sexuality in the 1930s) described many new species and varieties of cynipids. But those are not the main reasons I have stuck with them for so long. Instead, the approximately 1,400 species of cynipids intrigue me because theirs are among the most extravagant galls, assuming the shapes of sea urchins, stars, baskets, bowls, clubs, horns, and coral, to name just a few, in endless color combinations.

Cynipids rely primarily on oaks, sycamores, and certain members of the rose family as their hosts. Most of those wasps are quite small, some the size of a comma on this page, while others are about the size of a housefly. They are lilliputian architects, masters of grand alien designs, keepers of biological secrets that have defied naturalists and scientists since Hippocrates wrote about galls’ medicinal properties nearly 2,400 years ago.

A shell of plant tissue forms around each cynipid egg soon after it is deposited. However, the gall doesn’t really begin to grow around the shell—or group of shells—until after the hungry larvae hatch and begin feeding. Through their chewing, the wasp larvae release complex compounds that redirect the growth of different plant tissues to form the protective outer structures of the gall, its larval chamber, and its inner nutritive lining. While all cynipid nurseries have similar amenities on the inside, the gall’s outward shape, color, and size is unique to each species. Galls form on every plant organ—including roots, flowers, and fruits—but most seem to appear on twigs and leaves, where high metabolic activity occurs over a short period of time, particularly in the spring.