Gall Wasps In Trees

I followed the narrow trail up a ridge into the foothills of the southern Appalachian Mountains. It was fall and the tangy sweet-and-sour scent of decay permeated the cool morning air. Oak and hickory blanketed the hills, and their fallen leaves crunched satisfyingly beneath my boots. After several miles the trail entered a grove of scarlet oak, where red foliage stood out among the oranges and browns of the surrounding forest. It was a lovely spot to rest, so I shucked my pack and plopped down at the base of one of the trees.

As I gazed into the crimson crown above, I noticed dozens of brown blobs almost hidden among the colorful leaves. They were the size of golf balls and attached to the tips of twigs like finials. I knew that the blobs were galls-essentially growths caused by some irritation to the plant tissue-but I had no idea what they were made of or what caused them. I snapped one off for a closer look at home.

Anyone who spends time in the woods has encountered these peculiar growths that afflict almost every tree and plant species, from roses to redwoods, from orchids to oaks. They come in an astonishing array of forms; some look like red BBs, others resemble miniature pineapples. Some are almost indistinguishable from fungi, while others are nutlike in appearance. To further complicate matters, almost anything can cause them, including insects, bacteria, viruses, and fungi.

But if you’re looking to gaze at some galls, there are a few you can easily identify. Oak apples, the galls I spotted on the scarlet oaks, are hard, green growths, 1 to 3 inches in diameter, that turn brown or red as they age. They can be found on almost every species of oak from California to Florida and are sometimes so numerous that the tree appears to be bearing a crop of undersized apples.

Because oak apple galls are so common, they have been studied for years. In fact, my office is adorned with several oak galls, each of which has a tiny round hole in its shell-the key that unlocks this galling mystery.

Many of the larger galls, the ones hikers are likely to notice, are caused by solitary wasps of the family Cynipidae. Entomologists have counted more than 700 species of cynipid wasps in the United States and Canada, most of which form galls on oak trees. They have also learned that the eggs of cynipid wasps do not create the gall; only after the larvae have hatched does the gall begin to form.

Cut open an oak apple gall and you’ll see that the inside is filled with a fine, fuzzy material. In the center is the egg chamber, clearly defined but most likely empty. When the wasp larvae hatched, chemicals in the their saliva caused the plant tissues to swell and the gall to form. The growing grubs fed on this handy smorgasbord until they matured into full-fledged wasps, then bored a hole in the gall, and flew off to inflict more damage. Scientists still don’t know exactly what chemicals cause galling, and not much is known about the wasps themselves.

In part that’s because cynipids are such small, secretive creatures. Unlike the social wasps, such as yellow jackets that like to set up shop in trail shelters, solitary wasps are downright reclusive. They don’t have stingers or descriptive names. In fact, most of them don’t have common names at all. The wasp responsible for the oak apple galls I keep in my office is known only by the ever-so-catchy Amphibolips confluenta.

Because they are so small, cynipid wasps are virtually impossible to observe in the wild. But I was determined to see one, so I visited Ken Ahlstrom, an entomologist and curator of insects for the North Carolina Department of Agriculture.

“Look at this,” Ahlstrom practically gushes. I peer into the microscope. A specimen is mounted on a sliver of stiff paper. Magnified, A. confluenta appears as a shiny, hunchbacked creature with a dark brown exoskeleton and oversized, transparent wings. Ahlstrom hands me the mount. The wasp is so small that it is nearly invisible, no more than a speck at the tip of the paper. He squints at the mount. “They’re tiny buggers, aren’t they?” But tiny as it is, A. confluenta, which is about a quarter-inch long, is huge compared to some gall wasps. One, the oddly named Alaptus magnanimus, is the world’s smallest insect, no bigger than the period at the end of this sentence. Because of their microscopic size and cryptic ways, gall wasps are some of the least-studied creatures in the animal kingdom.

In The World of the Wasp by Joy Spoczynska, I discovered just how quirky these insects are. Many species alternate generations, Spoczynska tells us. They produce two broods a year. The first, known as the “unisexual generation,” is composed of females only. The second brood is made up of wasps of both sexes.

In some species, the females live their entire lives on one oak tree. In late summer, they emerge from the snug comfort of the oak apple galls in which they were born, attract males, mate, crawl down the trunk, lay their eggs in the tree’s roots, and die. When the eggs hatch, a root gall forms. The next spring an all-female generation emerges. They crawl up the tree and deposit their eggs in the buds at the end of oak twigs. When the unfertilized eggs hatch (through a process know only to virgin female wasps), an oak apple gall forms, and the cycle begins again.

To learn more about gall wasps, some scientists have moved their research from the forest into the laboratory. Unfortunately, the wasps don’t always cooperate. In her own work, Spoczynska tried to get wasps of the all-female generation to lay eggs in oak twigs in her laboratory. In her book, she recounts her many failures then challenges her readers: “Why,” she asks, “don’t you have a go?” Naturally, I had to pick up that gauntlet. Which is why I have oak apple galls in my office.

First I found an oak apple gall with no holes anywhere to be seen and put it in a jar. Since I lacked the fine muslin Spoczynska suggested for closing the opening, I opted for a screw-on top. I jabbed a few tiny holes in it with an ice pick to let in fresh air. The jar sat in a corner of my office for weeks. A few days ago, a pin-size hole appeared in the gall where the wasps had gnawed their way out.

Had I visited Ken Ahlstrom first, I would have known that even fine muslin for those tiny parasites would be like a chain-link fence to mosquitoes. The holes I punched in the lid must have looked like gaping skylights. But I haven’t given up. I’m incubating another batch in a jar covered with facial tissue.

And if that doesn’t work, I can always be content with sitting under crimson oak trees, pondering the unsolved mysteries of nature.

Phillip Manning lives in Chapel Hill, North Carolina. His most recent book is Orange Blossom Trails: Walks in the Natural Areas of Florida (John F. Blair, 800-222-9796).