Go to relax under a spreading oak tree this autumn and you may discover a somewhat unwelcome surprise. That oak may be buzzing with large, black and white hornets. What’s going on? The answer is a bit complicated. But rest easy: the wasps are seeking sugar water, not flesh!
What is that insect? The bald-faced (or white-faced) hornet (or wasp), Dolichovespula maculata, is a social wasp in the family Vespidae, along with yellow jackets, paper wasps, and other local species. They live in colonies of a few hundred individuals and build large hanging nests consisting of layers of cells for larvae surrounded by a papery shell. Bald-faced hornets have a sting that is only mildly toxic to their insect prey. Instead, the sting is used to defend against vertebrate intruders, which they do aggressively. According to Justin Schmidt, whose pain scale for insect stings derived from his own intimate experience, bald-faced hornet stings rate a “level 2” pain (level 4 is most intense), similar to that of a honeybee or yellow jacket. Schmidt describes the sting as “rich, hearty, slightly crunchy. Similar to getting your hand mashed in a revolving door.” The worst of it lasts for about 5 minutes.
What is the plant? The leaves and twig in this picture belong to a bur oak, Quercus marcocarpa. Bur oak is an important hardwood of Midwestern forests. Trees can live up to four hundred years and grow as large as 10 feet in trunk diameter. As a member of the “white oak” (as opposed to “red oak”) group, bur oaks have leaves with rounded lobes and acorns that are relatively low in tannins and mature in a single season.
If that’s an oak tree—are those round things acorns? Acorns—the fruits of an oak tree–consist of a nut set in a cap. Bur oaks have the largest acorns of any North American oak (up to 3.5 cm diameter), with a deep, bumpy, and heavily fringed cap attached at the end of a longish stem. The balls in this picture share none of those traits. Instead, they are relatively small (about 1 cm), have no cap, and appear directly along the twig. These are not, in fact, acorns. Rather, they are galls—abnormal plant growths induced by “irritation” of the plant. Most commonly, that irritation comes in the form of an insect laying its eggs, and the gall serves as an incubation chamber for insect development. However, galls also may be induced by insect feeding or by infection from bacteria or fungi, and they can occur on a variety of plant parts, including leaves, stems, and buds.
What induced these galls to grow? Oak bullet galls, as these are called, develop in response to a small wasp in the family Cynipidae (the gall wasps) laying its egg under the bark of a twig. Cynipids are a diverse group, with more than 100 species in Ohio. (The most famous oak gall researcher probably is Alfred Kinsey, author of the influential Kinsey reports on human male and female sexuality in the mid-20th century, who examined millions of galls and gall wasps before switching to his better-known studies.)
The wasp species that causes galls on bur oaks is Disholcaspis quercusmamma, the rough bulletgall wasp. Galls form in mid-summer, and each one houses a single wasp larva. In autumn, the larva metamorphoses into a self-fertile (asexual) female adult. She chews an escape tunnel out of the gall and then lays eggs in dormant oak leaf buds. In spring, male and female gall wasps hatch from those eggs. (Interestingly, each individual asexual female produces either only male or only female offspring). Those males and females mate, the females lay eggs in the twigs, and the next round of bullet galls begins. Theories about how the insect induces the tree to produce a gall include roles for mechanical signals, chemical activation, and viruses. The actual mechanism remains a mystery.
Why is the bald-faced hornet visiting the gall? Bald-faced hornets are omnivores. During much of the summer, they eat mostly bees, flies, and other insects. But they also consume sweet liquids like flower nectar and fruit juices. Some of that sugar water is returned to the nest to feed the larvae. Bullet galls take advantage of that sweet tooth. As the gall develops, it creates a specialized tissue called a nectary that secretes a sugary droplet of liquid. Wasps love that treat, and a large oak with thousands of galls can provide a rich banquet of sugar water.
Why should the gall attract wasps? Oak bullet galls are exposed and conspicuous, with a nutrient-rich nugget (the wasp larva) inside. So, the wasp larvae are vulnerable to predation by birds like chickadees and woodpeckers. The nectar is the price the gall pays to “hire” a guard—the bald-faced wasps—for the wasp larvae. Of course, it’s the tree, not the wasp, that pays the cost of the nectar. However, bullet galls seem to have just a minor impact on the host oak.
Conclusion: What starts out seeming like a relatively simple—if unwelcome—observation—some wasps hanging out in an oak tree—turns out to encompass a combination of complex biological processes. The gall wasp alternates between asexual (all-female) and sexual (male/female) generations. The placement of an insect egg under the bark of an oak twig induces a specific deformity. That growth—a plant gall—secretes sugar water that attracts larger, stinging wasps that protect the gall wasp larva inside. So many complications! And so many as yet unanswered questions! All of which might have led Alfred Kinsey to ponder: which is more complicated, the wasps and the trees, or the birds and the bees?!
Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.