World Of The Plant Galls

photograph by Tony Oppersdorff
text by Kyrill Schabert

You find them in the woods on low shrubs or fallen on the path from the canopy above: spiny excrescences, hairy balls, pinecone- or flower-shapes. They are the plant galls caused by insects, fungi, bacteria or viruses. Some harmful to the host, others benign or rarely (as in the case of the tropical fig wasp/fig mutualism) beneficial, these worldwide plant symbionts play an unsung and still not completely understood role in forest ecology. Galls are essentially a controlled tumor, a “sink” into which the host plant sends proteins and carbohydrates, a sort of enslavement for the benefit of the gall maker.

Our favorite, the oak apple gall, is made by the gall wasp (family Cynipidae) in the tree tops of oaks but is found at your feet, having broken from its leafy mount above. Slice open the thin outer shell and you find the wasp grub at the center supported by the spongy filamentous interior. This inconspicuous member of the bee/wasp/ant tribe (the so-called social insects or hymenoptera) has a wonderfully complex life cycle, here described by the German artist/naturalist Walter Linsenmaier (1972): “The female of the summer generation implants her eggs in the ribs of oak leaves, and at each place stabbed there arises a spherical gall within which a larva is growing. The generation developing therefrom consists solely of females, which emerge in winter and lay their eggs in buds, which turn into bud galls. In summer, these yield the new, once again bisexual, generation.” In describing reproduction in another gall wasp species, he writes, “So an unfertilized female lays only a small number of eggs and waits until she has copulated with one of the males that develop from them before she deposits her extensive store.”

Devote yourself to the study of galls and you become a cecidiologist, as did Brian Rehill, a Ph.D. in entomology, who says flat out, “I love galls.” His specialty, the witch-hazel gall made by an aphid, involves teasing out the mysterious hormonal interaction between plant and insect. Fieldwork might find him lying on his back in the late snows of early spring observing the pinhead-sized female aphid waiting on an unfurled witch-hazel leaf to start her gall home right when the nascent leaf begins to grow. She is known as the fundatrix (“foundress”). His patience is rewarded with seeing the mother aphid pivot around, inserting protein “inclusion” bodies (which look suspiciously like viruses) into the leaf to raise a dome of tumorlike plant tissue up and over her. Within this chamber, she bears by live birth (viviparouly) a generation of virgin females, who eventually grow wings, and may disperse to another tree and bear the third, sexual generation. The matriarchal endeavors of these aphids result in male and female offspring and subsequent mating. Third-generation females lay their eggs in tiny crevices of the bark, difficult but not impossible to see. Most aphid reproduction is known as parthenogenesis, virgin birth, performed in the complete absence of males, a strategy practiced by other invertebrate and microbial life forms. (In some cases such as a group of microscopic animals called rotifers and the more familiar mite family, males have never been observed.)

Rehill has since moved on to study how white oaks (Quercus alba) are being affected by climate change. As the Eastern US becomes more urbanized, forests grow more fragmented, and he wants to know if enough genes are shared among white oaks to meet some of the coming threats due to longer, warmer growing seasons: new diseases, new insect pests, more frequent extreme weather. This may seem like a departure from galls, but the oaks of North America have one of the richest gall faunas on earth, and using gall insects as sentinels of change has its advantages. They present readily identifiable evidence of their presence in the form of their galls, which can last years after they have hatched and departed. Many gallers are more readily identified by their galls than by looking at the actual insect. Midcoast Maine delineates the current northern limit of this species.

Next time you see an oddly shaped plant growth on stem or leaf, remember that even this subtle irregularity of nature figures into the local ecology in important ways still to be discovered. Even the mechanism of gall formation is unknown, and may hold keys to the way that plants develop into a myriad of shapes. The interconnected webs of fauna and flora in our environment are invisible but integral to a healthy world.

“World of the Galls” is taken from Best Nature Sites: Midcoast Maine, a collaboration between writer and photographer Tony Oppersdorff and writer and editor Kyrill Schabert.