A few Thanksgivings ago, my then-10-year-old daughter and I went for an afternoon stroll through a power line right-of-way through steeply sloping woods to the Winooski River. As we moved through the tall scrub, Lauren’s interest was drawn to the golf ball-sized swellings on desiccated goldenrod stalks.
As usual, she had many really good questions: what were these woody spheres on dead plants; why did some have holes; what did they look like inside? We pocketed a few and continued our walk. The soft silty river bank was peppered with footprints left by raccoons, herons, skunks, and deer that prompted more questions. By sunset we had made it through the Muddy Brook Natural Area and back out onto the gravel road. Our catch of the day remained in our pockets until after dinner.
The spheres we collected were goldenrod ball galls. A gall forms when, in spring, a fly species (Eurosta solidaginis) lays a single egg into the growing goldenrod tip. These flies are picky. They lay their eggs only on three of the many goldenrod species growing in our region. Once a larva emerges, it burrows to the center of the still-growing stalk. The plant responds by thickening its stem to grotesque proportions, often ten times the original diameter, comparable to a human growing a basketball around a wrist.
The hungry larva fattens up on plant material inside its spherical castle, excavates a small chamber in the center and digs a tunnel towards the surface. Then the larva transforms radically forming pupa. Months later, using the tunnel it created, the adult fly will emerge and complete its cycle by laying eggs in new goldenrod plants.
A careful snip with pruning shears provides a window into the strange world of goldenrod galls. My college students and I trek out to gather hundreds of them each spring. We learn that galls, quite frequently are invaded by other species. Parasitic wasps (Eurytoma gigantean and E. obtusiventris) insert their eggs through the thick protective wall. The hatchling wasp larva consumes the fly and makes use of its former home until its own spring emergence. The predatory beetle Mordellistena convicta follows a similar pattern.
Birds also consume both the goldenrod gall fly larvae, and the larvae of their wasp and beetle usurpers. You can tell a crisp chiseled hole left by a woodpecker from the untidy mess left by a chickadee.
Despite the waves of attackers, more than enough flies survive to found the next generation. In the laboratory, we experiment by warming galls, to simulate an early spring. Fewer galls kept in the dark hatch; those kept in the freezer never hatch.
While it might seem that the thick gall would insulate the flies from winter extremes, their true survival mechanism is far more interesting. Insulation works when there is heat to conserve, but tiny larvae and pupae with slow metabolisms don’t generate very much warmth. Instead, goldenrod gall flies rely on antifreeze properties of their tissues to prevent ice crystals from rupturing their cells.
When my daughter and I opened our pocketed galls, we found fly larvae in more than half of them which made for some interesting observations after apple cider and pie. Whether for a classroom, an educational stroll, or as winter foodstuff for hungry birds, goldenrod galls have a fascinating story to tell.
Declan McCabe teaches biology at Saint Michael’s College. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation.