"Imagine yourself living in a globelike room with greenish walls bulging outward and upward and then arching in to meet above your head," the naturalist Edwin Way Teale wrote. "Imagine such a room constructed of succulent, edible material, forming a house that at once provides food and shelter, plenty and protection. That is what you would find if you traded places with one of those gall insects that now live in the globular swellings on the stem of my hillside goldenrods.
Those galls began their story last spring. After a female goldenrod gall fly, Eurosta solidaginis, deposited an egg on the terminal bud of a growing goldenrod plant, the egg hatched in about ten days. The larva immediately bore down into the stem. The chewing action and the larva’s saliva, which is thought to mimic plant hormones, caused the goldenrod’s stem to thicken.
Soon, runaway cell division triggered by the larva formed a dense, round growth on the stem called a gall. This provided both food and shelter as the larva grew up. After going through three stages, called instars, the larva is ready for winter.
The larva will not leave the gall yet, though. Instead, it will excavate an exit tunnel in the gall to use in the spring, leaving just the outermost layer as a door. The larva must chew the exit tunnel now, because once it pupates into an adult, it will not have chewing mouthparts. The larva will then retreat to the center of the gall and fill its cells with glycerol, a cryoprotectant that protects cells from damage due to freezing. This is where we are in the story right now.
I often stop to examine the dried, brown galls as I ski or snowshoe through snowy fields. If you open a gall in the middle of winter, you may find the larva, surrounded by the debris excavated from the exit tunnel. They make good fish bait, and are protein-rich snacks for downy woodpeckers and chickadees.
You can tell which bird attacked a gall by the size and neatness of the hole. Downy woodpeckers have a thin, sharp beak that neatly excavates the tough dry material. Chickadees have a blunter beak, and make large messy craters. In contrast, the larva’s own exit hole (if it makes it through the winter) is tiny, perfectly round, with no rough edges, and no concave excavation pit.
Predators help determine the size of the galls. Downy woodpeckers select larger galls to attack, probably hoping for a larger grub. In areas where downies are common, flies with smaller galls survive better, and smaller galls are more common.
Birds are not the only creatures who exploit the gall fly larvae, though. A parasitic wasp, Eurytoma gigantea, uses its long ovipositor to penetrate the gall wall and lay an egg inside. The newborn wasp larva first eats the fly larva, and then continues to feed on the gall tissue until it pupates. The wasp’s ovipositor is only so long, so they prefer to lay eggs in smaller galls. Therefore, in areas where the wasps are common, galls tend to be larger. Where both downies and wasps are common, middle ground is found.
(To see for yourself why goldenrod gall fly larvae appeal to so many predators, I recommend that you taste one for yourself! The concentration of glycerol makes them slightly sweet. I have met many middle school and college students who are willing to accept that challenge! Let me know if you decide to join this Cool Club, too!)
After two weeks as a pupa in the spring, the new adult gall fly will emerge. It will walk up to the goldenrod plant and look for a mate. After mating, the female fly will leave in search of a new goldenrod stem in which to deposit her eggs.
Female gall flies are quite picky about which type of goldenrod to lay their eggs on. Certain fly “races” prefer certain species of goldenrod. The female can tell goldenrod (genus Solidago) species apart by “tasting” the plant with chemical sensors on their feet, antennae, and even in her ovipositor! When the right plant is found, a few eggs are laid, and the story beings again.
Once predators, parasites, or metamorphosis empties the galls, they become habitat for a variety of insects. Springtails, wasps, solitary bees, beetles, and ant colonies have been discovered using the galls. Next time you go for a walk or a ski in a field of goldenrod, take a moment to imagine yourself spending the winter in the globelike room of a goldenrod gall.