A recent stroll at an out-of-state park helped me realize just how fast the summer is coming to an end.
While most tree leaves still hold their green color, somehow they look tired. Ragged, weather beaten and insect-chewed, they still are capable of synthesizing sugar, but their efficiency has been extremely curtailed.
The grasses and forbs also are evidencing the change of season. Some are dried up completely, while others are fading and tipping over. Where just a few weeks ago impassable swaths of greenery halted most would-be hikers, now a person can pick their way through meadows less hindered.
Maybe that's why the goldenrods (Solidago sp.) stood out so much last week on that hike. Some species flower quite late and most are rather green and robust still. Once a person starts noticing them in numbers, one can't help but become aware of strange little globes, about the diameter of a half dollar, right in the middle of some goldenrod stems. We've all seen them. They appear as if the plants underwent some kind of weight training for a brief period or were injected with steroids.
In reality, the curious enlargements along the stems of goldenrods are the result of the plants' reactions to intruders. That intruder is the goldenrod gall fly (Eurosta solidaginis).
The parasitic fly is quite small and rarely flies and so spends most of its time walking. Most of that walking is done on - you guessed it - goldenrods. It lives as an adult for only about two weeks. During that time it does not eat, existing only to mate and lay eggs.
The fly's entire life cycle is dependent upon goldenrod plants but apparently not all of them. Stevens lists 13 species of goldenrod for North Dakota (Handbook of North Dakota Plants). Of those, only the late goldenrod and the Canada goldenrod are used as hosts.
The female fly deposits eggs in the growing tip of a young plant. In about 10 days the egg hatches and the larva immediately begins to feed. The chewing process and the parasite's saliva - thought to mimic a plant hormone - cause a reaction in the goldenrod which results in an odd little out-of-place ball (botanically, a gall) along the stem.
For nearly a year larvae will be confined to their cozy round domes, eating and going through three growth phases. In late fall, the grub will dig an exit tunnel leaving only the skin of the gall for a door. It's at this time something amazing happens. The larva makes glycerol. That's right; the same compound used in everything from aircraft deicing fluid to toothpastes is produced by the insect. In this way, it can survive freezing winter temperatures as low as -40 degrees.
As warmer spring weather approaches, the insect enters a pupal state where it rests while morphing into an adult. Two weeks later the fly exits the bulb to begin the cycle again.
As is normal in the natural world, the fly attracts many predators; some even target the gall-enveloped grub. A couple of parasitic wasps prey on the small worm as well as at least one beetle.
Some birds are also privy to the meal hiding within the gall. Chickadees are known to feed on them as well as one of our common woodpeckers. I've seen downy woodpeckers pecking away at goldenrod galls in the middle of winter, quite far from the nearest tree. As incongruous as this might seem, the chance at a juicy morsel is apparently worth leaving the safety of trees and risking predation to themselves.
Some neat school science demonstrating winter survival can be easily achieved by collecting galls in late fall, extracting the larvae, and putting them in the freezer. When class starts, plop the hard little cream-colored grubs on the table and within minutes they begin to warm and squirm. Repeat as necessary.
Look for the mysterious spheres anywhere goldenrod grows. What's inside is really not a secret. Even savvy, old ice fishermen have been known to tap into them for bait.