Goldenrod Gall Flies and the Payoff of Living With the Wild
I have found that any honest attempt to understand an ecological phenomena is a journey down a wormhole. I found myself down one of those wormholes this past week. As I’ve mentioned there are several large goldenrod plants, probably of the species Solidago altissima, growing along my back fence. A few of these developed galls, spherical tumor-like growths, this past summer. I knew these galls were formed by some insect but that’s all I knew and only recently, growing bored with winter, did I pay them much attention. Last week I decided to cut one open.
The outer shell of the gall had the hardness of soft wood, much harder than I expected. But it wasn’t dense, it felt like a cross between a golf ball and a whiffle ball. The inside of the gall consisted of a small hollow central cavity surrounded by variously colored tissue which had the texture of a very old dried sponge. The central cavity was empty expect for some loose crumbly material (which later research indicates may have been frass aka insect poop, or also possibly waste from tunnel excavations) and for a moment I wondered if the insect had already left or been eaten. Then I saw that a little tunnel ran off from the central cavity and it was in this that a larval grub was tucked away.
Grubs are gross. I’m sorry. So I had real mixed feelings when it started to pulsate. On the one hand, cool development! But I was obviously repulsed at the same time. The movement could have been instigated by the warmth of my house or the physical disturbance of having its home carried around and then cut open. I carefully scraped out the rest of the gall but there was nothing else of note.
At this point I was full of questions: What kind of insect was it? How does the larvae end up in a goldenrod stem and how long does it stay there? Does it make galls in plants besides goldenrods? How does the gall get formed? How did it get to my yard? I turned to the internet where I uncovered so much more than I was looking for. Apparently this is a classic subject organism for intro ecology and a very well studied parasite. There are volumes of great information available on the goldenrod galls. Let me try and summarize some of it without things spinning out of control.
The larvae is most likely (my dissection was not detailed enough to rule out a similar looking larval wasp which will be discussed later) a member of the species Eurosta solidaginis which is in the fruit fly family Tephritidae. Eurosta solidaginis, commonly referred to as the goldenrod gall fly, is a parasitic specialist which lays eggs only on the stems of goldenrods. The adult flies are less than 1 cm in length and poor fliers, mostly getting around by walking (so how did they get to the goldenrod in my yard!?). They live as adults for only two weeks in which the big highlight is mating on a goldenrod bud. The male attracts the female with a courtship dance, swaying from side to side, and then these tiny flies take up to an hour to finish copulation (Abrahamson & Heinrich). After this the female inserts her ovipositor into the plant and deposits an egg. When the larvae hatches chemicals from its saliva induce excessive localized growth in the stem, producing the gall which the larvae will eat from the inside throughout the summer. At some point in the fall the larvae prepares for the following spring’s exit by excavating a tunnel up to the outer shell, producing anti-freeze compounds in preparation for a winter that it will spend in a small hollow capsule blowing around in freezing winds, and finally going dormant. In spring the larvae transforms into an adult, chews through the last bit of the tunnel, and begins its two-week adult life.
Crazy life cycle right? That, as they say, is just the beginning. The goldenrod gall fly can only reproduce on goldenrod plants. In turn there are not one but two species of wasps that can only reproduce on the goldenrod gall fly. They have different strategies. Eurytoma gigantea females use a long ovipositor to reach into the gall and lay eggs inside the gall, where the wasp larvae eat the fly larvae; Eurytoma obtusiventris deposits eggs directly inside the fly eggs or larvae and the wasp larvae induce early pupation in the fly, then use the pupae as winter shelter while eating the fly from the inside out come spring. There is also a beetle species, Mordellistena convicta, which lay their eggs on goldenrod stems and whose larvae sometimes but not always get in on the goldenrod gall fly larvae predation act (Abrahamson & Heinrich).
Birds also peck open the galls to eat the fly larvae, the Downy Woodpecker most notable among them. Downy woodpeckers target large galls, presumably because a bigger gall equals a bigger larvae. In areas with lots of Downy woodpeckers natural selection favors goldenrod gall flies whose larvae’s saliva produces smaller galls. This discourages woodpecker predation. Except the wasps mentioned above happen to prefer attacking smaller galls (easier to insert their ovipositor into) so in areas with more wasps the gall fly starts to produce larger galls, and anyway fly saliva is not the only determinant of gall size. The genetic makeup of an individual goldenrod plant will incline it to produce larger or smaller galls. This inspires a question I could not find addressed. In areas with lots of woodpeckers is selection favoring goldenrods that produce larger and larger galls to encourage predation of the flies parasitizing them at the same time selection on the flies is pushing for smaller and smaller galls? And vice versa in areas heavy with wasps? Or are these species of goldenrods such robust, fast-growing plants that the gall flies have a negligible impact on their growth?
In trying to come to a basic understanding of this tiny fly’s life cycle I couldn’t help but be reminded of Jon Muir’s observation, that “when we try to pick out anything by itself, we find it hitched to everything else in the universe.”
The goldenrod gall fly, its goldenrod host, and its many predators may not at first seem like the kind of story that informs and gives meaning to our lives. Looking for a parable for the virtues of a sturdy home built with solid walls? You may be tempted to look elsewhere than the humble Eurosta solidaginis larvae, swaying in the breeze in a wasp-laden field. But should we not be humble in our dwellings? Is it not wise to remember that a dwelling is called upon to safely and securely house its occupants, be they perched in a hollow cavity slumbering over January snow drifts or preparing dinner at the kitchen sink? Skip on the gall fly and you may miss the reminder that flexibility in the allocation of our resources to meet our environment is critical, and that the adaptability to carve out a place in the world, no matter how small, is the real victory. The dormant larvae adapted to generations of heavy downy woodpecker predation, its gall reduced, the walls around it thin, has lived an entire life dedicated to surviving the fall and winter to emerge in the spring.
Is the goldenrod gall fly a model of virtue and economy? Does it deserve our pity that it lives but two weeks as an adult, only to provide an appropriately safe gall for the next generation? Our respect? Of course Eurosta solidaginis is not designed as a parable for home building. It does not exist to provide economic lessons to us. It is simply part of a natural tapestry from which we have the privilege of selecting threads to inform, qualify, and narrate our daily lives. If we tell enough of these stories, tell them faithful to our ecological understandings, humbly aware of their limitations, and indiscriminate between the significance in eagles, tigers, redwoods, flowering perennials, wasp and beetle larvae, zooplankton, and everything in between, then I believe we may realize the grand prize: understanding ourselves as lifetime members of an impossibly vast club of the living, all carving out reproductive cycles in every way imaginable at the expense of and benefit to all the other members. Now that’s a payoff, but it starts with the source material. Plant some goldenrods y’all.
Resources and works cited:
The Abrahamson and Heinrich page summarizing their work at Bucknell University is an excellent public resource on goldenrod gall flies and includes an identification guide for determining what you find after opening a gall. There’s information there about the flies that I didn’t include in the post, including evidence that female goldenrod gall flies have the ability to “taste” the difference between and show preferences for different goldenrod species, which may lead to further speciation of the fly, very cool. The Hilton Pond Garden Center article includes information on two additional species that form galls on goldenrod plants in case this whole situation wasn’t complicated enough for you.
Abrahamson, Warren G. and Pual Heinrich. “The Solidago Eurosta Gall Homepage: A Resource for Teaching and Research.” Accessed 2 February 2015. <http://www.facstaff.bucknell.edu/abrahmsn/solidago/main.html>
Hilton, Bill Jr.. “All Galls Are Divided Into Three Parts (At Least In Goldenrod),” This Week at Hilton Pond: The Hilton Pond Center for Piedmont Natural History. Accessed 2 February 2015. <http://www.hiltonpond.org/thisweek051001.html>