The Biologist Is In: The Gall of Those Goldenrods

Tuesday, October 13, 2015

The Gall of Those Goldenrods

S. canadensis flowers.

Goldenrods (genus Solidago) are blooming prolifically in southern Minnesota. This patch is near the building I work at, but I find them blooming in parks, along roads, and in my own yard. Though they are considered weedy and potentially invasive, I like the look of the flowers and they make an interesting subject for study due to the various animal species which interact with it.

One of the more studied interactions is between the Goldenrod and the Goldenrod Gall Fly (Eurosta solidaginis). This fly causes the Goldenrod to grow hard bulbous galls in the main stem that are referred to as "ball galls". The galls are easy to collect during winter, so ample material is available for teaching or research purposes.

S. canadensis flower galls.

Though I have found a few of the ball galls, the Goldenrod patch near my work didn't seem to have any. The patch was full of another type of gall, however. These galls are formed at the top of the stem and look like a tight cluster of leaves, forming a flower-like head structure. Because of the appearance, they are referred to as "bunch galls", "rosette galls", or "flower galls".

Goldenrod flower gall.

If you take one of these flower galls and dissect it carefully, it has an internal structure very similar in organization to the flower of a sunflower plant. There is a broad flat disk with leafy structures (florets or leaves), on top of a pithy core which widens as it approaches the base. Ok, I admit it is a bit of a stretch, but there is enough similarity to have the name "flower gall" make some sense.

When I first saw these galls, I had assumed they were caused by an infection with a fungus that was using the flower-like structure to trick insects into carrying its spores to other plants. (Check out the Cedar Apple Rust gall for an example of a fungal "flower" used for spore dispersal.)

R. solidaginis larva.

When I cut open the Goldenrod flower gall, I found several small insect larvae. They had no identifying markers, aside from their characteristic flower gall. After doing a bit of research, I found these were larva of the Goldenrod Gall Midge (Rhopalomyia solidaginis). These small flies are parasites of a specific Goldenrod species, Solidago canadensis.

I've had limited success finding information about the life-cycle of the Goldenrod Gall Midge. The few references I've found which talk about the insects' life cycle refers to two life-cycles per year. The larva I found represent the summer juveniles, which will pupate into fall adults. These fall adults will lay eggs in the ground (or the base of the Goldenrod plants?), which then quickly hatch and overwinter as larvae. These larvae would then grow quickly and pupate in spring to make the later generation of adults to infect the Goldenrods and produce the flower galls. I've known that the life-cycles of parasites often include multiple hosts or stages of growth, but I didn't realize this one was going through its complicated life right under my nose in the wild-flower patch.

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