|S. canadensis flowers.|
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.|
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.|
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.
- Goldenrod: en.wikipedia.org/wiki/Goldenrod
- Galls: en.wikipedia.org/wiki/Gall
- Goldenrod gall fly:
- Goldenrod gall midge:
- Cedar Apple Rust: