Monday, December 9, 2013

"I find you galling", said the goldenrod to the fly.

Winter weather has hit Mid-Michigan.  The roadsides and meadows look brown and devoid of life, but if you look closer you can find signs that life, for some species, continues on through the winter.

At first glance, a patch of goldenrod looks dead and devoid of life.  Closer inspection reveals signs that life is present, if not visible.  Goldenrod is the host plant for many species of insects including midges, wasps, and flies.  The larva of some of those species cause galls to form on the plant.  Looking at the photographs below several of those galls are visible.

Goldenrod patch - look closely and you can see at least two types of goldenrod galls

The first type of gall is formed on the tip of the plant by the larva of several species of midges.  This type of gall is known as a bunch gall, rosette gall, or flower gall.  The gall is caused by chemicals released by the midge larva which cause the plant to grow abnormally.  The gall serves as both food and protection for the developing midge.

Goldenrod bunch gall

A second common goldenrod gall is the goldenrod ball gall.  This gall is cause by the larva of the Goldenrod Gall Fly (Eurost solidaginis).  This gall is created as a reaction to chemicals is the saliva of the feeding fly larva.  The larva will remain in the gall all winter long.  The larva produces chemical that act as an antifreeze - allowing it to freeze without damaging its cells.  The larva remain a state of suspended animation all winter long.  When the weather warms in the spring, the larva will "come back to life" and pupate for a period of time before emerging as an adult fly.

Goldenrod Ball Gall - caused by the larva of the Goldenrod Gall Fly
As a kid I remember collecting dozens of these galls to use the larva as bait for ice-fishing.   

Several goldenrod ball galls

Each of these galls is approximately 1 inch across.

Goldenrod ball gall - approximately 1 inch diameter

If you cut open the gall, the larva can be found curled up in the center.  Before the winter, the larva will chew an exit tunnel to just beneath the surface of the gall.  This allows the adult midge to easily emerge in the spring.

Goldenrod ball gall - cut open to reveal the larva in the center and its exit tunnel
Goldenrod Ball Gall with larva removed to show chamber in the center of the gall and the exit tunnel.
Just for fun, I took several pictures of the larva through a digital microscope at 2X magnification.

As the larva warmed up it started to become active, squirming around on the microscope. Here is a short video of it squirming around.

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