Down with the thistles

Canada thistle (Cirsium arvense), an invasive species from Eurasia, is common in this area.  However, so too are its enemies.  Even though the thistles have died down for the winter, I have found several Canada thistle bud weevils (Larinus planus) in the yard.  These weevils seem to be seeking out spots where they can stay sheltered until the spring.

A Canada thistle bud weevil (Larinus planus) sheltering on the lavender bush.

Another of the thistles' enemies has found a very cozy way to spend the winter -- within galls on the thistles' own stems. 

A gall on a Canada thistle stem.

As I was clearing dead plants out of the garden, I came across a couple of dried thistle stems interrupted by these large galls.  I was curious to see what, if anything, was still inside.  Cutting open one of the galls, I found several very odd larvae.

Inside the gall there were several larvae of the Canada thistle stem gall fly (Urophora cardui).

The larvae did not have anything resembling legs or even heads.  They also failed to show any sign of life.  Thus, I initially thought that I must have cut them in half.  However, on closer inspection, I determined that they were all whole.  Although strange to me, this form is normal for Canada thistle stem gall fly (Urophora cardui) larvae at this time of year.  In the fall, the larvae finish growing and suspend their development.  Only in the spring will they pupate and then emerge from the gall as adult flies, ready to parasitize a new generation of thistles.