You know all those round galls you see on Canada and tall goldenrods (mostly the latter I think), the ones that are particularly visible in the winter? They are made by this fly. She lays her eggs in the stems of goldenrods and after a short while, when the egg has hatched, the stem, induced apparently, by chemicals secreted by the larva, forms a round ball around it. They overwinter as larvae and emerge right about now and begin the process all over again. Of course, many larvae don't make it, as they are a very popular food source for red squirrels and birds such as woodpeckers and chickadees. For all the galls we see, these flies are surprisingly difficult to find.