They don't occur on all cherry trees, but they are abundant when present. They drop with the leaves, and the tree appears to have never been visited by these gall makers. Sometimes a willow has so many, it seems there's a gall at the end of every branch. Both the larva and the gall develop during the warm months. The larva becomes dormant in winter and completes its growth the following spring.
Many insects take advantage of the napping larva's large nest and move in.
Some of the uninvited guests live as inquilines, others as parasitoids. In one study, researchers found 23 galls contained 564 insects, only 15 of which were the original gall makers. Before the snow gets too deep, look for galls. Cut open the different kinds of galls you collect and try to find the gall makers. Collect goldenrod galls in late fall or early winter to observe the antifreeze effects of glycerol on goldenrod gall fly larvae.