Fletcher Wildlife Garden. This is the fly responsible for all those ball-shaped galls you see on goldenrods. The balls form around the larva of this fly, and in winter you may see mice, squirrels, woodpeckers, other birds, busily probing the galls for the larva which provides food in the depths of winter. Woodpeckers make little round holes and use their long bills to extract the larva. Squirrels gnaw away until they have chewed the gall in half exposing the larva.