Worms? Monsters? These are “finger galls”.
Insects or mites feeding on a leaf, or laying eggs on or injecting eggs into part
of a plant, cause galls to form. The leaf tissue reacts by increased production of normal plant
hormones and localized plant growth.
The outcome is an abnormal plant structure called a gall. These look like tiny wiggly "fingers" protruding from elm leaves.
Gall formation usually occurs in late spring during the
accelerated growth period of new leaves, shoots, and flowers. This is when insects might be feeding on plant tissue, or emerging in their life cycle to lay eggs. The gall-making insect develops inside the gall and the
gall continues to grow as the organism feeds and matures. In autumn and winter, some birds feast on
the insects they find inside galls, especially on goldenrod.
