Answer: I appreciate your caution about using a systemic insecticide. It sounds like you've done your homework! Usually insect outbreaks are cyclical, meaning that they are more intense in some years than others. Trees seldom suffer permanent damage from defoliation unless they lose leaves for two consectutive years.
The likely culprit in your case is a sawfly called Profenusa collaris. The larvae will pupate and spend the winter in the soil under the tree, and adults will emerge at around the time the buds on the hawthorns begin to unfold and blossoms begin to open. The adult lays eggs inside the leaf tissue, so they aren't vulnerable to surface sprays or predators such as birds. However, there are two known natural enemies, the parasitic wasps Pexoporus tenthridinarum and Trichogramma minutum. A release of these tiny, non-stinging wasps synchronized with sawfly egglaying may reduce the population next year. You can order T. minutum from Gardens Alive, 5100 Schenley Pl., Lawrencburg, IN 47025, ph# 812/537-8650. I also suggest that you speak with an arborist or urban forester at your agricultural extension office (ph# 301/590-9638) for precise instructions, or other suggestions. This might even be a good project for the kids to study to learn about ecology! Best of luck!
Q&A Library Searching Tips