Consult an arborist if you're thinking about thinning, and note that you'll need a permit to thin a City-owned tree. The Tree Section does have its own pest control service. If your street tree is abnormally infested with caterpillars or other insects, you can ask the Tree Section to spray it. However, relief may not be immediate. Insects are only vulnerable at certain stages of their lives.
Caterpillars have to be sprayed in their first or second molts, for example.
Spraying later won't work because the caterpillars will have stopped eating. By the time you notice big caterpillars, it may be too late. You should call the City earlier next year to get on the spraying calendar. Fungus outbreaks are seasonal. Plane trees and Modesto ashes, not native to the area, are susceptible to anthracnose.