In severe outbreaks, Eastern tent caterpillars (aka bag worms) can defoliate entire trees. For those who are not squeamish, heavy gloves can be worn to rip out the developing tents with their caterpillars. If spraying is essential, use Bt (Bacillus thuringiensis) as soon as the silk tents are observed in early spring. It is a natural insecticide that only affects caterpillars. Bt is harmless to humans and animals. It must be sprayed on leaves the caterpillars will eat. Bt must be used in April since only young caterpillars are highly susceptible to this insecticide. It is available under many trade names including Thuricide, Dipel, Caterpillar Attack, Biotrol, etc. (read more)