Take a tip from you hydrangeas--when they start to bloom, it's time to spray for a troublesome little pest called the bagworm.
They don't bother hydrangeas so much, though. They stick to arbor vitae and plants with that type of foliage.
Bagworms eat the arbor vitae and form their cocoon from the plant material.
Timing is everything if you have an arbor vitae! Sooner is better to act.