Bagworms, Thyridopteryx ephemeraeformis, are known to feed on more than 120 species of conifers and deciduous trees and shrubs. They most are commonly found on arborvitae and junipers.
At this time the bagworms are mature. The best approach is to handpicked the bags and destroyed from fall through spring, thus removing the eggs.
Bagworms can be controlled by spraying the foliage next spring (June) with insecticides after eggs have hatched and small bags are seen on the trees.
For more information on bagworm -https://extension.entm.purdue.edu/publications/E-27/E-27.html
Is there a specific (window) or time frame in which to spray or is sometime in June good? Earlier the better? Is there an alert I can sign up for? Also should I spray yearly going forward as they are quite bothersome? Thank you very much!
You need to inspect your trees to see if you are finding the small bags on the trees.The best time to apply an insecticide is while the larvae are still small (less than 1/2-inch long). Small larvae are more vulnerable to insecticides, and feeding damage is relatively minor. Earlier is not better. We want to bagworm larvae to be feeding to come in contact with the insecticide.
There isn't really an alert you can sign-up for. Maybe contact your local Extension office to see if they have had reports of bagworm activity in the spring.
I think the best approach to deal with them is to monitor your plants. Clipping off the bags and destroying them when possible. In some years you may not have a problem and insecticide sprays would not be necessary.