Hello and thank you for contacting us!

The most effective time to spray is June.  So you have perfect timing to try and manage the bugs.  At this time the eggs have hatched and it is before the new crop of bagworms have had time to build their protective cases. A good cue to go by is when the catalpa trees and Japanese tree lilacs are blooming.

Typical life cycle starts with male bagworms fly from their cases in September through early October to mate with females in their cases. Then both die off, leaving behind anywhere from 500 to 1,000 eggs per case. Those overwinter on the plants, and the eggs hatch the following May through early June.

Picking as many bags as you can is the best thing you can do. Just don't fall off the ladder.

Parasites often bring an outbreak under control, so this doesn't mean all of your arborvitae are goners. You may want to consider spraying the attacked arbovitae and its close neighbors now and next June with Bt or Spinosad (non-chemical controls) or with such chemical insecticides as permethrin, Sevin or bifenthrin. If you really don't want to risk more damage, spray all your trees   

Or you could just take the inspection approach and check your trees carefully next June to see if any little worms or bags are starting to show up. Then spray accordingly.

After you think the bugs are gone you will probably want to fertilize and keep an eye on the plants   Just don’t fertilize right away   If there is too much damage fertilizer could stress  the plant  

I hope this helps and please let us know if you have any more questions   

Good luck!