What is making your arborvitae brown? Bagworm is the likely culprit. Bagworm is the second greatest cause of damage to arborvitae in Massachusetts, outdone only by deer. The bagworm larva starts feeding in June, when it can go unnoticed. Insecticide spraying must be done in June before the larva is too large. This larval stage builds a structure made of arborvitae tissue held together with silk. The silken house camouflages and protects the larva. It is hidden from view and also protected from insecticides.
Bagworm damage can appear rapidly, even though they may have been feeding for months, the damage final reaches a critical mass. In July or August the damage appears overnight. Remove, kill and discard of the nuisance pest., best way place in a pail of soapy water, then a sealed plastic trashbag left on a hot surface. Call for a free inspection if you suspect bagworm on your arborvitae.