There are four species of arborvitae leafminers in the Northeast, although the arborvitae leafminer Argyresthia thuiella is the most common in New York State. Mines start near the end of a branchlet in the scale-like leaves and extend into other branchlets.
Injury begins in the summer and reaches a climax in the fall.
Death of mined branchlets often occurs giving the tree a sickly appear- ance. The greatest injury probably occurs to hedge rows and ornamental plantings. The adults are active from late May to early June. Many tiny glittering moths may call attention to an infestation. After mating, the females deposit eggs on the inner edges of the arborvitae leaves.