Argyresthia thuiella (Packard) - Larva(e)The adult moths are small (wingspan 5-6mm), silvery tan in appearance, and are active starting approximately mid-June into early July. Eggs are laid on the foliage, from which the larvae soon hatch, and then they chew their way into the leaves. These entrance holes are only noticeable under magnification. Larvae will feed within the foliage until autumn, whereupon they will over-winter within these mines. Feeding will resume in the spring and pupation will begin by late May. Browning of the foliage from all arborvitae leafminer species feeding activity will not appear until late January or February and is therefore often misdiagnosed as being winter injury.

Browned foliage tips that are hollow, contain frass (the dried, brown pellets of the caterpillar’s excrement), and perhaps a living larva are the key telltale signs of leafminer presence. On the other hand, if the brown tips are not hollow, then the cause is due to winter injury, disease or some other factor.