There are four species of arborvitae leafminers in the Northeast, although the arborvitae leafminer, Argyresthia thuiella, is the most common in New York State. The major food plant is arborvitae (Thuja). The adults are tiny (3/8 inch wingspan) white to light gray moths with brown markings on the forewings.The larvae or caterpillars are 1/8 inch in length with a light green (sometimes with a reddish or yellow tinge) body and a shiny black head (Figure 1.).
Mined leaves have at first a translucent or straw color, later turning brown (Figure 2 & 3.). 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 appearance. 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. The young larvae enter the leaves and mine or excavate between the leaf surfaces (Figure 4.). The larvae overwinter in the mines and resume feeding for a short while the following spring. Pupation occurs in late May and the adults emerge soon after (Figure 5.).
Under forest conditions, leafminers are usually kept in check by natural control factors including tiny hymenopterous (wasp) parasites. Parasites are usually not abundant in specimen trees.
On ornamentals remove and destroy infested leaves from fall to early spring. Treat for larvae with acephate, carbaryl, or malathion: one application in mid-May (150-260 GDD), a second in early August (1800-2200 GDD). Or treat for the moth stage in mid-June (533-700 GDD) with permethrin. GDD = growing degree days.
Pesticide and management recommendations obtained from: Part I Guide to Pest Management Around the Home, Cultural Methods and Part II -- Pest Management Around the Home, 2005-2006 Pesticide Guidelines, Miscellaneous Bulletins 139S74I and 139S74II, Cornell Cooperative Extension Publications.
This publication contains pesticide recommendations. Changes in pesticide regulations occur constantly and human errors are still possible. Some materials mentioned may no longer be available, and some uses may no longer be legal.