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Arborvitae Leafminers
Argyresthia sp.

Description
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.

Injury
Mined leaves have at first a translucent or straw color, later turning brown. 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.

Life History
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. 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.

Management
Under forest conditions, leafminers are usually kept in check by natural control factors including tiny hymenopterous (wasp) parasites. On ornamentals it may be necessary to spray at times to control an infestation. Either the insecticide ace- phate, carbaryl or diazinon may be used for control. One application should be made in early June. A second application in July when adults are active (tiny light-colored moths flying close to the shrub or tree often seen best when foliage is disturbed); or 7 to 10 days after adult activity is observed. Follow manufacturer's directions carefully when using any pesticide.

Some control may also be obtained by cutting off infested, discolored tips in the fall and destroying them.

5/72 Prepared by: Carolyn Klass
1/90 Slightly Revised Senior Extension Associate
Department of Entomology
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. All pesticides distribut- ed, sold or applied in New York State must be registered with the New York State Department of Environmental Conservation (DEC). Questions concerning the legality and/or registration status for pesticide use in New York State should be directed to the appropriate Cornell Cooperative Extension specialist or your regional DEC office. Read the label before applying any pesticide.