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There are four species of arborvitae leafminers in the Northeast, although Argyresthia thuiella is the most common. Mined arborvitae (Thuja spp.) leaves first exhibit a translucent or straw color which later turns to brown. Mines start in the scale-like leaves near the end of a branchlet, extending rapidly into other branchlets. Injury begins in the summer and reaches a climax in the fall. The death of the mined branchlets gives the tree a sickly appearance. The greatest injury probably occurs to hedge rows and ornamental plantings. Description: The adults are white to light gray moths with a wingspan of 10 mm (1/8 inch) and brown markings on the forewings. The larvae are 3 mm (1/8 inch) long caterpillars with a light green (sometimes with a reddish or yellow tinge) body and a shiny black head.
Life Cycle: 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 briefly the following spring. Pupation occurs in late May and the adults emerge soon after.
Control: Under forest conditions, leafminers are usually kept in check by natural control factors including tiny hymenopterous (wasp) parasites. Cultural: Some control may also be obtained by cutting off infested, discolored tips in the fall and destroying them. Chemical: It may be necessary to spray ornamentals in order to control an infestation. Check for currently registered insecticides. One application should be made in early June, with a second application in July when adults are active or 7 to 10 days after adult activity is observed. Follow manufacturer's directions carefully when using any pesticide.
Adapted from the Cornell Cooperative Extension , 1999
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