We have gotten several inquires this year regarding the arborvitae leaf miner, Argyresthia thuiella. This leaf-mining pest is a caterpillar (Lepidoptera) that primarily feeds on arborvitae (Thuja spp.).
The adults are tiny, silver to gray moths with a wingspan of only 1/3 inch. The forewings contain brown and black markings. The legs and abdomen are light brown. The moths can be found flying around in June. The female moths deposit pinkish, scalelike eggs on the tips of plant foliage over a 5-week period. Eggs hatch in 2 to 3 weeks into very small (1/8-inch-long) yellow-green hairy caterpillars or larvae with a reddish tinge and shiny black head. The larvae burrow into the leaf scales and eat the inside of the leaf. Feeding injury starts in the summer and reaches a peak in the fall. Larvae normally tunnel into the growing points, killing affected tips. They start mining from leaf tips and move down toward the base. Their feeding causes the foliage to initially turn yellow and then brown, giving the plant a bleached appearance.
Foliage damaged from arborvitae leaf-miner feeding is very easy to distinguish from healthy foliage. Hold damaged leaves up to the light to see the larvae and frass within the leaf tissue. Although plants generally survive attack, continued feeding by heavy arborvitae leaf-miner populations may kill twigs and branches. The larvae overwinter within the mined leaves and feed for a short period of time in the spring before entering a pupa stage, which lasts approximately 3 to 5 weeks. Adults chew an exit hole from the mined leaves in late spring to early summer. There is one generation per year in Illinois.
Arborvitae leaf miners are attracted to Thuja planted in shady locations, so placing plants in the sun will help alleviate problems. In addition, pruning out heavily infested twigs or branches and placing debris in a sealed bag or other container removes leaf-miner larvae. Arborvitae leaf miner is highly susceptible to natural enemies, such as parasitic wasps (parasitoids), because they cannot escape being located within the plant tissues. However, the natural parasitoid populations may not provide sufficient control. As a result, the use of insecticides may be warranted. Insecticides recommended for controlling arborvitae leaf miner include abamectin (Avid), acephate (Orthene), and chlorpyrifos (Dursban). These materials are able to penetrate through the leaf surface and kill the larvae.