Arborvitae leafminer

The arborvitae leafminer (Argyresthia thuiella) belongs to the family Yponomeutidae. It is a significant pest of coniferous trees, particularly the genus Thuja (arborvitae). These small moths can cause substantial aesthetic and physiological damage if their population remains uncontrolled.

The primary host plants include various species of arborvitae, as well as some types of cypress and juniper. The damage is exclusively caused by the larvae, which mine the interior of the scale-like needles, leading to the browning of the foliage and eventual decline of the branch tips.

The life cycle typically consists of one generation per year. Adults emerge in early summer, typically June, and lay eggs on the needles. The larvae then burrow inside the foliage to feed. They remain within the needles throughout the winter, causing significant internal damage before pupating in the spring.

The most visible sign of infestation is the yellowing or browning of foliage tips, often mistaken for drought stress or fungal disease. Close inspection reveals small boreholes and internal hollows within the needles where the larvae have consumed the plant tissue.

Management strategies focus on a combination of cultural and chemical practices. Chemical control involves applying insecticides during the peak adult flight period in June to prevent egg-laying. Additionally, removing infested branches during the dormant season helps to reduce the local population density.