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Thuja mining moth

Thuja mining moth; the caterpillars of this moth eat the chlorophyll from the scales of thujas, making them transparent.

Caterpillar Thuja mining moth, photo: Gyorgy Csoka - CC BY 3.0 us

Thuja mining moth (Argyresthia thuiella). The three-millimeter-long caterpillars of this moth eat the chlorophyll (tunnels) from the scales of Thujas, making them transparent. In case of serious deterioration, a thuja is eaten completely bare.
The Thuja mining moth is native to Canada and North America and has been active in Europe since the 1980s.
The caterpillars hibernate in the Thuja leaf. During the spring, they become active. The caterpillars pupate in May; moths emerge in mid-June, laying eggs in the leaves of Thuja. From the eggs hatch the caterpillars that eat their way from the tops of Thuja leaves to the branches over the course of the summer.

Where to find

Control

Prune away the affected branches.

Prevention

Regular pruning helps prevent infestation by the Thuja mining moth.
There are species of thuja’s that are less susceptible to infestation by the caterpillars of this moth.