Arborvitae leafminer

French common name: Mineuse du thuya
Scientific name: Argyresthia thuiella (Packard)
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Lepidoptera
Family: Yponomeutidae

Distribution

Quebec, New Brunswick, Ontario, Manitoba, British Columbia, Prince Edward Island

Damage, symptoms and biology

The foliage of cedar becomes gradually discoloured, going from pale yellow to brown, and then the shoots die as a result of larval mining in the needles. The galleries containing the larvae and their frass can be easily detected by shining a light on the shoots or holding an infested shoot up to the sunlight. The first autumn frost accelerates foliage deterioration and the twigs become bare the year following the infestation.

Severe attacks over several consecutive years cause twig death and, occasionally, stem death. Infested trees are usually able to renew their foliage later during the growth season.

Severe attacks over several consecutive years can kill twigs and, occasionally, sterms. Infested trees are usually able to renew their foliage later during the growth season.

After hatching, the young larvae mine into the foliage where they begin their development. This causes the foliar tissues to turn brown. After overwintering, the larvae resume feeding in the spring by extending their mines toward the interior of the crown. They pupate and then change into adults between late May and late June.

Other information

Reported for the first time in Connecticut in 1921, arborvitae leafminer infestations were first recorded in Canada in 1940, in Ontario, with other outbreaks occuring in this province a few years later along with some in the Maritimes. In Quebec, the first outbreak occurred in 1962. A second, more severe infestation lasted from 1967 to 1973, killing a large number of stems. Salvage cuttings had to be carried out in some stands in southern Quebec in order to save the lumber.

To limit the damage to ornamental trees, infested twigs need to be pruned off during the winter or in early spring. If the infestation has been going on for several years, the most damaged stands should be harvested to salvage the wood before secondary pests can move in (xylophagous insects or pathogenic fungi). Wood debris resulting from this operation should be burned.