The Arborvitae Leafminers (Argyresthia thuiella) is a small moth whose larvae burrow tunnels inside the leaves (twigs) of thuja trees to feed and overwinter. Since early spring, the damage caused by the thuja leaf miner has become increasingly visible in the town of Pincourt.

This insect primarily attacks cedars (Thuya occidentalis). Discoloration, browning, or drying out of the tips of branches are often observed—typical signs of its activity.

• Preventive Measures

  • Balanced fertilization — Keeping cedars healthy and vigorous makes them more resistant to attacks.

  • Watering during drought — A stressed tree is more vulnerable to insects.

  • Avoid light sources near hedges — Adult moths are attracted to light and may lay their eggs there.

  • Encourage natural predators — Parasitoid wasps, ladybugs, and praying mantises help control populations. Encourage their presence by growing a wide variety of plants.

• Physical and Biological Control

  • Prune in early spring — Cut off and discard yellowed or dried-out tips to remove caterpillars before they develop.

  • Prune during the summer — Keep the hedge dense and compact while removing new infested shoots.

  • Dispose of infested debris — Never compost contaminated plant material; throw it away with the trash.

  • Use insecticidal soap — This can reduce the number of adult insects, but it does not eliminate eggs or larvae that are already present