Cedar leafminer: protect your cedar hedge this season
Is your hedge attacked by the cedar leafminer? We offer organic treatment — no pesticides
What is the cedar leafminer?
The cedar leafminer (Argyresthia thuiella) is a small moth whose larvae tunnel into the leaves (twigs) of the cedar tree to feed and overwinter. It completes one life cycle per year.
Symptoms to watch out for:
- Twig tips that turn brown in autumn or early spring
- Foliage that takes on a bronze or rusty hue, from the outside to the inside of the hedge
- Small larvae visible inside severed twigs (greenish-white larvae, brown head)
- Very small adult butterflies (about 5 mm) visible in June when laying eggs
Important: Browning caused by leafminers is similar to that caused by drought or other pests. If you have any doubts about the diagnosis, a virtual consultation with our team can confirm the presence of the leafminer before planning a treatment.
Biological treatment by trichogramma
Trichogramma (Trichogramma minutum) are tiny parasitoid wasps that attack the eggs of the cedar leafminer. They parasitize the eggs even before the larvae hatch — interrupting the cycle of infestation at its source, without chemical pesticides.
Why this treatment:
- Effective at the source — acts on eggs before larvae hatch
- Organic — no chemicals, no risk to pollinators, pets, or children
- Targeted — Trichogramma specifically attacks leafminer's eggs, without disrupting your garden's ecosystem
- Preventive and curative — effective both in protecting a healthy hedge and in stopping an infestation in progress
The application window is short. The treatment should be applied during the leafminer's laying period, usually in June. Outside this window, the treatment is not effective. That's why it's important to plan your treatment now.
Why intervene now
The cedar leafminer is a cumulative pest. The damage of one year weakens the hedge for the following year — a weakened hedge is more vulnerable to other stresses (drought, frost, other pests). A hedge that has been severely damaged over several consecutive years may require complete replacement.
Trichogramma treatment is a modest investment compared to the cost of a hedge replacement. For a 50-linear foot hedge, the treatment costs $135 including shipping.
Acting early in the season — before spawning — is always more effective than intervening after the larvae have caused visible damage.
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FAQCedar leafminer and trichogramma treatment
The first symptoms are browning of the tips of the twigs, usually visible in the fall or early spring. To confirm, open a few brown twigs: if small greenish-white larvae with a brown head are present inside, it is the leafminer. If in doubt, our free virtual consultation helps confirm the diagnosis before investing in treatment.
Yes. Trichogramma are tiny parasitoid wasps that do not sting and only attack the eggs of certain lepidoptera (moths). They have been used in organic farming for decades. There is no risk to humans, pets, plants or pollinating insects.
The treatment must be applied during the leafminer's egg-laying period, usually in June in Quebec. It's a short window — about two to three weeks. Outside this period, the trichogramma does not find eggs to parasitize and the treatment is ineffective. Planning ahead is key.
The cedar leafminer completes one cycle per year. In areas with high infestation pressure, annual treatment is recommended to maintain effective protection. In less affected areas, treatment may be sufficient to reduce the leafminer population to a non-damaging level. Your specific situation can be assessed during the virtual consultation.
No—browned, dead twigs will not turn green. The treatment stops the infestation and protects the new shoots for the coming season. If the hedge has significant damage, parallel fertilization can stimulate the growth of new twigs and help the hedge recover faster.
Yes. In addition to the trichogramma treatment, PURcèdres offers a fertilization service to stimulate recovery after an infestation. If your hedge is too damaged to be saved, we can also evaluate uprooting and replacement options with you. Request a virtual consultation to take stock of the condition of your hedge.
Steps in the treatment of cedar leafminer
A simple, organic and effective process to protect your cedar hedge.
ONLINE QUOTE OR VIRTUAL CONSULTATION
You describe your hedge and its approximate length via our online form. If you have any doubts about the diagnosis — browning, uncertain cause — our team can confirm the presence of the leafminer during a virtual consultation before planning the treatment. We then confirm whether trichogramma treatment is appropriate for your situation and send you your quote.
ORDERING AND DELIVERY
Trichogramma are living organisms packaged on trichocards — small cards containing parasitized eggs ready to hatch. PURcèdres coordinates a one-time delivery in early June. Upon receipt, the trichocards can be kept in the refrigerator until the time of each installation.
INSTALLATION OF TRICHOCARDS
Installation is simple and requires no expertise. Starting in mid-June, you hang a trichocarte every 12 feet along the hedge — and repeat once a week for 4 consecutive weeks. This deployment, which is staggered until mid-July, ensures an active presence of trichogramma throughout the leafminer's egg-laying period. A clear installation guide is included with your order.
HATCHING AND ACTION
Within 4 to 5 days of each set of cards being placed, the trichogramma hatch and disperse in the hedge in search of leafminer eggs. They parasitize the eggs even before the larvae hatch — interrupting the cycle of infestation at its source, without pesticides, without professional on-site intervention. No additional handling is required between installations.
EXAMPLECEDAR HEDGE AFFECTED BY THE LEAFMINER
