The cedar leafminer (Argyresthia spp.) is a small, moth-like insect that poses a significant threat to cedar trees, particularly Eastern white cedar (Thuja occidentalis), across Southern Ontario. This tiny and often overlooked pest can cause widespread browning and dieback of the foliage, reducing the aesthetics and new growth. Eventually, with multiple seasons of infestation, complete fatality of the host is highly likely.
In Southern Ontario, cedar leafminers have become more prominent in recent years due to milder winters and changing weather patterns, which allow more of the insect population to survive and reproduce. This is particularly concerning for municipalities and landowners who rely on cedar for windbreaks, privacy screens, and ornamental landscaping.
Cedar leafminers are most active in late spring to early summer. Adult moths lay eggs on cedar foliage, and the larvae tunnel into the leaves to feed. As they eat their way through the internal leaf tissues, the leaves turn brown and die. Affected branches may look scorched, and in severe infestations, trees may appear entirely discolored. While a single year of infestation rarely kills the host, repeated infestations can severely weaken it and make it more vulnerable to other pests, diseases, and other stressors.
The most proactive practice to help control the cedar leafminer involves the removal of infested branches and/or an appropriate IPM (integrated pest management) program. Pruning and removing infested branches can help reduce populations, as can promoting tree health through proper watering and appropriate spacing to reduce stress. Insecticidal sprays can be effective if applied at the right time. Applications are usually conducted when larvae are active and feeding in late May or early June.
As climate change continues to influence pest populations in Ontario, awareness and early identifications are essential to protecting our cedars from the long-term impacts of the cedar leafminer.
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