Although there are a few species of cedar leaf miner in Ontario, Argyresthia thuiella is the most common. The major plant food for this pest is cedar (Thuja).
The adults are tiny (3/8 inch wingspan) white to light grey moths with brown markings on the forewings. The larvae or caterpillars are 1/8 inch in length with a light green (sometimes with a reddish or yellow tinge) body and a shiny black head.
The larvae mine out the pith of foliar tips starting in the fall and continuing during the spring. The mined tips turn at first to a translucent or straw colour, later turning brown. The damaged foliage is hollowed out and crumbles easily between your fingers, whereas winter damaged foliage does not. The mines start near the end of a branchlet in the scale-like leaves and extend into other branchlets. Cedar leaf miner damage is denoted by a sharp margin between the healthy green tissue and dead brown tissue. Injury begins in the summer and reaches a climax in the fall. Death of mined branchlets often occurs giving the tree a sickly appearance. The greatest injury probably occurs to hedge rows and ornamental plantings. To find the larvae, gently tear the leaf scale along the margin between the green and brown tissue. Look for a tiny green or yellow larva or pupa in this area in the spring.
The leaf miners seem to attack from the bottom up. This photo shows a severely defoliated cedar.
The adults are active from mid to late June. Many tiny glittering moths may call attention to an infestation. After mating, the females deposit eggs on the inner edges of the cedar leaves. The young larvae enter the leaves and mine or excavate between the leaf surfaces. The larvae overwinter in the mines and resume feeding for a short while the following spring. Pupation occurs in late May and the adults emerge soon after.
Under forest conditions, leaf miners are usually kept in check by natural control factors, including tiny wasp parasites. On ornamentals there are a few options.
- Populations of overwintering larvae may be reduced by shearing the infested tips and then collecting and destroying the tips before June. It may be necessary to spray at times to control an infestation.
- Systemic insecticide can generally not be used any longer.
- One exception to systemic insecticides- on cedars with trunk diameters of 4" or more Acecaps could be used.
- Control of the adults with an application during peak flights may reduce the amount of eggs laid, but since the adults are flying moths, they do not provide an easy target..
- The insect injury is more pronounced in dry years. Deep watering of the cedars is important.
- Fertilizing will also help the damaged plants to recover. Deep root feeding in the fall is recommended.
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