GardenLine | Common Saskatchewan Pests | Lilac Leafminer

Bonnie Willie

The lilac leafminer is an introduced insect that feeds primarily on the leaves of lilac and green ash trees. Privet, a species not commonly grown in Saskatchewan will also play host the lilac leaf miner.

Damage:

The first noticeable sign of damage is a small linear tunnel (referred to as a mine) on the underside of the leaf surface. As the larva grow inside the leaf, light green to brown blotches will appear on the leaves' upper surface. The blotches develop in response to the larva eating the inner layer of the leaf. Gradually the blotches grow larger, turn a darker brown and become brittle. The larva then exits the inner leaf surface, rolls the leaf up and begins to feed on the upper leaf surface within the curled portion.

Description:

The adult lilac leafminer is a slender, greyish-brown moth reaching approximately 6 mm in length having a wing span of about 10 mm. The forewings are covered with irregular yellow patches. The eggs are very tiny and transparent are therefore not usually noticed. Small green larva emerge from the eggs, but as they mature, they change to a pale yellow with a yellow-brown head. At maturity the larva will appear flat and reach approximately 8 mm in length. The larvae will then pupate in the soil within a small whitish cocoon.

Life Cycle:

There are 2 generations of G. syringella per year on the prairies. The insect overwinters in the soil as pupae. The first adult moths emerge in late May to early June. The adults will then mate. After mating, the female will lay eggs in batches of 5-10, along the midrib and on the undersurface of a leaf. Each female will lay approximately 100 eggs. The larvae hatch in about 1 week and they begin tunnelling into the leaf to feed on internal tissue. They feed inside the leaf for a 3 week period before emerging and then curl up the leaf to begin feeding again. They feed here for about 10 days, then drop to the ground on a silken thread. They will then spin a cocoon and pupate under leaf debris.

The second generation of adult moths emerge in early August, mate and lay eggs. The larvae hatch and feed until mid-September. They will then drop the ground, form a cocoon and pupate over the winter months.

Control:

The lilac leafminer is best controlled by simply removing the leaves as soon as damage is noticed. Collect and dispose of leaves and leaf debris in the area. If the infestation is severe, and hand removal impractical, a systemic insecticide such as Cygon (Dimethoate) and be sprayed on to the affected lilacs. Cygon is not registered for use on green ash. When applying any insecticide, always read label directions carefully and use all safety precautions suggested by the manufacturer.

REFERENCES:

Insect Pests of the Prairies, by Hugh Philip and Ernest Mengersen; Alberta Agriculture; 1989.

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Sustainable horticultural information, offered free of charge to the public with the support of the University of Saskatchewan Extension Division, the Department of Plant Sciences and the Provincial Government.