Elm Leafminer

Leafminers are larvae of sawfly wasps that feed in between the epidermal layers of a leaf. As the larvae feed they consume the plant tissue leaving clear mines that, when they die, turn brown and crispy. Usually by the time the damage is observed, the larvae have already left the leaves and are pupating underground. Control for elm leaf miner needs to be applied before the mines develop. Repeated defoliation by leaf miners can impact the health of infected trees.

 
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Elm Leafminer

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The elm leaf miner is a common pest throughout eastern North America.

This insect feeds inside the leaf. The larvae tunnel through the leaf forming blotches and discolouration.  As the miners move to the outer edge of the leaf, the leaf turns brown. The larvae finish feeding in late June or early July and then fall to the ground where they pupate.

Whitish with pale brown heads, the elf leaf miner larvae measure about 6mm in length.  They overwinter in the soil and produces a brown papery cocoon. In the spring, they emerges as an adult sawfly.

Pesticides can control the spread of the elm leaf miner. Injected into the trunk after the leaves are fully formed, the pesticide will repel the larvae for about 2 months.

Natural predators can also help. Ground beetles, braconid wasps and ichneumonids all prey on the leaf miner during various stages of development.  To attract these predators, introduce plant species such as evening primrose, evergreen eunymous, baltic, boston or english ivy, fennel or rue.