Each spring the adult fly stage of the Elm Leafminer deposits larva into the leaves of our Elm trees. This larva has the potential to devastate the leaf canopy of some of our most statuesque trees.

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.

Signs of Leafminer Activity

• Small whitish spots (haloes) on leaves in May/June
• Blotch-like mines appear in May/June
• Mines join together into tan and papery patches
• Mined areas of leaves may fall out and leave holes in the leaf
• Brownish foliage that looks sickly
• Early leaf drop leaves tree canopies bare and stressed

With such a large number of Siberian and American Elms in the Denver area the population of Elm Leafminer is nearly always growing. The eventual damage is especially difficult on homeowners due to the loss of shade as the leaves quickly fall in early July and August. Control can be difficult to achieve, but more environmentally safe products are showing very good promise for the future.