QUICK TIP: Severe leaf miner damage can be mistaken as signs of Dutch elm disease by an untrained eye.  Be certain the problem is correctly diagnosed before treating!

The elm leaf miner is the larval form of an adult sawfly insect.  The elm leaf miner larvae are about 6 mm in length.  They overwinter in the soil and emerge in late May adults. The adults lay their eggs on the newest leaves.  When the eggs hatch, the young larvae feed on leaf tissue until they’re ready to pupate.  The larvae feed by mining their way between layers of tissue, munching and growing as they go.  The larvae are through feeding in late June or early July, cut a hole out of the leaf and fall to the ground to pupate. The insect goes through one life cycle per season.

Leaf miners are not typically a serious pest of healthy trees, as the infested leaves will be shed and new leaves will take their place. Treat leaf miners with a systemic insecticide such as Merit (active ingredient imidacloprid). This systemic is applied around the base of the tree and absorbed by the roots. When the larvae begin feeding, they ingest the insecticide and die.

Natural predators include ground beetles and braconid wasps. These prey upon on the leaf miner in various stages of its life cycle.   If you would like to attract natural predators, simply introduce plants such as evening primrose, fennel, rue, evergreen euonymus, or Baltic, Boston or English ivy to your yard.

In Minnesota the Siberian, red or slippery elm are most severely affected, although American elms are also attacked. Reduce overall stress on your tree by applying a layer of mulch 2-4 inches deep over the root zone.  Do not pile the mulch material against the trunk as this creates an excellent environment for decay fungi to degrade the trunk.  Mulch such as shredded cypress or cedar can keep the roots cooler and more evenly moist than grass or rock.   Fertilization may also be warranted.  Ask your arborist for suggestions.