The birch leaf miner is the larval form of the black sawfly adult insect. The larvae overwinter in the soil and emerge around late May as black sawfly adults. The adults lay their eggs in the newest leaves of birch. When the eggs hatch 7-10 days later, 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. They then cut a hole out of the leaf, drop to the ground, and turn into adult sawflies after 15-20 days. Although this life cycle is repeated three or four times throughout the growing season, it’s the first or spring infestation which is the most serious.

Birches occur naturally in moist, partly shaded environments. Thus, many birch planted in the urban forest are, by definition, out of their element. So, most birch are already stressed. Leaf miners are not typically a serious pest of healthy birch, as the infested leaves will be shed and new leaves will take their place. However, a “healthy” birch can easily move into the stressed category after a severe attack of leaf mining larvae! To further aggravate the problem, a stressed birch has been shown to be more susceptible to attack by the bronze birch borer. Birch borer infestations are lethal. It is therefore usually recommended to 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.

Most commonly attacked in Minnesota are the white or paper birch, although river birch can also be affected. Reduce overall stress on your birch 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. 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.

Majestic can help you manage your Birch Leaf Miner problem. Call today!