Are your elm trees looking a little sad this summer? You’re not alone. Across the Denver area this time of year, many an elm (American and Siberian) are displaying yellowing and brown, blotchy leaves. While it’s not an encouraging sight, fear not because it is not necessarily fatal. These symptoms are commonly a result of elm leaf miner, or Kaliofenusa ulmi if you want to impress your friends. In its adult form, the miner is a stout black wasp but the brown or yellow serpentine and then blotchy patches seen on leaves are a result of the larvae.
The miner spends the winter as a full grown larvae beneath the soil of a previously infested elm tree and adults form in late winter. Once new leaves emerge on elms in the spring, the adult females lay eggs by inserting them into the middle of the leaves. A keen eye can detect this by noting small, white spots present on leaves in May. Once hatched, the larvae develop, causing the serpentine mines between the surface layers of the leaves that eventually bleed into one another to cause the blotchy effect. If you happen to pick up one of these leaves off of the ground in early- mid summer, notice that these blotches are full of dark powdery substance. This is frass produced by the miner larvae as they feed. Once mature, they’ll cut through these papery blotches, dropping to the ground to tunnel and form cocoons to repeat the life cycle all over again.
So, what can be done? There are natural and chemical treatments. If you want to let nature take its course, birds prey on the miner once they have dropped out of the leaves and into the ground. However, this means the miner has already gone through the life stage causing the most aesthetic damage to the leaves. We favor systemic insecticides or those that are applied to the roots and transported throughout the entire tree via the cambium. Spraying the leaves directly is an option, but some risks to consider are pesticide drift from spraying high into the air, especially if the tree is located near a school, water or public space. Applying a systemic pesticide in the spring or fall will ensure the miner is poisoned at the time of either feeding in the leaves or before emergence in the spring.
Some things to do in the meantime, since we are in the midst of summer, is to ensure proper watering, mulching and a deep root fertilizer application by a professional to encourage the remaining leaves to retain vigor and thrive throughout the rest of the summer. If you’re not sure what your elm tree is displaying, contact us for a diagnosis.
