Help! We Have Leaf Miners In Our Birch Tree. I…
Holly Asked
HELP! We have leaf miners in our birch tree. I think the damage has been done too this late in the game/season, but is there anything I can do to help and to prevent the return year after year? The leaves are already mined out and the tree seems to be okay, but it looks awful. There are a few untouched leaves and then mined leaves are still on the tree. What do you suggest?
The Gardener’s Answer
Hi, Holly: The birch leaf miner, Fenusa pusilla, is a sawfly that can be very destructive to susceptible birch trees. These insects are native to Europe but very well established here in the states, specifically the northeastern states. Understanding their life cycle is important in terms of control. During the winter months they survive as pupas a couple of inches below the soil wrapped in a cocoon. When spring arrives the small black fly-like adults emerge from the soil, mate, and the female lays her eggs inside the new foliage. The eggs hatch anywhere from seven to 10 days and the larvae begin to feed. At this time the small, white, flattened larvae are visible within the foliage. They feed for approximately two to three weeks and then, cutting holes in the leaves, they drop to the ground and the cycle continues. There are two generations per year but the first generation causes the most damage because it attacks the new, tender spring growth. The second generation emerges later in the summer. There are a couple effective means of control when it comes to leaf miners, the first being a foliar spray such as Sevin. This is feasible for smaller trees and should be applied around the first of May and then again 10-14 days later. To control the second generation spray again at the beginning of July. If the tree is larger a granular/liquid systemic is going to be the most effective means of control. Systemics works internally through the root system of the tree. It will take a bit longer to work as opposed to the foliar sprays because it takes time for the chemical to be absorbed up through the root system and into the foliage of the tree. Systemics should be applied late April and then again late June. Good cultural practices are important as well; make sure all fallen plant debris is removed from the area. It can take a couple of years and several applications to get this under control.
