The cycle begins when the adults emerge from the ground in mid-May, after overwintering as pupae. The females lay eggs in the young leaves of the tree. After the eggs hatch, the larvae feed for about two weeks. They then drop to the ground to pupate and a second generation of adults emerge in mid-June. Eggs are again laid in young leaves and the larvae feed for about two weeks. This is the generation that will overwinter in the ground as pupae.

The feeding damage is usually not noticed until the leaves begin to turn brown. By this time, it is too late to control the insects, as they have usually already stopped feeding and have dropped out of the trees.

Controlling Birch leafminer is not always necessary for the health of the tree. Most trees can easily tolerate having 30% of the canopy affected by the insect. Vigorous trees, that are well watered, can even tolerate up to 60% damage. Healthy trees also resist miner damage.

To protect the appearance and/or health of the trees that have been heavily infested in previous years, it is important to accurately time the treatment to coincide with the onset of feeding by the larvae. Early feeding signs appear as light green or whitish halos on the leaf in mid-May, usually around the 15th. It can happen earlier or later, depending on how quickly the Spring has warmed up. A systemic insecticide, like Orthene or Lindane, applied to the foliage is an effective control. It is generally not necessary to control the second generation of leafminers, as their damage to the tree is minimal.

Be sure to read and follow all label instructions when applying an insecticide.

For further information, read online our care sheet on "Insecticide Basics", and "Chemical Basics".

Revised: 03/10/98