Birch leaf miner (Fenusa
pusilla) is a common pest of
birch trees in our
area. It invests trees in the early spring and eats
between the layers of the leaf causing them to turn
brown and drop off early.
The main hosts of this
insect are the paper, gray, European white and cutleaf
birches. Black, yellow and river birch seem to be less
susceptible to attack.
leaf miner adults are a black sawfly about 1/8 inch
long. They emerge from the ground in early May when the
leaves are about half to fully expanded. Often, the
adults may be seen hovering around the tree or moving
around on the foliage.
Females insert their eggs
into the upper surface of the leaf. Eggs hatch about 7
to 10 days later and the larvae begin to feed or "mine"
in the tissue between the upper and lower leaf surfaces.
This produces the characteristic blistering or
translucent spots on the leaves.
Initially, larvae feed
singly, but as they develop, nearby mines on the same
leaf may join together. This creates larger hollow areas
in the leaf. The larvae feed for about two weeks and
then chew an exit hole. They drop to the ground and
pupate for several weeks.
A second adult generation
emerges from the ground in late June to early July but
damage is usually restricted to the newly developing
leaves on the top of the tree. A third generation may
occur in late August. Mature larvae spend the winter in
the ground and emerge the following April.
Even with total
defoliation during extremely high populations, the
leaf miner rarely causes the death of birch trees.
However, a heavy attack that causes the loss of 50% or
more of the leaves causes a stress to the tree. Stressed
trees are weakened and are much more susceptible to
attack by the bronze birch borer which may eventually
kill the tree.
Cultural Controls: Keeping birch trees in good
vigor through proper fertilization and watering is the
first line of defense. Birches do not tolerate drought
well so watering during the hot, dry periods of the
summer may be the single most important cultural
The most serious damage is done by the first generation
so this is the time to implement a chemical control
alternative. If the first generation is controlled,
subsequent rounds do little additional damage.
A series of preventative
sprays may be effective for controlling damage on
valuable specimens. About early to mid-May as the new
leaves unfold from the buds, observe the leaves. When
small "mines" begin to appear in the leaves, make an
application of an insecticide labeled for this problem. Two applications
at 10 day intervals are usually sufficient.
There are also systemic
insecticides that may be applied to the root zone or
injected directly into the tree. Check with a commercial
pesticide applicator for these options.
companies may soil inject a product called Merit in the
fall for control of leaf miner.