My Boxwood’s leaves appear to have blisters on them. Is this
anything to be concerned about with my Boxwood?
Blisters on boxwood leaves are characteristic of boxwood leafminer (Monarthropalpus flavus) damage. These insects
overwinter as partially grown larvae in the leaves of
susceptible boxwoods (Buxus spp.). It is common for more than
one larva to occupy the same mine. Littleleaf boxwood (Buxus
microphylla) and common boxwood (Buxus sempervirens) are both
susceptible to boxwood leafminer, but individual cultivars of
both species exhibit resistance to this pest.
As weather warms in spring, the pale yellow larvae feed and grow
rapidly inside the leaves. They pupate in late April or early
May, taking on a bright orange color. Adult leafminer flies
emerge from infested leaves over a two week period as boxwoods
begin to put on new growth, just about the time weigela begins
to bloom. Females begin laying eggs in the upper leaf surface of
the new growth soon after emergence, inserting them deep into
the leaf tissue. She dies soon after laying the last of her
eggs. The eggs hatch in about three weeks, and the larvae begin
feeding inside the leaves. We have a single generation of
boxwood leafminer in the Pittsburgh area annually.
Boxwood leaves are also affected by psyllids.
A heavily infested plant may appear sparse and unthrifty. While
these insects are unlikely to kill your boxwood, they can reduce
its ornamental appeal. Infested leaves generally are smaller,
exhibit poor color, and drop faster than healthy ones.
should not be confused with another Boxwood pest in the
photo above: Psyllids
Control options include growing varieties of boxwood
reported to be resistant to boxwood leafminer. You can also
prune boxwoods prior to adult emergence or after egg laying to
mechanically remove the larvae and eggs. Insecticide
applications made in May while weigela is in bloom may reduce
the population of boxwood leafminers for next year.
Insecticides labeled to control boxwood leafminer include
Sevin (carbaryl), malathion, pyrethrins and piperonyl butoxide,
and Montgomery Garden Insect Spray or Captain Jack’s Deadbug
Brew (spinosad). Bayer Advanced Tree and Shrub Insect Control (imidacloprid)
is a systemic product that will kill the small larvae while they
are inside the leaves. It is best applied while the mines and
larvae are very small, sometime in June.