Contact: Eric Day, Manager, Insect Identification Laboratory
ENTOMOLOGY PUBLICATION 444-242, August 1996
Black locust is the preferred host for the larval stage but the
adults can be found feeding on apple, oak, birch, beech, elm,
cherry, and hawthorn. Locust leafminers are known to feed on
soybeans as both larvae and adults when populations are high.
Description of Damage
The primary and most conspicuous damage is caused by the
leafmining activities of the larvae. Larvae feed inside the
leaves on the inner layers. In the beginning the mine is small,
but it is gradually enlarged until most or all of the leaf is
affected. A single leafminer may attack several leaves. The mine
or blister is at first clear, but later turns brown as the leaf
dries out. Leaves damaged by the locust leafminer may turn
entirely brown and cling to the tree for some time, giving the
tree a blighted look. This damage is most noticeable in the late
summer along major interstate highways where it will seem that
miles of trees have brown leaves.
The adult is a small flattened beetle that is about 5-6 mm long.
The head is black and the thorax and the outer margins of the
elytra are orange. It has a prominent black dorsal stripe
running down the middle of the back. The elytra are deeply
pitted and have three longitudinal ridges each. The eggs are
flat, white and oval. They are laid in small clusters of three
to five on the underside of the leaves. The yellowish larvae are
flattened as well.
There are two generations a year in Virginia. Adults overwinter
in bark crevices and leaf litter of black locusts. They start
getting active in late spring about the time leaves start to
unfold. The adults initially feed on the lower surface of
leaves, skeletonizing and chewing small holes. After the eggs
are laid on the new leaf, the larvae feed gregariously in one
leaf, but eventually they create their own mine. They pupate in
the leaf and emerge in July. If the tree has grown new leaves,
the larvae will attack the new set. The second generation adults
will emerge in late summer and seek a site for overwintering.
The skeletonizing by the adults is usually very minor and control
is not warranted. The leafmining is usually only detrimental on
trees that are attacked repeatedly. In addition, black locusts
are rarely grown for ornamental or shade tree purposes and
usually have a low value. If control is desired, the best time
to treat is in late May or early June when the adults are active
and the mines are less than 1/4 inch in length. An insecticide
with some systemic activity will give the best control.
Soybeans rarely have locust leaf-miners reaching economic
thresholds and control is only warranted if defoliation of the
vegetative stages reaches 35 percent or more. Most common
soybean insecticides used at a low rate will give good control.
This beetle used to be considered a sporadic pest that would
cause outbreaks every few years. In recent years outbreaks have
occurred year after year in some locations such as borders of
highways. Low tree mortality is observed in these sites and
trees seem to be able to withstand repeated defoliation.
Prepared by: Eric Day and D. Ames Herbert, Department of
Entomology, Virginia Polytechnic Institute and State University,
Blacksburg, Virginia 24061-0319.