Western tentiform leafminer
Western tentiform leafminer
The western tentiform leafminer became a serious orchard pest
in some regions of Washington in the early 1980s. The use of broad-spectrum
insecticides used for codling moth control (Lorsban and Penncap-M)
resulted in the disruption of biological control of this pest.
Insecticides that could control the leafminer proved to be disruptive
to integrated mite control, that is their use caused increased
problems with spider mites. Research at the Tree Fruit Research
and Extension Center demonstrated that a small parasitic wasp,
Pnigalio flavipes, could in a majority of situations keep leafminer
densities below damaging levels if disruptive insecticides were
The western tentiform
leafminer completes three, and
sometimes four, generations per year.
Life History and Damage
The western tentiform leafminer completes three, and sometimes
four, generations per year. It overwinters as a pupa on the orchard
floor still within a leaf mine or loose in leaf debris from the
previous season. Adult leafminer are present in early spring and
deposit eggs on some of the first apple leaves that open.
be seen from the upper
are two distinct types of leafminer larvae, the sapfeeders and
tissue feeders. The sapfeeder larva bores directly into the leaf
begins feeding. These mines are only visible by examining
the underside of the leaf. Once the larva is half grown it transforms
into a tissue feeder and begins feeding on the cells associated
with the upper leaf surface. At this time the mines are readily
seen. Damage by the leafminer occurs from larvae consuming large
amounts of the active leaf surface, reducing the tree’s
ability to produce nutrients for growth. The feeding damage also
the tree’s ability to regulate water loss so under high
leafminer populations it becomes stressed. In situations where
are out of control it is common to find 6 to 10 mines per leaf
late in the summer.
Mines of sap feeders can
be seen only on the underside
of the leaf.
leafminer larvae are
wedge shaped and legless.
Larvae in the first three instars (stages) are referred to
as sap-feeders. They are highly flattened and feed by shearing
open cells inside the leaf with their sickle-shaped mouthparts
and consume the cell contents. The first sap feeder mines appear
in late April or early May. At first, the mine appears as a thin
line, often following a leaf vein. Later, it forms a blotch visible
only from the underside of the leaf.
leafminer larvae are cylindrical and have legs.
The fourth and fifth instars, the tissue feeders, have a more
normal caterpillar body shape with chewing mouth parts and they
consume the upper part of the leaf just to, but not through,
the epidermis. As a result, the mines can be seen by examining
the upper leaf surface. Tissue feeders appear in early to mid-May.
To accommodate its changing body form, the fourth instar larva
increases the depth of the mine by spinning silken threads and
attaching them to both sides of the mine. As the threads dry
they contract, arching the mine and giving it its characteristic
pupa of the western tentiform leafminer is light tan
when newly formed (bottom) but turns darker before the
The pupa is the stage of western tentiform leafminer
capable of surviving winter. However, in late summer, the leafminer
does not always stop development at the pupal stage. When this
happens adults emerge and another generation is begun, even though
most of it will not reach the pupal stage before the onset of winter.
This unusual biological phenomenon suggests that the leafminer
is not well adapted to our climate and may have originated in one
that was more mild.
Biological Control - Pnigalio flavipes
P. flavipes can
kill more than 90% of the leafminers in a generation.
Pnigalio flavipes (Ashmead), a small
wasp, is the most common parasite attacking the western tentiform
in the Pacific Northwest. Its life history is well synchronized
with that of the leafminer. P. flavipes can kill more
than 90% of the leafminers in a generation. Its activities against
are usually so effective that chemical controls are not needed.
It is an ectoparasite, which means it feeds externally on its host
rather than within the host’s body.
Parasitoid egg laying and
of the parasitoid Pnigalio flavipes are laid inside the
Adult parasitoids emerge in the spring at about
the same time as the leafminer. Female parasitoids search leaves
for mines and attack the larvae. Female parasitoids stings a tissue
feeder larva which causes it to be paralyzed and then lays an egg
in the mine on or near the leafminer larva. Parasitoids are in
the orchard two to three weeks before most tissue feeders and survive
during this period by attacking sap feeders, an activity known
as “host feeding.” When a female parasitoid finds a
sap-feeding mine, it will most often sting the larva, paralyzing
it. As its stinger is withdrawn a material is secreted that forms
a tube connecting the larva to the surface of the leaf. The parasitoid
adult then feeds on the contents of the sap-feeder larva through
this tube, like sucking on a straw.
Pnigalio larva is pictured feeding on
a western tentiform leafminer larva.
When the parasite’s egg hatches, the maggot-like
larva crawls to the paralyzed leafminer and begins feeding. Emerging
adults escape from the mine by chewing a small, circular hole through
the leaf. This characteristic hole can help determine the level
of parasitism in an orchard.
This information was
derived from Orchard Pest Management - A
Resource Book for the Pacific Northwest. Edited
by: Elizabeth H. Beers, Jay F. Brunner, Michael J.
Willett and Geraldine M. Warner. Published by Good
Fruit Grower, Yakima, WA, 1993.
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