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Western tentiform leafminer

Phyllonorycter elmaella
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 not used.


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.

Tissue feeding mines can
be seen from the upper
leaf surface.

There are two distinct types of leafminer larvae, the sapfeeders and tissue feeders. The sapfeeder larva bores directly into the leaf and 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 reduces the tree’s ability to regulate water loss so under high leafminer populations it becomes stressed. In situations where leafminers 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.

Sap feeder

Sap-feeding 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.

Tissue-feeding leafminer larvae are cylindrical and have legs.

Tissue feeder

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 tentiform appearance.

Pupa

The pupa of the western tentiform leafminer is light tan when newly formed (bottom) but turns darker before the adult emerges.

 

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 leafminer 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 the leafminer 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 host feeding

Eggs of the parasitoid Pnigalio flavipes are laid inside the
leafminer mine.

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.

Parasitoid larva

A 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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