My Larches Look Sick. What Could it Be?

By Melissa Fischer, Eastern Washington Forest Health Specialist, Washington State Department of Natural Resources, melissa.fischer@dnr.wa.gov

I have noticed quite a bit of damage to western larch foliage this season.

Upon close inspection, I have found that much of it is due to the larch casebearer, an invasive species of moth introduced into the United States in 1886 from Europe. In the larval stage, the larch casebearer damages both our dominant Western larch (Larix occidentalis) and the more eastern and northerly tamarack (Larix laricina) by defoliation.

Lifecycle

The larch casebearer (Coleophora laricella) has one generation a year, with adult moths emerging from the end of May through early July. After mating, the females lay between 50 and 70 eggs singly on larch needles. After the eggs hatch, the larvae bore directly into larch needles.

The larvae will develop through four instars (developmental stages between molts) prior to pupating. They will mine a single needle for about two months, during which time they will develop from the first to second instar.

Once hollowed out, the larvae will make a case from the needle (hence the name “casebearer”) by lining a portion of it with silk and chewing it free from the rest of the needle. The larva will reside within this case through the third instar, feeding from mid-August to late October.

In the fall, larvae leave the foliage before needle shed and attach their cases to twigs, overwintering within the case as third instar larvae. In the spring, the third instar larvae will begin feeding again. They develop into the fourth instar, and then pupate inside their cases around late May.

The cycle begins again when the adults emerge from their pupal cases.

How do I know if its larch casebearer damage?

Casebearer damage to larch foliage can be seen in the early spring. The tops of mined needles will look straw-colored, curl over, and/or look wilted (Image 1). By early summer, the foliage will turn reddish-brown.

By mid-June to mid-September, much of the damage visible in the spring will be concealed by green foliage that appears when new shoots elongate and/or if a second crop of needles develops. Mining in late September may brown the trees again, but by then the tree has completed its growth, so damage is minor.

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Image 1 A. Larch casebearer damage to western larch contrasted with an undamaged tree in the foreground and B. Close-up of larch casebearer damage to foliage of western larch.

The casebearer itself can be seen in the early spring attached to needles within their cases. Later in the spring, pupal cases can be found attached to needles or hanging from the ends of silk off larch trees. This can be quite a spectacular display if the tree has been heavily infested. The cases are straw-colored and less than a quarter of an inch long (Image 2). Many landowners describe the cases as looking similar to grains of rice.

The adults can be found around June and are pretty nondescript, being less than ¼ of an inch long and silvery (Image 3). If you look closely with a hand lens, you may see that the ends of the wings are fringed (Image 4).

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Image 2. Larch casebearer cases attached to western larch needles. The needles in this picture are quite long for western larch, but these particular trees were open-grown and well-watered.

Cases may again be seen from the end of August through to the next season on needles or overwintering on twigs.

The larch casebearer is not the only cause of damage to larch needles. Larch needle blight (Hypodermella laricis, Image 5) and larch needle cast (Meria laricis), both fungi, can cause similar damage. Larch needle blight damages needles in the spring and needle cast affects needles in the summer. Close inspection of the needles themselves will help you determine whether the damage is caused by the casebearer or a fungal infection.

Should I worry?

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Image 3. Larch casebearer adult: side view

Larch casebearer damage can look quite serious, but one year of damage is typically not something you need to worry about. Larch trees are capable of putting out another flush of needles within the same season and, because they are deciduous, they will refoliate the following spring.

In addition, the larch casebearer is especially vulnerable to a suite of parasitoids, especially two European parasitic wasps, Agathis pumila, a braconid, and Chrysocharis laricinellae, a eulophid, that were introduced in the early 1960s as biological control agents. Both parasitoids are well established and very successful at reducing casebearer populations. Studies have shown that either wasp can parasitize over 90 percent of the larch casebearer population in an infested area. Samples I collected this spring in Eastern Washington had the same results.

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Image 4. Larch casebearer adult: Notice the fringe at the end of the wings.

If your larch experience continued heavy defoliation for five or more years, you may begin to see the trees decline. Evidence of decline will begin with branch dieback. After a few years, entire branches may begin dying, followed the next season by epicormic branching along the trunk. Within another one to two years, the tops of the affected trees may die.

Soon after these symptoms appear, tree mortality may occur. Trees weakened by continued defoliation are also susceptible to other insects and disease, such as western larch borer and Armillaria root rot.

There are no known silvicultural controls for larch casebearer. Insecticides over large landscapes are not economically practical, but may be advisable in high-valued stands or individual trees. Typically, natural controls are effective, particularly parasitoids.

In addition to parasitoids, prolonged cold and wet weather in the spring, with frosts after the larvae have emerged, can also cause considerable damage. Droughts that last into the late summer causing needles to dry out and fall off also reduce populations. Needle blight and needle cast also have the capacity to reduce the larvae’s food supply.

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Image 5. A. Larch needle blight damage to western larch and B. Close-up of larch needle blight damage.
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Image 6. Parasitoids of larch casebearer that emerged from samples taken in Eastern Washington, 2019 A. braconid and B. Eulophid.

For more info:

Pederson, L. 2006. Management Guide for Larch Casebearer. Forest Health Protection and State Forestry Organizations. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5187464.pdf

Tunnock, S. and R. B. Ryan. 1985. Larch Casebearer in Western Larch. Forest Insect & Disease Leaflet 96. U.S.D.A. Forest Service. https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev2_043608.pdf