Montana Field Guide

The Larch Casebearer Moth is a small moth introduced from Europe to North America in 1890s. Caterpillars of the species are leaf miners and feed on the needles of both native and introduced larch species. Infestation of a host tree typically leads to death of some of the foliage, and severe infestations may cause reduced growth rates of host trees, but rarely death of the infected individual. Although large outbreaks were once common, biological controls have limited the impact of this species to smaller and more local outbreaks.

The species is present in Montana wherever larch are found, which for the two native species Subalpine Larch (Larix lyallii) and Western larch (L. occidentalis), is Northwest Montana.

Adults are unremarkable small moths around 6 mm in length with no conspicuous markings and narrow wings fringed with slender hair-like scales (Tunnock and Ryan 1985) and may be difficult to distinguish from other moth species based on general morphology and appearance. Adults are present in high densities on host trees in early summer, and groups of small silver colored moths on or around defoliated larch trees are diagnostic of an infestation of this species. The “cases” from which the species derives its common name are easier to identify. These structures are formed by caterpillars to overwinter and then pupate and are often found in groups on needles and twigs of host trees between August and the following June. Cases are less than 0.6 cm in length, initially straw-colored and rectangular becoming light-gray and cylindrical during pupation (Pederson 2010).

Several other pests or pathogens cause superficially similar defoliation and needle color change in larch trees, so these attributes are not diagnostic. Damage caused by the fungal pathogens Larch Needle Cast (Meria laricis), and Larch Needle Blight (Hypodermella laricis) can appear similar to that caused by the Larch Casebearer Moth. Larch Sawfly (Pristiphora erichsonii) can also cause similar damage to needles, but the presence of chewed needles can be used to identify this species (Pederson 2010).

This species is a larch specialist, and is only found in association with these trees. It appears to colonize native and introduced species in the Genus Larix. However, the casebearer moths have not been observed on the Subalpine Larch. It is possible that the elevations this larch species occurs at are above the upper threshold for the moths (Tunnock and Ryan 1985), but it has been suggested that the moth will spread to this species from low elevation stands (Arno and Habeck 1972).

As the species is closely tied to larches, it is found throughout their range in Montana.

The Larch Casebearer Moth is a larch specialist. The caterpillars are leaf-miners and feed on the needles (Tunnock and Ryan 1985).

Across the specie’s life cycle, it is associated with larch host trees. The species produces a single generation each year. In the western US adults emerge beginning in late may through early July. After mating, the female deposits between 50 and 70 eggs on the needles of a host tree. Eggs are placed singly and usually on the underside of the needle. About two weeks after deposition, larvae hatch and burrow directly into the needle on which they were laid. For two months the larvae mine the needle, switching to adjacent needles as each becomes unsuitable. Beginning in August through October these larvae create the characteristic cases from hollowed out needles. The species overwinters in its third instar in a case attached to a stem. As the host tree refoliates in the spring, the larvae emerge and continue feeding. Pupation occurs between May and June (Tunnock and Ryan 1985).

Repeated defoliation by Larch Casebearer Moth larvae can result in the stress of the tree, damage, reduced growth rates, increased susceptibility to other pathogens and in some cases of repeated attacks over years, death of host trees. Given these effects, management actions to control the species have been implemented across its introduced range. Two parasitic wasps, Agathis pumila and Chrysocharis laricinellae have been successfully used as biological control agents to reduce the density of this pest (Tunnock and Ryan 1985).

For information and resources on plant pests and diseases see the USDA's Animal and Plant Health Inspection Service's Plant Pests and Diseases Profiles