Larch Casebearer

The larch casebearer (Coleophora laricella), a native of Eurasia, was accidentally-introduced into Massachusetts by 1886. It was discovered in the western U.S. in May 1957 by David Fellin and Richard Schmitz near St. Maries, ID. They were students at Oregon State University en-route to Missoula, MT, for summer employment with the Forest Insect Laboratory. On a downgrade south of town, they observed discoloration of western larch foliage and collected some case-bearing caterpillars feeding on the needles.

The numerous native parasites adapted to the casebearer and several European hymenopterous species were released in an effort to control it (photo). However, for the next ten years, the casebearer continued to spread over the range of western larch and every spring the larch forest looked as though it was dying. The phone in my office would ring repeatedly as the public wanted to know what was wrong. At one point, the Forest Service regional office decreed that larch should be discriminated against in favor of propagating other tree species, although no convincing evidence existed that needle mining was having any impact due apparently to inherent characteristics of this deciduous conifer and the nature of the mining itself.

Life cycle:Larvae over-winter on the stems, branches and trunk of the host tree. In the spring, they will crawl to the newly emerging needle tips to feed. Pupation occurs in late-May into early June and the adult moths will be flying by late-June into early July. Eggs are laid on the needles and new caterpillars appear soon thereafter (Approximately 2375 – 2805 GDD). Newly emerged caterpillars make cases for themselves but do not create much injury to the host during that year. Before the needles fall from these deciduous conifer hosts, the larvae will migrate to the woody portions of the plant to over-winter. There is one generation per year. GDD for the adult flight period is approximately: 700-1200.

The caterpillars are very cryptic and are only detectable to the trained eye. New injury in the spring is obvious and should be monitored for and detected prior to it becoming severe. Branches can be lightly shaken for the presence of adult moths during their flight period. Adults are grayish and have a wingspan of only 8 mm.

Damages:Casebearer larvae injure trees by mining within the needles during spring, summer, and fall; but spring feeding on the new foliage causes the most damage. The hollowed-out needles turn yellowish-green; and by early summer, severely defoliated trees take on a reddish-brown cast . From mid-June to mid-September, however, western larch can lose its brownish color and look greener when new shoots elongate or if a tree grows a second crop of needles. Mining in late September may brown the trees again; but by then, the tree has completed its growth, so damage is minor.Larch can withstand repeated defoliations better than most other conifers because it drops its leaves in the fall, refoliates each spring, and can produce two crops of needles during a growing season. After 4 or more years of severe defoliation, however, stressed larch grow shorter needles and fewer second-crop needles per fascicle – that is, fewer needles grow out of one bud. When this happens, instead of feeding on the average 24 needles it usually mines, each larva needs to feed on about 76 needles to complete its development.

Control: Colonies of larvae may be seen on needles from late June through August. Small larvae are cream colored with brown heads. Mature larvae are gray-green along the back and white beneath with shiny, jet black heads. They are wasps about three-fourths inch long. During the winter tough, papery, brown cocoons may be found in the duff. Adult sawflies appear in early spring. They are about three-eighths inch long with a characteristic orange band around the abdomen. Eggs are laid in new shoots causing them to curl.Defoliation is similar to that caused by the larch looper.

Light damage may be confused with larch needle cast, larch casebearer, larch budmoth, or larch needle blight damages. Close examination will reveal distinct differences.Some natural management methods include: Parasites, predators, disease organisms, weather and competition. The aforementioned factors all regulate the abundance of the larch sawfly. No practical silvicultural or chemical measures to manage the sawfly in the forest have been developed. Prospects for management of the sawfly through stand management are limited because the sawfly appears to attack all types and age classes of larch.

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