Pest Alert: Top Kill in Western Larch

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

Between June 2018 and November 2019, forest health specialists in North Idaho received 12 reports of top kill and mortality in western larch that appears to have been caused by a species of moth. Larvae collected from infested western larch were identified using DNA analysis. The closest match was a moth in the family Tortricidae, Cydia rana; a species native to the eastern U.S. The presumption is that the species found in Idaho is Cydia laricana. Cydia laricana is a closely related western species that was described infesting western larch near Missoula, Montana, more than 100 years ago. Unfortunately, no specimens are available for DNA comparison.

Although native, very little is known about the biology of C. laricana. This species has not previously been documented as a mortality agent and is therefore not well-studied. It appears that C. laricana tunnels into the wood to feed, develop, and possibly pupate. Following pupation, adult moths emerge, mate, and lay eggs. Similar moth species usually have a one- or two-year life cycle per generation. In Montana, adult specimens have been collected in May, suggesting that emergence and subsequent attacks on new trees may occur in spring.

While C. laricana has only been found infesting western larch in Idaho, it has been documented in both western larch and Douglas-fir in Montana. Infested western larch in Idaho have been found in both pure and mixed-species stands, sometimes scattered throughout the stand and sometimes found in small patches. Infested trees are between 3-14 inches in diameter and no more than 30 years of age.

The first symptom of attack is yellowing from the top down. The top kill progresses down the stem, sometimes killing the tree. Signs of attack include small canker-like areas (flattened, sunken, loose, cracking bark and viscous sap) on the main trunk and branches, and the presence of frass (excrement from the moth larva).

Management

Insecticides cannot be recommended due to lack of information on the C. laricana lifecycle, which is necessary in order to identify an appropriate application timing. Sanitation by removing and destroying (chipping or burning) infested trees may help reduce populations. Thinning may not be effective at increasing stand resistance, as a number of reports originated from young stands that had recently been thinned.

If you suspect Cydia laricana may be infesting your western larch in Washington State please contact Melissa Fischer, DNR’s Northeast Region Forest Entomologist, at melissa.fischer@dnr.wa.gov

For additional information and pictures, please see the Idaho Department of Lands’ complete factsheet: https://www.idl.idaho.gov/wp-content/uploads/sites/116/2020/02/Cydia-Fact-Sheet_12.6.19_2.pdf