Western Tent Caterpillar

Malacosoma californica (Packard)

Host(s) in the Pacific Northwest:

Red alder (Alnus rubra), cottonwoods (Populus spp.), willows (Salix spp.), crabapple (Malus diversifolia), and various fruit trees. Ash (Fraxinus spp.), birch (Betula spp.), hazel (Corylus spp.), hawthorne (Crateagus spp.), and Pacific madrone (Arbutus menziesii) are also reported hosts.

Damage: feeding on leaves, larvae feed as a colony and construct a silken tent in the branches.

Current Status & Distribution in Alaska (2022 Update)

Western tent caterpillars were observed in Hyder by Forest Service staff conducting vegetation surveys along the Salmon River. The caterpillars and their tents were found on willows along the stream bank at multiple locations, indicating the population has been established there for several years. The previously known range of western tent caterpillars extends into northern British Columbia, but with established populations found in Ketchikan, Metlakatla, and Hyder it is possible the range has expanded. Two research grade observations of western tent caterpillar were recorded on iNaturalist.  

Western tent caterpillars were reported in Southeast from Annette Island, Ketchikan, and Hyder in 2020. The brightly colored and gregarious caterpillars are not known to occur in Alaska; however, their native range extends into northern British Columbia. A survey of the Ketchikan road system took place in June 2021 to delineate the distribution of western tent caterpillars. Western tent caterpillars were found in red alder along the bike path for 0.25 miles starting at approximately mile five on the Tongass Highway, known as Mountain Point (Fig_western tent caterpillars). The tents were limited to small diameter, shrubby red alder. Within tree damage severity was low, and the number of infested trees at each point ranged from one to over 16. Reports from the public suggest the population has been established for at least five years.  No other western tent caterpillars were observed outside the main infestation area of Mountain Point and no new observations were reported in iNaturalist.org.  

The forest health protection team is asking members of the public to be on the lookout for the caterpillars or their tents.  Snap a picture and submit them to iNaturalist, this important data will help us determine how extensive the caterpillars are in Southeast Alaska. You can also contact Elizabeth Graham, be sure to include a location or geotag your photo!

Historic Activity in Alaska

Western tent caterpillars were accidentally introduced into Anchorage in 1988 and 2003 on nursery stock and since eradicated. In June 2020, reports from the public came in about defoliation and a large population of caterpillars in tents along the bike path in the Mountain Point area of Ketchikan, AK.  The species was identified based on the photographs as western tent catepillar (confirmation of specimens by a specialist are pending).  Western tent caterpillars are not known to occur in Alaska but are reported in nearby British Columbia.  It is likely the establishment in Ketchikan is a result of range expansion however further surveys need to be conducted to confirm.

Additional Resources

Ciesla, W. M. and Ragenovich, I. R. 2008. Western Tent Caterpillar. Forest Insect & Disease Leaflet 119. USDA Forest Service. Available here.

Pacific Northwest Plant Disease Handbook- Information on Western Tent Caterpillar on Alder species. Website linked here.

Pacific Northwest Moths- Western Tent Caterpillar Moth, Malacosoma californicum (Packard, 1864). Website linked here.

Content prepared by Elizabeth Graham, Forest Health Protection, elizabeth.e.graham@usda.gov.

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