Forest & Grassland Health

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.

Photos

Click on the image for a larger version.

Western tent caterpillar on red alder in Ketchikan, AK

Western tent caterpillar on red alder in Ketchikan, AK. Photo credit: Tom Heutte. 

Group of western tent caterpillars in their tent. Ketchikan, AK

Group of western tent caterpillars in their tent. Ketchikan, AK. Photo credit: Tom Heutte. 

Defoliation and larval tent of western tent caterpillar in Ketchikan, AK

Defoliation of red alder by western tent caterpillar in Ketchikan, AK. Photo credit: Tom Heutte. 

Current Status & Distribution in Alaska (2020 Update)

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.  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.  

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.

More western tent caterpillar images are available at the InsectImages.org.

 

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

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