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.
General Distribution in Alaska: Reported in Southeast from Annette Island, Ketchikan, and Hyder.
Current Status & Distribution in Alaska (2024)
No new activity was observed in 2024. An area in Ketchikan with infested trees was brushed along the highway and may have removed a portion of the population.
Ground Detection Survey Observations: None.
iNaturalist Observations: None.
Defoliation of red alder by western tent caterpillar in Ketchikan, AK.
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. There were additional observations in 2020 of western tent caterpillars in Southeast Alaska on Annette Island and Hyder. The species was identified based on the photographs as western tent caterpillar (confirmation of specimens by a specialist are pending). The brightly colored and gregarious caterpillars are not known to occur in Alaska; however, their native range extends into northern British Columbia. It is likely the establishment in Ketchikan is a result of range expansion however further surveys need to be conducted to confirm.
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. 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 Dr. Elizabeth Graham, be sure to include a location or geotag your photo!
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.
For more information, contact Dr. Elizabeth Graham, Entomologist, U.S. Forest Service, elizabeth.e.graham@usda.gov.