Infestation of an unknown species of caterpillar.
Bernice Nicholi writes,
Big bunches of unknown species of caterpillar infestation, would like to also know if they are a native species to Alaska. If so, why so many?
The caterpillars posted here look similar to a June 2014 observation out of Chalkyitsik, about 550 miles to ENE of Grayling. There thousands of caterpillars covering an area of about 100 square feet, all clinging to the leaves of the nettle. This observation has been shared with the UAF Cooperative Extension Service program and US Forest Service.
Review the Butterflies and Moths of North America website, a Aglais milberti observation dated August 13, 2016, was verified on April 29, 2017 by Norbert Kondla of a sighting of Milbert's Tortoiseshell in St. Mary's, Alaska, just to the south and west of Grayling on the Yukon River. M. Tcheripanoff
iNaturalist – The species of butterfly, "Milbert's tortoiseshell, range includes all of Canada and Alaska south of the tundra, all of the western United States and most of the eastern United States." Checkout iNaturalist's, Recent Observations mapping tool on there website of observations of Aglais milberti ranging from as far as the eastern coast of Canada to the Pacific Northwest on up to Alaska.
BugGuide – Species Aglais milberti - Milbert's Tortoiseshell - Hodges#4433 | Iowa State University, Department of Entomology