Observation: There were thousand of caterpillars on the nettle covering an area of about 100 square feet. It was a mass of moving black swarm eating the plants down. We want to know if this is a potential problem to the local vegetation and if this is a native butterfly or an invasive species. La'ona DeWilde
UAF Cooperative Extension Service Consult: The Milbert's Tortoiseshell (Aglais milberti) butterfly has a range that spreads through all of Alaska, Canada, and most of the eastern and western United States. They are commonly found in wet marsh areas, having two broods from May to October. Laying up to 900 individuals on the underside of host plants leaves, and as larva they would forage on the leaves of the Nettle plant. UAF Cooperative Extension Service, Jessie Moan, IPM and Gino Graziano, Invasive Plants consult were able to determine what the swarm of caterpillar species was foraging in the field. USDA Forest Service, Forest Health Protection, Tricia Wurtz has identified the caterpillars as being a native species to the region.