Treason applies to non citizens as well.

https://supreme.justia.com/cases/federal/us/83/147/

1. Aliens domiciled in the United States owe a local and temporary allegiance to the government of the United States; they are bound to obey all the laws of the country not immediately relating to citizenship during their residence in it, and are equally amenable with citizens for any infraction of those laws. Those aliens who, being domiciled in the country prior to the rebellion, gave aid and comfort to the rebellion were therefore subject to be prosecuted for violation of the laws of the United States against treason and for giving aid and comfort to the rebellion.