I was wondering if you could help me identify this wasp at all. I'm not sure if they're any experts on American species but it would be great if I could get some help on working it out. I think its a Megarhyssa macrurus but thats an uneducated guess...
Any help you could give would be much appreciated.
According to The Ecology, Behavior, and Biological Control Potential of Hymenopteran Parasitoids of Woodwasps (Hymenoptera: Siricidae) in North America by David R Coyle and Kamal J K Gandhi (pdf here) the only Megarhyssa species in Alaska is Megarhyssa nortoni nortoni. In On Some Genera of Pimpline Ichneoumonidae J H Merrill (text here) writes on pages 132-133 (M lunator = M macrurus):
In M. nortonii the yellow spots on the sides of the abdomen
are rounded-oval, while in the closely related M. lunator and
M. greenei they form angled bands. In M. nortonii there is a
dark stripe extending from the vertex to and including the man-
dibles, in M. lunator there are two dark lines running from the
antennae to the mandibles, and in M. greenei these lines are
absent. M. nortonii can be distinguished from M. mexicana
in that the latter has a pale-yellow ground color, with black
markings on the abdomen. The wings of the latter are clouded
at the tips, and possess no areolet, neither are there any dark
markings on the face. M. nortonii may be distinguished from
M. atrata, humida, canadensis and nitida by the fact that in these
latter forms the greater part of the surface of the body is black,
with white, yellow, or fuscous body markings.
And as your photo shows rounded spots on the side of the abdomen and not angled bands, everything fits!
And also (page 133):
Megarhyssa nortonii is widely distributed throughout the
United States, Canada, and Alaska. Specimens taken from the
Pacific coast ranging from Alaska down through California,
exhibit in general much darker color markings than those taken
east of the Rocky Mountains.
That explains why your individual is quite dark.
There is also a species of Rhyssa in Alaska: Rhyssa alaskensis/persuasoria, but it hasn't the round spots of Megarhyssa nortonii nortonii on the sides.