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Photo#31541
Western Tiger Swallowtail Caterpillar? - Papilio rutulus

Western Tiger Swallowtail Caterpillar? - Papilio rutulus
Eugene, Lane County, Oregon, USA
September 12, 2005
Size: 1.25"
My wife found this within a few feet of an Aspen.

Images of this individual: tag all
Western Tiger Swallowtail Caterpillar? - Papilio rutulus Western Tiger Swallowtail Caterpillar? - Papilio rutulus

I think you're correct -
the larva is supposed to resemble that of the eastern tiger (below), plus Populus (including Aspen) is listed as a host plant here.
I suggest you move these images to the guide, as we have no larval images there yet and these would be a helpful addition. We don't get so many submissions from the west coast, as you may have noticed.

 
Pale Swallowtail -- a distinct possibility
I hope I am only making things more interesting by saying that this swallowtail larva could also be that of a Pale Swallowtail (Papilio eurymedon). Western Tiger and Pale Swallowtail larvae are hard to tell apart and their depiction (by actual pictures) in Butterflies of British Columbia makes them look nearly identical. One distinguishing characteristic given is that: "The blue center of the Western Tiger Swallowtail eyespot is about 1mm, that of the Pale Swallowtail is 0.5mm." Also the Pale Swallowtail larva is described as "apple green, except for the whitish underside." The Western Tiger is described and appears as all green. So ... can you measure those blue eyespots? (before they turn completely black as the larva approaches the pupal stage).
Excellent pictures by the way -- I have been hunting for one of these creatures for a while now with no success.

 
Western Tiger Swallowtail vs Pale Swallowtail
Also please check the links below for comparison:
Pale Swallowtail
Western Tiger Swallowtail
Incidentally, this is an excellent photographic reference. Its creator, Dr. Tatum, an astrophysicist, used to be my physics professor at the University of Victoria (but he doesn't know that).
Butterflies and Moths of Southern Vancouver Island

 
Interesting indeed!
I don't have any references for the west coast, I'm afraid. Still, the website says "I know of no way of distinguishing the immature stages of this butterfly [Pale Tiger] from those of P. rutulus, except that the preferred foodplant of eurymedon is Alder" - maybe these images can be left as Western Tiger for now, since they were presumably feeding on Populus (Aspen in this case).

BTW there is one sure way to find out - raise the caterpillar to adulthood - not very hard to do. See forums for some discussions on how to go about it - or see Caterpillars of Eastern Forests.

 
Raising the caterpillar
I already let it go, and I was unable to track it down again. Another clue might be that I have never seen the Pale Swallowtail in my yard, but plenty of western tiger swallowtails.

 
Moving items to the guide
I cannot figure out how to move items to the guide, can you do it if you are only a contributor?

 
I did it for you
Yes, you can move images to the guide. You tag them, then go to the right page (Images) and move tagged images there. You'll get used to it after a while.

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