I am fairly sure that this is the caterpillar of the Bedstraw Hawk Moth (Hyles gallii), a species which is found across Canada, the USA, Mexico and also throughout Europe and Asia. It feeds mostly on Fireweed (Epilobium angustifolium or E. montanum), and Bedstraw (Galium verum or G. mollugo) but also on a number of other plants. This one is on the leaf of a Fireweed plant – the territorial plant of the Yukon and one of the most abundant.
The caterpillars of this species have a number of different ‘colour morphs’ including green and black, which can make them difficult to distinguish from some of the other, more reliable members of the Hyles genus.
‘The Illustrated Natural History of British Moths’ by Edward Newman gives a far better description of this variation, which is as follows:
“The caterpillar is smooth, bluish-green above, inclining to pink beneath, sometimes brown and sometimes black, but always having a pale, almost yellow, line down the middle of the back, and a row of ten conspicuous eye-like yellow spots, on each side; the head is green, brown, or black, according to the colour of the caterpillar, but the horn above the tail is invariably red.”