Moved from Trichiosoma.

These images agree with Norton's original description of the species (p. 44), which notes the long and dense setae on the thorax, lack of any testaceous color to the antennae, and darker wings (black-yellow rather than testaceous - this seems to be particularly notable along the costal margin).

Moved from Cimbicid Sawflies.

A friend posted a photo of what is, I'm reasonably confident, this same species from Truckee a week ago. With those mandibles, I think male Trichiosoma is a great bet, but I'm not familiar with Cimbicids. I've read what little I could easily find, and it seems like these usually go after hardwoods, with triangulum known to use Prunus. As my friend's was crawling around on her porch, I'm hoping that the hostplant is somewhat nearby in her or her neighbors' yards. I'd be curious if your site had much in the way of, say Sorbus, Prunus. For what it's worth, she said hers was under Salix, but if that's the host, that hardly narrows down the search!