According to both Eichlin & Duckworth (1988) and Engelhardt (1946) Synanthedon chrysidipennis is found at higher altitudes (at or above timberline). This individual was definitely found at an altitude below 700 m in Artemisia tridentata scrubland. The official description for the suggested species does not entirely match the pictured individual, nor the males i collected so far. Forewings and legs are suffused with red-orange scales, not yellow ochre as seen in this image. Anal tuft is pure yellow, not a mixture of black and yellow. There is also no mention of the medio-dorsal yellow stripe on the thorax. The female pictured in Beutenmuller (1901) looks quite different too.

Nevertheless, if this individual turns out to be a yellow, low altitude morph of S. chrysidipennis then that is one exciting find on many levels!!