Epargyreus clarus huachuca
'Arizona' Silver-spotted Skipper
(Note: The following assumes huachuca - not californicus - is the correct name for this skipper in the southern parts of the state.)
Flies from May to September, probably in two broods; all my photos of this uncommon butterfly are from mid-May. It ranges from Arizona (and elsewhere in the SW) into Anza-Borrego and just beyond, as at Cactus Spring Trail (where I've seen it a few times). (In the San Bernardino Mountains (e.g. Big Bear) and northwards, we have subspecies californicus.) Its host is legumes, with Amorpha fruticosa and lotus species mentioned by Garth and Tilden (1986; pp.176-77). The Monroe's (2004) Anza-Borrego book lists this butterfly in the "migrant and stray" section (p.105), but I've seen it a few times over the years, with Horsethief Creek being a particularly good spot. (It likes riparian areas and canyons.) I have photographs from hikes in May of 2006, 2012 and 2016. The suspected hosts can be abundant along the trail, but I can't confirm that it's breeding there.
Jim Brock has photos of huachuca larvae on the Butterflies of America website. Refer to my californicus page for life-cycle notes that should apply to this subspecies as well. Evidence that clarus is breeding on Cactus Spring Trail - and isn't just straying some years - may be valuable information. Is it there each year, using the False Indigo as a host? Is there a second brood in late summer? Conditions seem favorable, but I don't know the answers.
When Bruce Dixon named huachuca in a 1955 paper in the Entomological News (66(1):7-9), he pointed particularly to the upperside forewing, where the small golden spot in cell Cu1 in huachuca is separated from the large spot above it in M3, in contrast with most nominate clarus. It would be great to see more of these from Cactus Spring Trail in order to check this diagnostic spot on the upperside forewings and rule out the possibility that these are californicus straying from the north. But we have huachuca in Anza-Borrego, the park border of which is just a few miles to the southeast.