Almost seventy species of butterflies have been recorded at Snow Mountain Ranch (SMR), a 2000-acre slab of shrubsteppe with an elevational range of 1000 feet, just west of Yakima in central Washington. Until recently, the Desert Checkerspot, Chlosyne acastus, was not on the list of butterflies recorded at SMR, but since 2020, it has been seen commonly on Washington Fish and Wildlife (WDFW) land immediately west, within yards of SMR property. Some individuals have also been seen within the SMR property. Closer examination of this Desert Checkerspot population revealed that there were two seemingly distinct ‘forms’ on the WDFW land. One, matches all the identification criteria for C. acastus and appears typical for the species (Fig. 1). The other form is darker (Fig. 2), quite distinct from the typical form and is similar (albeit darker) than individuals of a segregate of the Northern Checkerspot (Chlosyne palla) population that is recognized from the eastern end of the Columbia Gorge (type locality, Wasco) (Fig. 3), 75 miles due south from SMR. Both forms occurred in similar numbers during May for the past two years in a restricted area of the numerous shrub-steppe coulees that characterize the WDFW land.

Figure 1. Female (left) and male (right) Desert Checkerspots (Chlosyne acastus) at SMR-WDFW May 2022 (Photos: Tanna Knouse)

Do both Desert and Northern Checkerspots occur in the same habitat at SMR-WDFW? Northern Checkerspots are generally butterflies of montane areas, except for the eastern Columbia Gorge segregate (CGS) that occurs on lower elevation lands (~1500-2500 ft), very similar to the elevation of the area the Northern-type form occupies at SMR-WDFW (~2000 ft). Perhaps the SMR-WDFW population represents a northward extension of range of the CGS segregate of Northern Checkerspots?

Although the coloration and markings of the two Checkerspots at SMR-WDFW strongly suggest the presence of two species, I wanted further evidence that these are indeed two species. Especially since Jon Pelham was skeptical! So, I captured a female of the putative Northern Checkerspot in late May 2022 to try and get her to lay eggs so I could see and rear the caterpillars. Initially, I confined her with Rabbitbrush (Ericameria nauseosus) a primary host plant of Desert Checkerspot. After a week, no eggs had been laid, so I removed the Rabbitbrush and gave her Showy Fleabane (Erigeron speciosus), a known larval host of Northern Checkerspots. Within hours, she laid eggs! I reared ~20 larvae that diapaused as third instar larvae in July and then overwintered. Nine larvae resumed feeding on Erigeron (also the ornamental Aster, Symphyotrichum novae-angliae) in mid-March producing final instar larvae and pupae by early April (Fig. 4). My caterpillars had prominent orange-red spots dorsally which is characteristic of some populations of Northern Checkerspots, while mature larvae of Desert Checkerspots sometimes lack these. However, considerable variation occurs in larvae of Northern and Desert Checkerspots and as Dave Nunnallee and I noted in ‘Life Histories of Cascadia Butterflies’, it may not be possible to reliably differentiate the larvae and pupae of these two species. However, the fact that my female putative Northern Checkerspot would only lay eggs on Asters and not Rabbitbrush and the larvae fed on Asters, is a little bit of evidence, I think, that the ‘mystery’ checkerspot is in fact the CGS of Northern Checkerspot.

Figure 2. Putative Northern Checkerspots (Chlosyne palla) female (left) and male (right) at SMR-WDFW, May 2022 (Photos: DG James left and Tanna Knouse right)

I hope to rear these checkerspots again, this time rearing Desert Checkerspots alongside ‘Northern Checkerspots’, to confirm differences in host plant choice and utilization. In a further twist to this mystery, recently published genomic studies suggest that the Desert Checkerspot is actually a subspecies of the Northern Checkerspot, rather than its own species! If this is the case (and there is skepticism about this), then we would have a situation of two subspecies of the same species living in the same habitat with the same seasonal history, but utilizing different host plants. I’m not sure if this would be unprecedented but I think it would certainly be rare.

Figure 3. Eastern Columbia Gorge segregate of Northern Checkerspot (Chlosyne palla) female (right) and male (left). Taken from butterfliesofamerica.com. Photos by Kim Davis and Mike Stangeland.

Figure 4. Final instar larva (left) and pupa (right) of putative Northern Checkerspot (Chlosyne palla) from SMR- WDFW (Photos: DG James)

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