Go to the USDA plants ddatabase - http://plants.usda.gov/cgi_bin/topics.cgi?earl=advquery/adv_query.html, ignore the first search-term boxes and scroll down to where you can select your state, then further down where you can enter Viola as a genus. Further down you will find a 'Display report now' button; hit it.
[You could go further down still and specify rhizomatous as 'growth form' to try to restrict it to stemless species, but that gets hits only on Vv. sororia and pubescens; other growth forms don't help either.]

That should give you 23 hits (inlcuding subspecies), which you can work through to reject those which are not blue and not stemless. OK, OK - I'll do it - that gives you:
- V. affinis
- V. cucullata
- V. palmata
- V. pedata
- V. pedatifida
- V. sagittata
- V. sororia
- V. viarum

You can try to shorten that list by including your county in the search criteria as well.

Regarding your fritillaries' larvae, I expect if they have a choice they will not bee too fussy which species they dine on - one violet tastes pretty much like another, and many lepidoptera larvae are not that fussy (though some are very fussy, I know). However, if pushed, I would go for those closest to V. sororia, viz. Vv. affinis, cucullata, palmata.

Here is a link that might be useful: USDA Plants Database advanced query

Thanks Mike! I tried looking up information on that site but wasn't able to get it nearly as narrowed down as you were.

I've only seen V. pedata, V. pedatifida and V. sororia on host lists, of the ones you found.

Most places just list violets as host plants for fritillaries, but James Scott ('Butterflies of N. America') gets more specific. I don't know if it is necessary or not. This is what he says about 3 of the fritillaries where I live:

1 - Meadow Frit. (Boloria bellona)- Viola sororia, pallens

2 - Great Spangled Frit. (Speyeria cybele) - Viola rotundifolia, papilionacea, palustris, adunca var. bellidifolia; assoc. with V. canadensis

3 - Regal Frit. (Speyeria idalia) - Viola pedatifida, papilionacea, lanceolata, pedata

I bought some V. pedata and have what might be Viola sororia. I was looking for v. papilionacea, and found the discussions here about papilionacea/sororia.

I'm just going to hope that I've got what I need at this point. I wouldn't have any idea of where to buy the other kinds anyway.

Hi Trisha,

To generalise: It is rare for field guides and scientific papers to tell you what certain species of butterfly *don't* eat. Having listed what they *do* eat it is all to easy too make the false jump to presuming they are the *only* plants they eat. That applies whether the plants are named at species level or higher. If you do find references that point you to certain species of food plant, as you have, it is obviously worth trying to get those, as you have wisely done. But I think you have a 90% chance you'll be OK with Vv. pedata and sororia for your frits.

By the way, adult females of some species of butterflies 'taste' potential food plants with their feet, to make sure they have selected a suitable one, before laying their eggs. You can sometimes see them doing this - 'pawing' at a leaf with their front legs. I don't reckon I'd be much good at identifying plant species by rubbing my feet on them :) ...Though I might be able to hazard a guess in the case of nettles, poison ivy, thistles, etc., I suppose.

Mike