There's a weed in our lawn that they like a lot. It's a plantain with narrow leaves. I thought it was plantago lanceolata but I didn't realize there are so many types of plantain. P. virginica and p. purshii also have narrow leaves and are found in Missouri. Whatever it is, it's growing in our hayfield too so there's plenty of it for the buckeyes to eat. You might be able to move them off of your snapdragons onto your weeds. : )
I found this list of plantains we have in Missouri:
 Heartleaf Plantain (Plantago cordata)
 Common Plantain (Plantago major)
 Rugel Plantain (Plantago Rugelii)
 English Plantain (Plantago lanceolata)
 Bracted Plantain (Plantago aristata)
 Salt-and-pepper Plant (Plantago Purshii)
 Hoary Plantain (Plantago virginica)

I've seen more Buckeye butterflies in my yard this summer than I ever have before. Now that my sedum has started blooming they've been flocking to that. This is Matrona.

Here is a link that might be useful: plantago lanceolata

I see them around here so I must have what they eat. Before your photo last summer, I didn't notice them. You are so tolerant to let them eat your snapdragon. I am noticing dragonflies now and they are as beautiful as butterflies. Whenever I get a book that I can follow, you will be getting dragonfly pictures.

Helen, I get a lot of dragonflies here too.