what is eating the leaves of my sweet potatoes in Zone 4b?

I started them from store-bought in FL - had good success - planted the best in growing medium and carried them in my car driving to Wisconsin. Hardened them off and then only planted the best - when time was fully right - into black grow-bags with well-amended and balanced soil ... about June 20, and no frost since then.
Some pest is eating the leaves to death! Making tiny holes, and some leaves are completely removed. I can't find any bug. I know they don't share bugs, but my Irish potatoes are bug-free and nearby. My whole garden is pest-free except for flea beetles on the brassicas.
I knew it would be a stretch to get sweet taters this far north, but I started them so early and was not expecting a pest in this locale.
Thanks for any advice to newB in Zone 4. AND best regards to all!

Just taking a stab based on (a) being now in Wisconsin, (b) not having the normal insects you would have in Florida, and (c) not seeing any bugs.......by DAY!   Cutworms are known to take whole leaves, but if the leaves were already advanced when the feeding began, they my just cause chew holes in the leaves.    If possible, try going out with a flashlight around midnight or a bit later and inspecting the plants.  A description of the worms can be found here:   http://labs.russell.wisc.edu/vegento/pests/black-cutworm/

Note that they did not mention sweet potato at that weblink, but that's likely because they are infrequently grown this far north.  Other bulletins from the south note that cutworms will eat sweet potato foliage.

Thanks, John. Experienced with cut-worms - don't think here. Actually have good growth from the base and new leaves. Kinda funny ... whatever it is is only choosing random leaves to make LACE, lol. All plants have only one or maybe 2 leaves that are "holey" or missing.
Good idea to look very early or late in the day ... slugs not a problem here, but they do exist.
Best regards to you 2019 season!