Thanks for your question.

I am having difficulty in identifying this caterpillar due to a lack of clarity and resolution in the picture you sent.  Unfortunately until I can make an identification, I cannot really provide you intelligent information on remedial procedures you might take.  Please send me some additional pictures that possess a higher level of clarity with better focus..

Many thanks.

Thanks for your response with newer pictures. I believe that I can now help you.

In the upper left picture, there are two adult Sumac Flea Beetles (Blepharida rohls) engaged in the process of making baby flea beetles. Take a look at the following:

http://www.minnesotaseasons.com/Insects/sumac_flea_beetle.html

https://wiki.bugwood.org/HPIPM:Sumac_Flea_Beetles

https://bugguide.net/node/view/2284701/bgimage

The two bottom pictures show larvae of Sumac Flea Beetles. These larvae are largely responsible for the destruction of the leaves though adult beetles also contribute. Look closely at these larvae. At their posterior ends, you can see three yellow-white stripes. In between the stripes is blue-grey coloration. Now look at what appears to be the heads of the larvae. If you look very closely and perhaps even enlarge things, you can observe black protuberances. These protuberances are a very interesting feature and constitute what is called a “shield defense”. This shield defense consists of frass, which is a polite term for “poop”. The larvae cover themselves with their own feces so they become undesirable as food for predators. Perhaps more than you wanted to know! See:

https://wiki.bugwood.org/HPIPM:Sumac_Flea_Beetles - :~:text=Sumac flea beetle larvae protect,and many other potential predators.

https://fmr.org/naturenotes/unassuming-little-beetles-shield-defense

http://springfieldmn.blogspot.com/2013/10/sumac-flea-beetle.html

https://extension.usu.edu/pests/ipm/ornamental-pest-guide/arthopods/leaf-beetles-weevils/sumac-flea-beetle

Now the good news is that these beetles are nearing the end of their life cycle. There should be relatively fewer and fewer larvae present, which means that most of the leaf destruction is over. Not to worry about your sumac. It should survive into 2024 provided that you water it continuously this fall up until the ground freezes. The existing larvae will soon develop into adults. The adults will then winter as such until next spring (2024) when they will emerge, mate, lay eggs, and then the fun starts all over again. Right now, your best strategy is to eliminate as many larvae and adults as possible to minimize the number who might otherwise over winter. Look into using “sticky cards”. It will also be extremely important that all leave and plant debris from around your sumac and ferns are removed and disposed of in trash. Do NOT compost it. This will eliminating over-wintering sites for adult beetles. The following will give you some information on these and other points:

https://extension.sdstate.edu/dealing-flea-beetles - :~:text=To manage adults, place yellow,to provide consistent population management.

https://extension.umn.edu/yard-and-garden-insects/flea-beetles - using-insecticides-3089113

https://www.gardentech.com/blog/pest-id-and-prevention/stop-flea-beetles-from-feasting-on-your-garden

https://getbusygardening.com/controlling-flea-beetles/

Good Luck; Have Fun; please get back to us with any further, related questions.

Glad to have been of assistance.  Good Luck!!  Thanks for consulting with us.