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I would use Bt, it works exceedingly well on chewing larvae. I had this happen years ago to a greenhouse full of ferns. One application of Bt, and the caterpillars were history.
There is a lot of controversy about Bt GMO products we are consuming. Hopefully, you won't be eating the fern. Bt is organic. This article discussed Bt: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/using-bacillus-thuringiensis.htm

You can cut the brown fronds off. This article should also help: https://www.gardeningknowhow.com/plant-problems/environmental/tips-for-saving-cold-damaged-plants.htm

This article should help: https://www.gardeningknowhow.com/plant-problems/pests/animals/get-rid-squirrels.htm

If it's just one or two fronds, it really shouldn't be anything major to worry about. The plant could be going into dormancy, be potbound or may be in need of some fertilizer at half strength. I would suggest keeping an eye on the plant, however, to see that it doesn't spread to any other fronds. You can also remove the yellowing fronds from the plant. Here is more information: https://www.gardeningknowhow.com/houseplants/boston-fern/trimming-boston-ferns.htm

Normally this would be an indication that the plant needs additional humidity. The brown fronds can be removed. Here is more information that may help: https://www.gardeningknowhow.com/houseplants/boston-fern/trimming-boston-ferns.htm, https://www.gardeningknowhow.com/houseplants/hpgen/raise-humidity-for-houseplants.htm

Boston ferns will not usually handle freeze damage well. However, you can put them into dormancy by placing them in a sheltered area ( a cool, dark place) like an attached garage, a closet or a basement. Water only once a month, but water it thoroughly when you do. It will look dead, but in the spring, take it back out and it should regrow fine.