Unfortunately, chewing damage pretty much looks the same.  Insects that feed on fern fronds include: sawfly and Lepidoptera larvae, cutworms, beetles, crickets and grasshoppers.  At this time of the year, it is more likely to be sawfly or Lepidoptera larvae than crickets or grasshoppers.

It is important to determine if damage is happening currently.  There is a possibility that the damage is complete.  None of these insects feed for the entire season.  Some of these insects only feed at night so you might have to go out after dark.

If it turns out that you can find larvae, you may be able to use insecticidal soap.  This is a product that you would buy, not mix up.  But read the label to make sure you can use it on ferns.  Then test it in a small area and look for damage after 24 hours.  Ferns are very sensitive and many pesticides can cause more damage than the insects.  

Thanks Gretchen. Do you think the larvae eggs were laid in the soil under the ferns this spring or last fall? Would it help to remove the top layer of soil and replace it next spring, since this is the second year in a row I've had this problem? Or were the eggs laid on the fern fronds themselves? I have many other ferns (same variety) in different locations around the yard that do not have this problem. If I dug up the nibbled ferns this fall (or now?) and replaced them with healthy ones next spring and replaced the soil, would that remove the possible eggs that might be laid? I did notice some ants on the fronds and large spiders with red bodies. There is one on the fern pic I sent to you.

Some cute little insect laid her eggs on the ferns this spring. Probably more than one, by the amount of damage.  Removing soil won't change anything. 

Whoever your chewers are, they prefer this fern.  Liking one does not necessarily translate into liking others.  Removing ferns does not solve anything.  Begin monitoring the ferns about two or three weeks before you saw the first damage.  As soon as you see your eaters, spray them with a pesticide and end their feeding. If you stay with your plan, you will lose plants and not solve much.  Ants and spiders are part of an outdoor habitat.  They are probably not contributing to or solving the fern problem.

Thanks Gretchen, I'll put a note on my calendar next spring. 

Good luck!  Wet springs mean more and hungrier munchers.