I have five large Giant Chain Ferns (3.-4' tall) that look like they are being eaten by some kind of pest. All of the fronds seem to have some damage: some are barely noticeable and others look like the right side of the picture.

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Can anyone tell me what might be causing this? They are growing in partial shade and watered every third day. I gave them Osmocote twice earlier this year. I'm located on the northern California coast where these are native to the Redwoods here. Thanks for your help.