Could be ground squirrels or rats. For some reason, these critters like the taste of many plastics. I have an extensive irrigation system and the only ones I have had chewed on are the 1/4" soaker lines, not the regular solid ones. I don't know what can be done to protect them. You may have to consider using live traps and then deal with the animal. In my area you aren't allowed to simply transport them somewhere and let them loose, so the alternative that the wildlife folks recommend is to fill a garbage can with water, dump the cage with the animal in the can, and dispose of them that way. You also need to determine whether the species of animal you're dealing with is protected in any way before using this method. Not fun, but you have to decide whether continuing plant losses from destruction of your tubing merits harsh methods.

hot sauce (Capsaicin) might work.

mblan- I tried that years ago by mixing the hot sauce into vaseline (it makes it last longer if you have rain). I used this on my new bamboo shoots which were getting gnawed off each spring by the local squirrel population. It worked for a while but I think they got used to the taste and went back to destroying stuff. However, it would be worth trying on something like tubing since it’s not a food source for the critters. It would be less problematic and less work than my squirrel eradication program ;-)

I’ve had trouble with rodents mostly chewing directly into the drippers, which obviously you won‘t want to cover. I always assumed they were after the water, not the plastic. I have had good success with a combination of frequent liberal use of a product called Repels All, a nontoxic granulated repellent that I sprinkle around the drippers, and also providing a ground level birdbath/watering hole in another part of the yard so they have an easier source of fresh water.

I went with 50 feet of the 1/2 inch diameter version of this wire guard. It easily slips over the existing 1/4 inch tubing. I am having problems with the feeder tubing from the base of the pot up to the emitters inside the container. So this wire guard will easily slip over that section on the side of the pot and provide another layer of protection.