I'm sorry to hear that you're encountering so much damage this year. The picture you have supplied is great, but I have a few questions. First, can you please measure the width of one of the single tooth-scrapes? That can help us understand the size of the incisors being used in the chewing, and thus help diagnose the species at work. Next, are the lines experiencing damage buried, on the surface, or hanging? Finally, is all the damage mid-line such as in your photo, or are you also having endcap/end-crimp chewing? If you are having line-ends chewed off, is the size and nature of the chewing similar in pattern (tooth scrapes) to what we see in your photo, or different?

Thank you! I'll look forward to hearing more!

Thank you for those followup photos and information. A few of the individual bites are clear enough to see that the animal's incisors are quite small. That suggests to me that some of the damage could be by voles. You mentioned seeing many squirrels however, some caught in the act of chewing. I'm assuming those are ground squirrels, although you have not mentioned tunneling damage by either squirrels or voles.

In less extensive (shorter lines) cases, we'd suggest "armoring" the lines with either metal (1/4 or 1/2" hardware cloth) or with relatively harder or much larger-diameter PVC, for instance. In your case however, especially because you mention that the intensity of the problem has spiked compared to past years, we need to think about 1) reducing populations of the chewers, both through habitat modification to lower the carrying capacity and safety of the area as well as directly via lethal means (traps and potentially toxic bait for ground squirrels, toxic baits in burrows of voles). 2) Is there a way to elevate your system similar to how wine grape growers do so? If you have California ground squirrels (AKA grey diggers), you'd still have the issue of them climbing up -BUT at least you'd be able to target the climbing opportunities (stakes or posts) and have a reasonable chance of excluding their access to the waterlines. The downside of elevated lines, which wine growers will tell you, is that coyotes are attracted to the end caps/crimps and unless those are protected, they'll chew them off which of course creates a whole new set of problems until the break is repaired.

So back to population control: The most effective chemical tools for managing the ground squirrels and voles are restricted-use. If you have a pesticide license through ODA already, then purchasing and applying those products will not be an issue. If you don't have a license, then in the short term it would likely be best to contract with a Wildlife Control Operator or pest company that IS certified and has the burrow-baiter tools, etc. Finally, vole populations are noted for being cyclic, and what you describe sounds like a high-population-peak sort of phase. That doesn't mean that they'll go completely bust and disappear next year, but populations generally build up to a very high-high and then reduce as behaviors and foods limit the numbers.

Those are my thoughts for now - Let me know if it helps to talk through more.