You need to find out what animal is doing this. Different animals are managed in different ways, so proper identification is key. If it is squirrel damage, you should be able to see the incisor imprints at the location of the damage. Are there any tracks? Sprinkle some white flour on the ground in an area protected from rain and see what tracks appear.
Why do squirrels chew non-edible things? Squirrels are classified as rodents and have well-developed jaw muscles and chisel-like front teeth that can chew through just about anything. These front teeth are kept sharp by gnawing on hard objects such as woody plant materials, seeds, and nuts. Since rodents' front teeth grow constantly, they must be filed down, or they will grow to the point where the animal cannot eat, and starvation will result.
You may be correct in your suspicions about squirrels being the plastic-chewers in your yard. According to the National Pest Control Association, squirrels are the number one nuisance animal in America.
Do you feed birds? The presence of lots of tall trees, utility poles and lines, and free food from a bird feeder can make a yard very attractive to squirrels. In general, squirrels have two litters of young each year, once in the spring and again later in the summer. You will want to do as much as possible to keep from attracting squirrels to your yard. You could stop feeding the birds, trim your trees to disrupt their treetop inter-yard freeway, consider having baffles put on the utility lines, or get an outdoor dog. You could also try using taste repellents like capsaicin (a hot pepper spray) on the plastic items. There are sticky substances registered for use with squirrels, but they are messy to use.
If the above recommendations don't appeal to you or fit your situation, the only other solution is to get rid of the squirrels either by trapping or shooting. In some communities, the animal control department will help with nuisance squirrels, or you could contact an animal damage control service if you don't want to do the trapping yourself. To learn how to find a qualified animal damage controller, visit Finding a Qualified Wildlife Controller
. Shooting of squirrels is legal only if you live in a rural area, and in most states, you are REQUIRED to get a permit for shooting or trapping.