Texas has these critters bad. I HATE THEM!!!!! If you live in the city they don't bug you too much but if you are out in the country, they come in herds of sucking waves. There are several different types of them. I resort to poison. The guy at the Living desert uses s Dawn dish soap in water. He puts 3/4 gallon in a sprayer and then squirts the soap in . He squeezes and counts to twenty. I have not tried this yet. I did not see any of those hickey marks on his cactus out in the backyard of his house. I use Dursban and soap ( I will run out of that bottle one of these years), but I am not eating my cactus. I have squirted them with raid. I hate them so bad. Do NOT do this on cactus other than Opuntias. Raid can cause bad tissue damage and kill a ferocactus. A friend suggested this as an option. I no longer follow that advise. I look for their eggs. They like to hang them in a row off the long needles. I feel so accomplished when I find them and strip them off the needle.

A couple weeks ago, I had, of all things, pill bugs eating the new growth on some smaller Echinopsis. Lucky, I caught on before they did too much damage. They only took out the growth points on two small offsets. Sevin dust dispatched them with haste. If you see these guys around, they will definitely munch on new growth if they have a chance. Here is a link that might be useful: Pics of pill bugs

Horses do graze out of boredom. They sell things that make it so your horse can only get a tiny bit of grass past the mouth cover for people whose horses boredom eat themselves obese on grass, because those horses couldn't be put into a grassy paddock otherwise. My guess is that if the horse has nothing to naturally graze off the ground because it's summer in Cali and the grass is all browned, the horse just found something else to carry out it's natural behavior on. We read posts on the internet, or watch movies, when we're bored. Horses can't do that, so this one probably is finding entertainment by trying out cacti.

yes, no, sometimes. You could do a couple of things, how attached are you to this cactus? If it were mine, I would un-pot, allow to dry out a bit and re-pot in better draining potting medium. Though I don't grow cacti per se, my succulents are of the non-spiny variety & I prefer straight up pumice without any additives in my little corner of the world. =) edited to add: I have killed plants in the past in 24 hours with over-watering or allowing plants to get rained on =/ Now I keep everything under the eaves of my house.