If you have never had these before, it is likely some odd infestation like tent caterpillars. There is little or nothing short of spraying poison all over that will get rid of them. Anything that kills caterpillars is also poisonous to people, and to kill such vast numbers of them will require a lot.
Caterpillar infestations only last a few days, although if these are the kind that we had in this area for a few years, they can eat all of the leaves and grass in the area (and I am talking about several a hundred square miles of area, not just one yard).
The caterpillars move in a "flock" until they get to an area with trees that they like. Then they climb up the trees, eating as they go, until they are big enough to pupate. They spin their cocoons, develop over time into a moth (nothing attractive), all hatch at one time, and then fly off or blow off to another area to lay their eggs.
If an infestation is bad enough (as you describe) to be a nuisance, it is likely too large to really do anything about.
Buy a product called Dipel. It comes in a powder form and isn't harmful to people or pets. Sprinkle a line around your patio, so that when the caterpillars crawl onto your patio they will have to cross the Dipel barrier.
Please identify the caterpillars first. You may have treasures of butterfly young. That would be a sight to see if they emerge as pure beauty. Don't kill butterfly larvae please. They are great pollinators.
Possible someone in the neighborhood has an attractant like milkweed. If is an obnoxious larvae your extension office can not only identify them but advise on the best, safest way to eliminate them.
Some obnoxious infestations are even controlled by arranging with the extension office. In fact, your extension office needs to know of this phenomenon. You do not want to spread poison on your house and contaminate the air, land and water. You and your pets could be affected.
I had the same problem. I took a can of Wasp and Hornet killer, the kind that sprays up to 20 feet, and I sprayed every one of the nest. It killed tham and I haven't had the problem any more. I do keep a can handy just in case.
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