First ID them, two find their nest and try to destroy it. I actually net them and stump them. I'm finding the European Paper Wasp is a big pest this year and last year, too.

Here is a link that might be useful: Species Polistes dominula - European Paper Wasp

I feel your pain!!!! I thought I was being smart and netted one of my large milkweeds. The cats din't much like the netting and they started crawling on the netting and support wires. I went out the other day to check the plants and I'll be danged if a wasp hadn't managed to eat one of the cats through the top of the netting. I shooed it away but the cat was already half eaten. Then I found that some of the little devels had managed to squeeze under the betting. when I opened the netting to try to get it out, more wasp flew in. In frustration, I just pulled the netting off and gave up on that idea. Now I have three small plants (3 footers) in pots so I just placed them out next to the other plants in the garden. Sure enough along came a Monarch and started laying eggs. I watched the plants closely for a couple days and when the cats hatched out I put the plants inside my small greenhouse. So far I have about 20 cats that I have managed to save. Just to be on the safe side I netted one of the pots that has ten cats on it and put an elastic band around the netting on the pot. The other two plants have cats but they are still small yet. I'm not too woried about any wasp getting in the greenhouse, but they can come in through the exhaust vent if they really want to. I'm going to get my husband to net the exhaust vent this weekend just to be on the safe side. There are four fans in there for air movement and a water wall cooling pad that so the wasp would really have to be determined to get in through that water wall...

heykid

Runmede, you got it. That's exactly what they look like. I've never had a problem with these before this year, and I've been butterfly gardening 10 years. Thanks for the info. I like your net & stomp idea, but I'm really afraid of getting stung. Do they seem aggressive or fly at you when you're trying to net them?

Heykid, oh how frustrating, going to such lengths to protect your babies and they still get them! One year I was rearing some Monarch cats in my wire mesh cage. I started noticing these tiny gnat looking things inside the cage. They turned out to be Chalcid Wasps, and they had already ruined several chrysalides. They were so tiny, they were able to fit right through the mesh. The rest of that year and the next year, I raised all my cats in the house. I'd never seen the Chalcids before, and thankfully I haven't seen them since.

It's so great you have a greenhouse to put your cats in. I've always wanted a greenhouse. I'll tell my husband that I NEED one to protect our babies! Thanks for the excuse!

I think I might have seen those wasps on the A. curassavica - but I thought they were yellow jackets. Is there a way to tell the difference? I brought in as many eggs and small cats as I could find but never saw any big cats on the plants, could be because of the wasps.

A few years ago my sister had one of those battery operated bug zappers that looks like a little tennis racket. Maybe one of those might work?

Yellow jackets will predate upon caterpillars, too, as well as other small and soft-bodied prey. They are something to watch when they find a population of rose sawflies, I can tell you that!

Terrene, I do have one of the tennis racket shaped bug zappers and have been very tempted to try it. I'm just afraid it won't be strong enough to kill the wasp and he will come after me!