Yellowjacket nest defied my first effort at elimination

If you can't get to Queenie, it's a war of attrition. Don't try sealing them up-they may find a way indoors. I bet a conduit hole has them right on the other side of the drywall.

I would keep spraying, maybe different products. They are tough but eventually will as succumb. You can also hang a piece of fish a few inches over a soapy water tra, and do a honey/apple juice trap. Good luck, and a baking soda paste will help with stings.

If you have access to CO2 fire extinguisher go out early in am( coldest part of day) , spray area with CO2 which will chill them. Open panel and use poison spray. CO2 will chill warm beer in nothing flat especially if the boss buys the extinguisher!

I have apple trees so fight yellow jackets every year. Ace foaming wasp/ hornet or ortho are my preferred brands of insecticide

Caulk the access hole shut.

This stuff is awesome, knocks them down immediately and from a distance, added plus it won't hurt your electrical box. https://www.sprayon.com/products/s00857_blast_em_wasp_hornet_killer

Get ground bee power and an applicator and spray the dust into the entrance hole. Insects coming in will bring the insecticide into the nest and in a couple of days the nest will be killed. The liquid sprays often kill the bees coming out and their bodies plug the exit forcing the rest of the nest to find another exit. My experience is that the second exit was in the kitchen!

Are you sure they're yellow jackets, a type of wasp?
If they're bees instead a local beekeeper may help to relocate them.

I use diluted dish soap in a sprayer on my last Yellow Jacket invasion.
You have to harass them a lot to get them to move, or as someone else mentioned, kill the queen.

I had a hard to reach nest up under my porch behind the lattice last year, there was no straight line of site to hit the nest with spray.
I was able to adapt a piece of plastic tubing (from a can of spray foam) onto my can of bee spray, with a little heat I was able to put a 90 degree bend in it. Early morning I was able to spray blindly through the lattice and up to the bottom of the deck. Took two tries and they were gone.

I'm with osagebow, they're probably in the wall or the box.

Good luck!

Between a barn, a big old house with lots of nesting spots, and 4 acres of lawn and gardens, I deal with a lot of bees, wasps, yellow jackets, and the like. Of all the above, the only ones I really fear are yellow jackets, as they're just about the most aggressive and nasty creatures ever created.

In your situation, I'd probably break down and call an exterminator, which I've only had to do on one other occasion in all my years. Short of that, forget the consumer products, and go pro: http://doyourownpestcontrol.com/yellowjackets.htm

I've used a few of their products for carpenter bee elimination, and their effectiveness just blows my mind, after years of frustration with products that can be purchased at Home Depot.

Are you sure they are yellow jackets and not paper wasps? I believe yellow jackets nest in the ground. Paper wasps build nests inside walls and under the eaves of a house, and are much less aggressive. Yellow jackets are a kind of wasp, about as aggressive as hornets, but I don't think they would build a nest behind an electrical panel. I had a nest of them in my front lawn last year. Every time I cut the grass I was getting stung. I finally located the nest by closing the windows on the car and driving back and forth over the area where I was getting stung. When I drove over where the nest was, there must have been thousands swarming out of the hole. They practically covered the windows of the car.

I bought some yellow jacket spray, waited until after dark, and saturated the hole with the spray and plugged up the hole. That took care of them. Before spraying, I looked down the hole with a flash light. The nest was huge. They tend to build in abandoned holes left behind by other animals. I wouldn't have bothered them if they hadn't been so close to the house and stinging me when I cut the grass.

The paper wasps often mistaken for yellow jackets are smaller and slimmer. Usually they won't sting you unless you bump into the nest or otherwise disturb them.

Some people mistake honey bees for yellow jackets. Bees are a lot less aggressive as well (unless you get a swarm of the dreaded Africanised bees AKA "killer bees", which I don't think have migrated this far north yet).

Maybe try hanging a trap somewhere outside the entrance?

They are likely nesting in the wall cavity. I don't think I'd try plugging their entrance - the ones that get stuck in there might end up going further in, rather than out.

I had a huge nest once in my Venmar exit pipe. 6" flexible pipe, plugged solid for about 4 feet. I happened to notice a couple going in & out when going by on my lawnmower. After stopping & watching them go in & out for a few minutes, I just left it alone until late fall after they had vacated, then I tore down the pipe from in the basement & replaced it. But we don't have real Yellow Jackets, these were yellow hornets.

Nothing wrong with calling a pro. Will cost you $150 - $200. I have one visiting my house today to take down a large wasp nest, built in a place where I can't access the entrance with a foaming spray. Second time I've had to call them in 3 years... but also second time in 40 years.

Had to call a pro last year myself. They had a nest inside a hollow wall cavity, and I couldn't get to them. He came, worked on things for about an hour, and that was the last I saw of them.

http://www.doyourownpestcontrol.com/yellowjackets.htm. Maybe one of these was used

Just got stung by two hornets today. Didn't notice they built a nest on my empty woodrack. Noticed real quick while being stung and seeing the nest a swarming. Getting a little itchy here. Fuggers

We've found that if we immediately put some vinegar on the bites the pain quickly goes away. We use a cotton ball or some such.
I believe this is because insect venoms are basic (high pH) and vinegar is acidic (low pH) so the vinegar neutralizes the venom.

german hornets look a lot like a yellow jacket- nest in ground- extremely aggressive when disturbed. This year after 4 years of systematically attacking wasp and hornet nests close or on/in various areas of my home I am pretty free of them. Problem I have now and the last couple years is Bald Headed Hornets. Two summers back huge nest some 40 ft up in a tree 30 ft from house, 20 gauge shot gun took care of nest. This year the nest is somewhere back in the conservatory area behind the house, I haven't found it yet. At least it is some 60 or so yards from the house so hasn't presented a big problem yet. But the little buggers are working the wood stacks for construction material.

I've cleaned up a few large nests of wasps and hornets both by rigging up a shop vac hose to point strait at the nest opening (About 1 inch away). I've set up the rig using a step ladder and some tape to hold it in place, another time some random bits laying around. Anyway, if you move slowly while setting up and don't make a bunch of noise, I found they pretty much ignored me until I turned on the shop vac. At that point they start running out of the nest to see what's happening and promptly get sucked into the shop vac. After a while they are more cautious and you have to bang on the nest a bit and then they really get pissed and pour out of the nest strait into the vac. I left it set up and running for a few hours and by then 98% of them have been sucked up. At this point you can dispose of the nest, spray it first if you like.
This works like a charm and there's that satisfying glee of listening to the "thuk" and then bouncing down the hose and smack into the vac... at the end they're so dazed that you dump them out and they're just barely moving their legs. Squish the whole bunch with your shoe or something and problem solved.

i was told that if they are yellow jackets the spray you use has to say yellow jackets or it won't work it will just daze them. as far as the powder that the pro uses leave the powder alone don't wash it off. i left the powder alone for the three years it stayed and it kept killing on contact. hey hogwildz if you were itchy your allergic you should have a epi pen. with each sting the side effects get worse. just spent 3.5 hours in the er with a swollen throat and tougue and almost passed out.