Yellow Jackets are tough to deal with especially during mealtimes. A screened in canopy or tent for your outside table helps.
We would take a plastic milk jug and fill it with about 2 inches of water. Cut a hole in the side towards the top for the yellow jackets to fly in. Hang a hot dog using a string from the lid of the jug. Hang the jug by the handle from a tree away from where you'll be sitting (5-10 feet away). Yellow jackets like to eat food while sitting on something so they will take a bite of the hotdog and then head for the bottom of the jug. They get caught in the water and drown.
You can buy yellow jacket traps at a hardware store like Lowes or Home Depot or at Walmart.
Make sure drinks are covered no open holes... yellow jackets will get in to the drink and be there when you go to take a drink...ouch.
