It's late fall on my farm. The time of year when yellow jackets can engage in extensive robbing of bee colonies. Reducing the size of entrances helps the bees to defend themselves. Reducing the population of yellow jackets helps.
Here is my simple, do-it-yourself yellow jacket trap. Smear some smelly food (such as canned catfood) onto a board, and suspend the board over a container of soapy water. I leave about 1/2 headspace between the water and the board. The yellow jackets flop into the soapy water and drown while trying to get to the food. It is very effective. I caught the first yellow jacket within 5 seconds of installing the trap. And by the next day had caught a hundred. The trap is not attractive to honey bees.