If yellow jackets are like the wasps we get here then in the spring, they are looking for protein for the queen. What we do is take an empty plastic icecream container and cut a few little door into it then we mix Carbaryl and cat food and put a blob inside, put on the lid and nail these to the tops of fence posts, the worker that have over-wintered take the brew back to the queen and kill the lot. We had a terrible problem a few years back, did the poison thing all around the farm for two years and have been clear of the little buggers for the last five years.
Ernie