I had two yellowjacket nests in the wood siding of my house this summer. Several cans of foaming spray had no effect since it couldn't penetrate far enough inside. I found a random forum thread where someone suggested using Sevin insecticide powder on the openings. It took a week and a half of daily applications using a paintbrush, but it seemed to be effective. Less expensive than calling an exterminator, and I have most of the can of powder left for next time.