Made an interesting observation yesterday that I thought I'd share. I was using a 30hp Kabota diesel pushing out an old fence approximately 50 feet from a 4 hives. These bees haven't shown any interest in me (while mowing etc) until yesterday. When I began loading the tractor and increased the rpm to around 2250, I was quickly surrounded and began receiving head butts on my face and chest (no stings). I immediately dropped the rpm back to 1500 and everything stopped. Just to test, I ran it gradually back up to 2150 and began receiving interest. At 2200 I began receiving a few head butts. I'm guessing the frequency @ 2150 was key to triggering them since I dropped back to 2100 and worked for three more hours in the same area without even a buzz. Might be handy information if you have some bees that don't like your equipment....vary the rpm a little and see if the problem doesn't go away.