Sometimes - No, they are a friend.
Now - to anyone who has lost a hive to YJ's, yes they most certainly are the enemy.
But, over the course of the summer and into this fall, I witnessed a phenomenon that made them friends. I, like many of you, have to fight the YJ's every year, especially in the spring and fall, to catch the queens.
This year we had more YJ's than ever before. Billions of them ... (ok, that's exaggerating) ... 1,000's anyways. And I began to invent more ways to catch them. But I noticed 2 things: 1) I didn't seem to be able to make a dent in the YJ population & 2) (completely unrelated - I thought) I wasn't having the Ant problem that my bees fight every year. (I've lost more hives to Ants than any other single cause).
One afternoon I was sitting in the Apiary - and I observed an unlimited supply of YJ's hovering around the ground near the hives. And as I waited for the inevitable hive-entrance conflicts, they never happened. The YJ's were too busy to bother the hives. I got on my hands and knees, and watched closely ... they were targeting the Ants with a relentless force, hovering low and grabbing every Ant they found. It was quite a sight - and they've been doing it all summer.
In a normal year, if I fail to spray the hive stand legs with an Ant blocker every 2 months, I'll lose every hive in 3 weeks after the blocker wears off. I haven't sprayed once this year - since the spring.
Anyways - I took down my YJ traps and left them bee. Hive entrance conflicts are occurring now, as the firest fall rains have knocked the Ants down in population - as always happens. (These ants can't survive rains - only a few make it thru the winter, but by the next fall, they are everywhere again).
In short - nature is amazing, and I feel that we know so little.