Seen a couple last couple summers. And yes, if they're around, they'll be on ailanthus, but they go after other trees too. They're annoying too -- look like they'd be easy to swat, but get close and they'll quickly jump and fly.

beng, if you see "a couple", soon, you will seeing multiple 1000's. They need a predator to eat them. I suspect that something is beginning to eat them around here. I saw infestations of them earlier in the Spring, but now, not so bad. I see an Ailanthus weed tree growing by my driveway, but not nearly as invasive as in NYC, and I am just outside the city. That dam! tree just thrives where everything else DIES! Same with Mimosa, I don't see that as much locally as I remember as a kid in NYC..., and they should be hardy here. Climate, generally, is milder, especially with respect to the length of Winters. My whole yard is populated now with Southern Magnolias, they have even started to self sow, and not any more "native" to NJ than Ailanthus or Mimosa!