At a lecture at the Stoneleigh (UK) convention almost exactly a year ago Dr
Jeff Pettis described experiments he had conducted to see how far the tray
had to be below the mesh for fallen living varroa not to get back onto the
bees.  At a half inch space they all got back.  At 2 inches none did, with a
numerical gradient in between these extremes.

He was careful not to say that the mites jumped the gap.  He was unsure how
they bridged the space, but offered the suggestion that proximity to the bees
above the screen provided the stimulus to make their way back up.

Chris