If you know where the nest is, try a few drops of Nippon (this sounds
unlikely, somehow, but that is its name. See:
http://www.vitax.co.uk/home/vitax.asp?product=14 for details) on a stone
near one of their routes. They take it back to the colony to feed the
others and it wipes them out. The main ingredient seems to be borax, and
I've heard they don't like chalk either, so maybe it's the alkalinity that
does for them?