The worst thing ants seem to do in the greenhouse is try to establish nests in floor standing pots.Rather unpleasant and not recommended for the plant. They colonise black fly onto stems too but that is more so outside than in the greenhouse.Keep the floor wet where possible as they like dry conditions.A puff of ant killer dust at their source helps too.I have just cleared the compost from my neighbours compost bin.Inside was a big ants nest, fascinating but disgusting too.