It is my understanding that they are airborne. If I do not keep misting my Phals. and Masd. I can just about bet on them moving in. They like dry air, definitely do not like the humidity. They also love Diffenbachia and a host of other plants - Schefflera, etc. If you are not currently misting I would take up the habit - just make sure to do early in the day and give the leaves time to dry off before dark. There are many remedies sold to remove them - I have only had good luck with a miticide and then starting the misting.