They crawl or can be transported by the wind. Spider mites can be detected if you shake some affected branches over a white paper. If little eight legged dots are moving around, you probably have spider mites. Verify with a magnifying glass. Heavy infestations sometimes can be detected because the plants have webbing over the needles and branches.
Mites are most active in the winter months.
They are already outside and since rainfall on the leaves washes them off, they are mainly a winter problem. Nurseries have overhead watering systems, so they don't have a major problem with spider mites. Take your plants outside, under a tree and blast off the mites with a strong jet of water from the hose. Other controls include Safer's Insecticidal Soap or a miticide such as Kelthane. Q. We have a millipede invasion on our screened porch every fall.