Spider mites are closely related to spiders. They are nearly microscopic in size and are not often suspected until the damage is visible, usually on thin leaved orchids such as Dendrobiums. The leaves will have a silvery look caused by the death of cells at the surface of the leaves due to the sucking of sap by the mites. This is most apparent on the undersides of leaves and sometimes it will look like there is dust or dirt on the top of the leaves.
Some spider mites spin a protective web which can cover the underside of leaves during a bad infestation. To verify that you have mites, spray the underside of the leaves with a fine water spray and then hold up to the light. The webs should be made visible from the spray. You may also be able to see the tiny bugs moving on the webs. These webs will keep any spray off the eggs and the adults if you do not destroy it manually.
They are particularly invasive during hot, dry weather and the life cycle only takes about a week during warm temperatures. Each female can lay three to five eggs per day and thus produce more than 100 eggs in about three weeks. It is very important to identify the problem early and take immediate action especially since you already have an infestation before you can see any signs of the problem. Keeping humidity in your growing environment is detrimental to mites since they prefer dry air. Hand washing of leaves or spraying with water will remove many of them from plants and kill them from the pressure of the water spray.
After you have determined that you have mites, it is very important to move quickly or they can kill you plant. Wipe the leaves on both sides with a damp cloth moistened with water and soap. This will destroy the webs and will manually kill the mites you touch. Then spray with a mixture of 409 Cleaner (one pint), rubbing alcohol (one pint) and water to make a gallon. Spray all surfaces of the plant as well as all surrounding plants. You will need to spray every fourth day for about a month to contain the infestation.
There are also chemical controls called miticides which can be used to control these pests, but most are not available to hobby growers. Horticultural oils such as Neem Oil will help control these pests, but their application during hot weather must be done carefully. Apply in the evening so that there is time for the oil to kill the pests without any direct sunlight on the plants.