Spider mite is a particularly difficult pest to eradicate as it readily develops tolerance to chemical controls and generally requires several applications of chemicals in order to eradicate all stages of the lifecycle (mites, nymphs and eggs).
Therefore biological controls such as the predatory mite Phytoseiulus persimilis are the most effective control against spider mite. This particular predator reproduces at twice the rate of spider mite. It is highly active under hot dry conditions, where just one mite of P. persimilis can consume 20 eggs or 5 adults per day! However, it requires a minimum temperature of over 16ºC (although daytime temperatures of 21 ºC are preferable) and there needs to be sufficient numbers of spider mite present for the predatory mites to feed on. Obviously, if you choose to use biological control then you cannot also use chemical control as this will kill the predatory mites!
There are a number of cultural practices that you can also use to help prevent and reduce outbreaks. Spider mite thrives in warm dry conditions, so damp down greenhouse floors and paths to increase the humidity. Regular inspection of the undersides of leaves will also help to detect the problem before it spreads. Pick off and burn any infested leaves when you find them. Finally, at the end of the growing season, make sure that you take time to really clean out your greenhouse properly as spider mites like to spend the winter hidden in dry crevices and plant debris until the weather warms up again in spring. For more information see http://www.thompson-morgan.com/pests/spider-mite