Last year, I had an infestation of spider mites that killed my cucumbers. I let the soil get too dry and with the heat in Southern California, it was a perfect breeding environment for spider mites. I lost my will to keep up with the plants after I tossed the cucumbers and forgetting to water. This year, I did some research and found out that neem oil is a natural pesticide that is also good for plants. Neem oil is an essential oil harmful to pests but not to humans (unless you're pregnant).
Neem oil is "harvested" from the fruit and seeds of the neem tree which is from India. It needs to be diluted with water before you can use it directly on the leaves otherwise, you risk killing or burning it. Because it is a natural and organic product, you do not get instant results like you would with chemical pesticides and multiple sprayings on top and underneath the foilage over a few days are needed in order to kill any spider mites. The reason being is the neem oil doesn't instantly kill the mites but instead messes with their body (i.e. breathing, eating, etc) which is why it takes a few days to get rid of them.
Just mix a tablespoon of neem oil with a gallon of water and couple drops of dish soap (the soap helps to keep the neem oil and water from separating) and spray on top and under the leaves of the infested plants every day until the spider mites have been eradicated. The mixture is only good for up to about 8 hours so be sure to make a new batch every time.