If you are growing strawberries, you will get spider mites (Tetranychus spp) in your crop at some point.
Even in a screened greenhouse, they are so small they fit through the screen. A fairly effective biological control for spider mites are predator mites.
Phytoseiulus persimilis is a fast moving and voracious spider mite predator. These are good for obvious mite infestations. If you apply enough of these, they can control heavier mite outbreaks. But once they have eaten enough that the spider mite population (adults and eggs) is too low to support Phytoseiulus persimilis, the predator population can crash. But that is not a reason to not use it, as it is effective.
Neoseiulus (Amblyseius) californicus is a spider mite predator that appears to be persistent in the crop. It can, apparently (although some dispute this), live on pollen when mite populations are low. Regardless, low levels of spider mite are usually persistent in a crop and N. californicus is feeding on those (keeping them in check). I have observed N. californicus in greenhouse strawberry several months after application, in plants that had begun to exhibit visible spider mite infestation. This is a good one to apply to your crop before you notice spider mite, once you have flowers (to support the predator with pollen).
If you observe the beginnings of visible spider mite infestations, and you can see N. californicus among them, know that spider mite reproduces faster than predators, so you probably want to quickly add P. persimilis to help knock back the spider mite population. There are other spider mite predators, and other controls, well described in:
http://www.ipm.ucdavis.edu/PMG/r734400111.html
Since it can take up to a week to acquire predator mites (insectaries often ship on preset schedules) it is prudent to treat spider mite outbreaks to reduce the population while you are waiting, when you have no predators already in the crop. I have done this using safer soap (2% dilution). Spray the outbreak area and the area immediately around it thoroughly every 3 days. This will eliminate most of the adults, and those hatching out. I have found that it is good to wash off the safer soap (with water) just prior to the application of predator mites because safer soap is tough on predators too. You will never kill all the spider mite eggs or adults with safer soap treatment so there will be food for the predators. Safer soap also makes berries taste bad so I don’t treat all the plants, just the affected area.
If you discover a serious, extensive infestation, and mites can explode if conditions are warm and dry, trying to control at that point with predator mites could become overly expensive, and may not work. Out of control mites can wipe out your crop. There are many chemical treatments for spider mite recommended in the UC Davis site and, to prevent out of control mites from taking out your crop, a chemical treatment may be necessary. I do not have a good answer as to when it is safe (for the predators) to apply predator mites after miticide treatment, but some treatments seem to provide several weeks of control so there is no need to apply predators immediately. Since predator mites don’t feed on the plants, they less affected by miticides. But be sure to inquire to the predator supplier you are using as to post-miticide predator application timing.
The short days and cool nights of winter seem to slow down the mite reproduction, allowing predator mites (and other biological controls) to keep up, which is great for those targeting a winter production season. But, as soon as days start to lengthen noticeably, spider mites can wake up and blow up quickly. So don’t be lulled into a sense of unguardedness by the lack of spider mites in December.
Unless otherwise indicated, all of the images in this section are from the UC Davis IPM site and are used with permission from the University of California Statewide IPM Program, Jack Kelly Clark, photographer.