Treatment for spider mites and do spider mites overwinter in outdoor pots and raised beds?
My longstanding Park Hill Denver backyard garden (mostly veggies) had a distressing infestation of spider mites this summer. My cucumbers, tomatoes (some varieties more than others) and eggplant were most affected, along with some marigolds and zinnias. Some of the plants were planted directly in amended soil (from my compost pile) in a raised bed and others were planted in 14" pots using potting soil. Treatment recommended by City Floral (who confirmed the problem was spider mites using a microscope), including regular watering, hosing the plants with a hard spray, and applying Bonide All Seasons Horticultural and Dormant Spray Oil on a 5-7 spray day schedule had little to no effect. I didn't have the heart to destroy to plants but I did regularly remove and dispose of affected leaves and branches, which seemed to at least slow the process. The plants all survived (and revived somewhat as the days grew shorter and the sun grew less intense) but their productivity was obviously affected. I have two questions: 1) is there anything more/different that I should have done to address the problem and is there anything more I can do to prevent a repeat of this problem next year?; and 2) should I dispose of all potting soil in the pots of the affected plants or can it be reused for potted flowers next year (my common practice is to use mostly new potting soil each year for potted vegetable plants but to reuse the old soil for flowers that are planted in pots). In other words, do spider mites overwinter in the soil? (Typically, the pots are left with their soil still in them in an unheated garage over the winter.) Thanks for your help!
Denver County Colorado
Overwintering depends on the species of spider mite. Some mites survive winter in the egg stage. The twospotted spider mite and honeylocust spider mite survive winter as adults by hiding in garden debris or other protected areas.
Since your infestation sounds rather extensive, you may prefer to rid yourself of this season's potting soil and wash your pots to remove any chance of a winter home for surviving spider mites.
Spider mites need arid conditions to thrive. Many of their predators need more humid conditions to be successful hunters. The advice you were given to spray your plants was excellent. You can do a great deal to manage spider mites by occasionally spraying your plants to remove the mites, disrupt web formation and delay egg laying.
Dr. Whitney Cranshaw has written an excellent article about spider mites, their life cycles and controls which you may find useful:
Thank you for your informative response and the linked article. Based on your advice and the article, I will redouble my jet spraying efforts next season and dispose of the potting soil from affected plants.