Hello, master gardener. Spider mites are my nemesis. I engage in a, mostly losing, battle with them every year over the the plants I try to grow in planters on my east-by-south-east-facing Riverdale balcony. They devastate my aristolochia and impatiens, and significantly damage my bobo hydrangea, morning glory and mint. Coleus and nicotiana seem to be more resistant. Insecticidal soap has not been effective. Neem works better, but I still have heavy losses. I’ve tried a dormant oil spray on the aristolochia without benefit, and lady bugs. At this point, I’m out of ideas, and I don’t want to use non-organic chemical pesticides. Are there mite-resistant shade annuals (or very cold-hardy perennials) I could try? Do mites overwinter in soil and would changing the top layer at the end of the season help? Thank you so much for ideas.
Very sorry to hear about your battle with spider mites.
Female spider mites overwinter in either the soil or on the host plant. So changing the soil would help in mite population reduction. See Spider Mites and their Control
In general, spider mites develop through 5 stages from egg to adult. There may be up to 20 generations in a season depending on the weather. The females become active in April and May when they seek out the undersides of leaves on suitable hosts. Each female may lay over 100 eggs. A single generation may require as much as 20 to as few as five days, depending on the temperature. These mites prefer hot, dry weather, exactly what we have been experiencing this summer. No wonder gardeners feel that they are fighting a loosing battle when it comes to spider mites.
The recommended treatment for a light infestation of spider mites is a spraying with a forceful stream of water including the undersides of the foliage. Do this for three days. If mites continue to be a problem, as it is in your case, spray with an insecticidal soap every three to five days for two weeks in order to interrupt the cycle of eggs hatching. Have you sprayed your plants every 3-5 days? Have you been spraying the underside of leaves?
Keep in mind that spraying with insecticidal soap may also kill beneficial mites and other insects that feed on the spider mites and keep them in check. Please follow the directions on the package.
For many trees and other plants, in order to control spider mites, the best treatment would be dormant oil (applied in late winter or early spring, when the plant is dormant). Timing of this product is imperative to it’s effectiveness. This is an excellent article on the Do’s and Don’ts of Dormant Oil
There are also two species of predatory mites, persimilis and fallacis as well as lacewing that are recommended for control. The persimilis is the main predator of spider mites and is the only one that will go directly into the webbing to eat them. One aspect that you need to keep in mind when introducing any predatory bug is that there is no residual pesticide residue on the plant leaves as this might cause a hostile environment for the beneficials.
Unfortunately, there is no specific list of spider -mite resistant shrubs. From my research I did find this article that states: ” the most effective spider-mite repellants are members of the Allium genus : leeks, chives, garlic, scallion, and shallot… These plants are not 100% guaranteed to keep spider mites away since the latter are so widespread and prolific, but they have been shown to have positive effects when interspersed among other garden plants.“