Spider Mites thrive in Hot, Dry Gardens

Stippling is a sign of spider mite damage. (Image: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org)

Article by, Nicole Sanchez,
Horticulture, OSU Extension Service, Klamath County

Current weather and smoke conditions are tough on people, plants, and pets. Woody plants grow slowly and drop leaves earlier in fall after prolonged smoke exposure. High temperatures cause tomato and pepper to abort their fruit. Time spent in the garden, usually enjoyable, becomes headache-inducing.

Meanwhile, concentrated on the undersides of leaves and in the newest plant growth, something is thriving: a lot of somethings. Hot, dry, dusty weather is ideal for spider mites. These tiny arachnids rival aphids in destructive and reproductive capabilities when conditions are just right- as they have been in the Klamath Basin and eastern Oregon over the last few weeks.

The term “mite” encompasses a wide range of tiny, often microscopic creatures related to ticks and spiders. Mites are often grouped with insects in pest management discussions and information. Spider mites, broad mites, eriophyid mites, and others each have slightly different life cycles and host (food) plants. Some, like clover and spruce mites, are specific to a few types of plants, while others, like the two-spotted spider mite, can feed and thrive on dozens of garden and landscape plants.

For a survey of the different mite types commonly found in Oregon, check out OSU’s web page on the topic: Mites | College of Agricultural Sciences (oregonstate.edu). Eriophyid mites are common in the Klamath Basin. This summer, and especially in the last few weeks, two-spotted spider mites have been on the increase.

In the author’s yard and greenhouse, spider mites are currently on tomatoes, cucumbers, chamomile, burning bush, and parsley, demonstrating the wide range of plant families suitable for spider mites. On a recent visit to an eastern Oregon vegetable and flower farm, spider mites were observed on peppers, eggplants, and beans.

Microscopic mites are easy to overlook, enabling them to build up large populations before fine webbing around leaf margins and between leaves makes their presence obvious. Resources for identifying and managing these tiny, plant-draining pests are outlined below.

Identification:
The signs of spider mite presence are often easier to see than the mites themselves. Look for leaf stippling- little spots where the leaf is lighter. Yellow or white spots are sign of mite feeding and removing a little of the leaf’s chlorophyll in the process. Feeding often starts on the younger or newer leaves. Another sign to look for is frass- fecal matter left on leaves. Frass looks like tiny dust or dirt particles, often caught in the fine webbing made by the mites and giving them the “spider” in their common name. As a spider mite population builds, webbing becomes increasingly easier to see. Continued feeding makes plant leaves look speckled with lighter or bleached out sections.  

Treatment:
If caught early, it’s possible to pinch our affected parts of the plant or use a soap solution to manage spider mites. For soap to be effective, it needs to cover the mites’ bodies. Since mite are often on the undersides of leaves, it may be necessary to move leaves around and upside down to get good coverage. Soap must usually be applied multiple times for effective control.

Severe infestations may require chemical treatment. Spider mite life cycle and cultural control info are offered in the “PNW handbooks”: Landscape pests-Spider mite | Pacific Northwest Pest Management Handbooks (pnwhandbooks.org) and suggestions for chemical control here: Chemical Control of Landscape Pests | Pacific Northwest Pest Management Handbooks (pnwhandbooks.org)

In brief, cultural controls include blasting mites off plants with water spray, removing affected plants, or creating a more humid environment for short periods. Numerous chemical products available to the homeowner are effective against spider mites. Most are contact products, meaning that the chemical must directly contact the mites’ bodies to work. It’s usually necessary to apply a thorough spray, more than once, for effective control.

Of special note when temperatures are above 80-85F, many products including insecticidal soaps can cause further damage to plants. As always, read product labels carefully. Apply product at exactly the rate outlined on the label to reduce pesticide resistance- spider mites are notorious for building up resistance when the same product is used repeatedly. The pesticide label will also have information regarding appropriate temperatures for treatment.

Cold weather and higher humidity are two natural controls for spider mites- winter will kill off the populations that have had the opportunity to build up over this hot, dusty, dry summer. Predatory mites are another form of control- and one that commercial growers increasingly rely on to control spider mites without the use of chemicals. For additional info on the use of predatory mites, check out Robin Rosetta’s article “When mite makes right” here: biological control of two-spotted mite | College of Agricultural Sciences (oregonstate.edu)