Why is my squash plant covered in webbing? My grape vine has lots of webbing and yellowing leaves, but I do not see any spiders. The leaves on my tomato plants are covered with light stippling, are turning yellow and falling off, and have tiny, moving dots on them – what are they? Based on calls like this to the Stanislaus County UCCE Master Gardeners' Help Line, many gardeners are currently dealing with an explosion of spider mites, a common pest of vegetables, fruit trees, berries, vines, ornamental plants and houseplants who can thrive in the hot, dry, dusty summers of California's Central Valley. 

What are Spider Mites?

With eight legs and a segmented body divided into two regions, spider mites are arachnids like spiders and ticks.  They are mites, members of the Tetranychus genus, which has more than 1,600 mite species worldwide. Tiny and difficult to see, spider mites generally live in colonies on the undersides of leaves of plants, where they may spin protective silk webs, giving them the “spider” portion of their common name. If the colonies are large, their dense webbing can cover the plant's leaves, stems and fruit. They cause damage to the plant by puncturing the leaves and sucking the plant cell contents. A small number of mites is generally not a problem on healthy plants, but very high populations can be damaging.

The initial damage generally appears as a stippling of light dots on the leaves. Heavily infected leaves turn yellow or bronze and fall off. To spread to new locations, these mites make use of “ballooning” by becoming airborne on a strand of webbing in the wind. If conditions are favorable, some spider mite varieties can hatch in 3 days and become sexually mature in as little as 5 days. A female can lay up to 20 eggs per day, live to 2-4 weeks, laying hundreds of eggs. This accelerated reproductive rate allows their numbers to grow rapidly, and to adapt quickly, becoming resistant to pesticides.

How To Control Spider Mites

Fortunately, spider mites have many predators, including lacewings, predatory thrips, minute pirate bugs, and even predatory mites. If infestations are not severe, these beneficial natural enemies can help keep spider mite populations low.

Keeping the dust down by planting ground covers, using mulches, and irrigating regularly also reduces the chances of getting spider mite infestations. Ensure your plants are getting enough water to reduce drought stress – stressed plants are more susceptible to harmful insects and diseases. If you do have a large spider mite population, apply water spray or mist to the undersides of the leaves at least twice a day to help decrease their numbers since they prefer dry conditions.

If mites have been determined to be the cause of the infestation and a pesticide is needed, use a less toxic insecticide such as those listed in the UC IPM Pest Notes: Spider Mites listed below.

For More Information

UC Integrated Pest Management (IPM) has additional information on how to identify spider mites, their life cycle, plant damage, and how to control: 

UC QUICK TIPS: https://ipm.ucanr.edu/QT/spidermitescard.html

UC PEST NOTES: https://ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnspidermites.pdf

Denise Godbout-Avant has been an UC Cooperative Extension Master Gardener in Stanislaus County since 2020