Spider Mites - Tetranychus
By SCMG Coby Lafayette-Kelleher
Baby it’s cold (and wet) outside. Too cold, one would think, for any self-respecting insect. It’s mollusk weather, so we expect to see worms, snails and slugs busy turning last season’s plant material into compost. But insects? This time of the year, they are keeping a low profile.
Don’t let your guard down, though. They’re out there, waiting. This is especially true of our bug of the month—Spider Mites. A point of clarification before we go much further: Spider Mites aren’t really bugs or insects; they are actually tiny little Arachnids. Thus, the “Spider” part of Spider Mites. Just so we’re clear.
Anyway, up close, really close, these tiny mites do look rather spider-y with their eight bristly legs and tear-drop shaped bodies. And, while you can see them clearly with a hand lens, it’s the evidence of their presence that usually gets your attention. Your first clue to the presence of Spider Mites is probably going to be a fine, silk-like webbing on leaves, twigs, and fruit of infested plants. They are particularly fond of vines, berries, and fruit trees, although ornamentals are vulnerable too.
Spider Mites feed on their host plant by piercing plant leaf cells and sucking out the contents. So, it isn’t surprising that leaf stippling is actually the first sign of damage to your plant/s. Stippling is subtle and easily overlooked. Consequently, most people don’t “discover” a Spider Mite infestation until the webbing starts showing up.
Spider Mites are colonial Arachnids, meaning that they occur in large groups; generally, an infestation suggests itself by the presence of mite activity on the under-side of plant leaves. Strong, healthy plants can usually hold their own against a few colonies of mites. But, bear in mind that in favorable conditions, mites can complete an entire life cycle in just one week. Really.
So, when all evidence points to the presence of Spider Mites, it’s time to take action. With moderate infestations, cultural controls can be quite successful in reducing and discouraging these mites. They like it dusty, so keep areas around plantings moist. A more direct method of controlling Spider Mites is the tried-and-true forceful blast of water on infested leaves and twigs.
If cultural controls fail or the infestation is severe, consider biological controls. Turns out Spider Mites have a number of natural enemies. The most significant are predatory mites in the Genus Phytoseiulus; common name Western Predatory Mite. These mighty mites are not much bigger than Spider Mites, but are extremely good at their job and once established tend to stick around as long as they have something to eat.
Predatory Mites are generally available these days through most nurseries. But, if your nursery doesn’t sell them, don’t despair. The California Department of Pesticide Regulation has a list of “Suppliers of Beneficial Organisms in North America” available on their website here.
Here in California, Rincon-Vitova Insectaries carries a number of Predatory Mites.
Let it be said though, that when it comes to Spider Mites, an ounce of prevention is worthy of consideration. These critters, like most “pests,” are opportunists. So, keep your plants healthy, well-watered and free of insect infestations. It’s a good idea too, to routinely look for stippling and check the under-sides of leaves for evidence of Spider Mites, such as webbing or colonial aggregations.
But don’t let your guard down, because while you are waiting out the winter, so are Tetranychus, safely tucked into leaf litter somewhere in your garden.
One more thing: a much longer and more detailed version of the material shared in this feature can be found at the University of California’s Online Integrated Pest Management Website.