Have you noticed an odd stippling on the leaves of plants in your garden lately? Or maybe a super-fine webbing on a favorite bush? These are two early signs of a spider mite infestation, common during the dry, hot months of summer.
Spider mites look like tiny, moving dots, mostly on the undersides of leaves. But with a 10x hand lens, you can easily see them. They feed on many ornamental plants, as well as fruit trees, vegetables, vines and berries.
The drought makes matters worse as water-stressed plants are even more vulnerable to attack. If you don’t control them early, spider mites can do massive harm as they suck the cell contents from leaves.
First, leaves become stippled with tiny, light spots. Next, the spider mites’telltale webbing starts to spread. Later, the leaves often take on a bronze color and have a dusty, dry appearance. Finally, the leaves turn yellow or red, dry up and fall off. For some useful descriptions of spider mites and their damage, go to this website:
The good news is that simple, natural controls are the best way to control spider mites:
Rely on the Good Guys Spider mites have many natural enemies, so the first line of defense is to create a garden that draws beneficial insects. A diverse selection of native and/or drought-hardy Mediterranean plants is best. You can also help Mother Nature by purchasing and releasing one of several commercially available mite species, such as the western predatory mite. Predatory mites must be released when spider mites are actually present, otherwise they will starve or migrate elsewhere. In the long run, best results can usually be achieved by creating favorable conditions for the many naturally occurring predators that feed on spider mites.
Blast the Mites Off The spider mites in my garden prefer to beleaguer my late mother’s beloved roses planted in barrels on the front deck. I haven’t had the heart to let the roses die for lack of water—drought or no drought. So, most mornings I diligently spray the roses with a quick, hard blast of the hose, concentrating on the underside of the leaves. This practice not only knocks off the mites and their eggs, but also removes the dusty conditions they love so much.
Choose Safer Treatments If all else fails, use selective products such as insecticidal soap or oil. According to UC IPM, “Both petroleum-based horticultural oils and plant-based oils such as neem, canola, or cottonseed oils are acceptable.” UC also suggests trying any number of plant extracts formulated as acaricides (pesticides that kill mites), including oils from garlic, clove, mint, rosemary and cinnamon. Test these sprays several days before applying a full treatment as they may injure some plants.
Because oils and soaps must contact mites in order to kill them, be sure coverage is complete, especially on the undersides of leaves. Don’t use these products on water-stressed plants or when temperatures exceed 90°F. Instead apply them after watering in the evening and spray the plants down with water the next morning. Repeated applications are sometimes necessary.
As with most garden pests, spider mites can be managed with a little planning and the use of eco-friendly techniques.
Rachel Oppedahl is a UCCE Master Gardener of Tuolumne County.