Have you noticed an odd stippling on the leaves of plants in your garden lately? Or maybe a super-fine, spider-like webbing on a favorite bush? These are two early signs of an infestation of spider mites, one of the most common garden pests during the hottest months of summer.
It’s high season for spider mites because their favorite conditions are hot, dry and dusty. The drought makes matters worse, as water-stressed plants are even more vulnerable to attack.
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.
If you don’t control them early, spider mites can do massive harm as they suck the cell contents from leaves. There is a common progression of visual symptoms of an uncontrolled outbreak. 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 more close-up photos of spider mite damage, as well as specific mite identification, go to http://www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7405.html.
The good news is, the best way to control spider mites is to employ simple, natural controls. The University of California’s Integrated Pest Management (IPM) program has actually found that application of broad-spectrum pesticides often causes outbreaks of spider mites. “Spider mites frequently become a problem after applying insecticides,” states UC Pest Notes. “Such outbreaks are commonly a result of the insecticide killing off the mites’ natural enemies but also occur when certain insecticides stimulate mite reproduction.”
So, put the Sevin back on the shelf. Here are more effective ways to manage spider mite infestations.
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, which feed on spider mites. These predatory mites don’t feed on foliage or become pests. However, they must be released when spider mites are actually present, or they will starve or migrate elsewhere.
Blast Them 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, to control mites, I diligently spray them down most mornings (forgive me, TUD) with a quick, hard blast from a hose with a pistol nozzle. 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 (a pesticide that kills mites), including oils from garlic, clove, mint, rosemary and cinnamon. UC cautions that, “These materials may injure some plants, so check labels and/or test them out on a portion of the foliage several days before applying a full treatment.” Because oils and soaps must contact mites in order to kill them, it’s important that coverage is complete, especially on the undersides of leaves. These products should not be used on water-stressed plants or when temperatures exceed 90°F, so apply them after watering in the evening and spray the plants down with water the next morning, after the pesticide has done its job and before temperatures rise. Repeated applications are sometimes necessary.
As with most garden pests, spider mites can be controlled with a little planning and the use of eco-friendly practices.
Rachel Oppedahl is a UCCE Tuolumne County Master Gardener.