Plant Pests 101: Spider Mites
Spider mites are a dreaded pest for house plant owners. They suck the juice from the cells in your plants. And they are mobile, so they can move from plant to plant. The worst part of this scenario is that they also reproduce quickly. Oddly, it’s both heartening and disheartening to know that Horticulturist Justin Hancock and Integrated Pest Management Manager Michelle Opela have had infestations of spider mites. If professionals have to fight off spider mites on occasion, then you shouldn’t take it as a personal failing if your plants fall victim.
How do you get spider mites?
How do you even get pests on indoor plants, you might wonder. Where do they come from? Hancock says some pests are simply inevitable. “While we may think of our plants as members of our households, they're ecosystems, providing habitat for a wide variety of beneficial microorganisms,” he says. “Unfortunately, they can also provide perfect habitat for pests.” Opela agrees. “Spider mites especially like the warm, dry environment that winter delivers,” she says. According to Hancock, spider mites are so ubiquitous indoors and out, you may find yourself with an infestation by simply “bringing a new plant home, by cutting flowers and bringing them inside, or even just potentially leaving your windows open if you have landscaping right outside,” he says. “They’re so small, they can even hitch a ride into your home on you or your pets,” he says. But one place they do not enter your home is through the soil in your houseplants. “I’m going to bust a myth,” says Opela. “Spider mites are not breeding in your soil. Don’t worry about treating your soil,” she says.
Lean in and look closely
“These little suckers are super tiny,” says Opela. “Look on the undersides of the leaves. That’s where they hang out,” she says. “If you have great vision, you may be able to see spider mites, appearing as tiny flecks in webbing,” says Hancock. “They are hard to see,” admits Opela. “But if you mist the plant, they’ll stand out,” she says. Or you can use magnifying tools. “You can verify they're mites, rather than just dust, by looking closer with a magnifying glass or jewelers' loupe,” says Hancock. (He recommends a magnification of 40x). Or you can use your phone camera macro lens attachments or a handheld Bluetooth microscope.
When you magnify them, do a leg count. A little entomological ah-ha: Mites are arachnids; they have eight legs. Insects have six. “There are many species of mites that attack plants, but one of the most common on houseplants is the two-spotted spider mite,” says Hancock.”It has earned its name because it has two dark spots in its back, like little saddlebags,” he says. Mites can be different colors. Season can affect their color. “In winter, they can get a reddish look,” says Opela. And what the mites are dining on can also affect what they look like. “Different plants can impact the coloration of spider mites,” says Hancock.
Look for leaf damage
Hancock says it may be easier to spot spider mite damage before you see the mites themselves. “These pests have needle-like mouths that jab into plant leaves, much the way a mosquito feeds off you,” he says. As they feed from the cells, the cells die. This leaves a stippled appearance, where the plant has tiny pinprick spots on the leaves,” says Hancock.
Look for telltale webs
“Honestly, you don’t know you have them until you see the webbing,” says Opela. “It doesn’t look like spider webbing,” she says. “It’s thinner and closer together. Spider mite webs are very fine,” she says. “The tiny webbing looks like cobwebs,” adds Hancock. “You’ll most commonly see it on the undersides of your plant’s leaves, but in some species and situations, the webbing may also appear on top of the leaf surfaces, as well as around the junction between the leaf and stem,” he says.
Spider mite behavior
Common indoor plant targets include palms, calathea, and English ivy, just to mention a few. “Palms are spider mite magnets,” says Hancock. But they aren’t picky eaters. “Unlike many plant pests, spider mites aren’t very fussy—they have a wide host range, which means they can attack dozens of different kinds of plants,” says Hancock. In addition to being widespread, spider mites are also mobile. “Spider mites crawl from plant to plant,” says Hancock, so one infested plant can spread them throughout your home. And they reproduce quickly. “Spider mites are really good at making babies,” says Opela. And they have a very rapid lifecycle. Eggs hatch in 2-4 days and nymphs develop in 2-4 days. That means you could leave for a week's vacation and come back home to a full-blown infestation.
How to treat plants
Hancock and Opela recommend these actions when you see evidence of spider mites. Quarantine the affected plant. Isolate a plant on which you’ve discovered spider mites. Spider mites will walk from plant to plant and you can stop an infestation of your whole plant collection by isolating the affected plant.
1. Wash the leaves.
“If your plant is small enough, wash the mites off in the sink or shower using lukewarm or room-temperature water,” says Hancock. He recommends taking extra care to wash the undersides of the leaves where mites are most likely to be. “For an extra dose of protection, wipe the tops and bottoms of the leaves with a wet paper towel, and then rinse the leaves in the sink or shower,” he says.
2. Treat leaves.
Use a horticultural oil (including neem oil) to control mite outbreaks. Use a horticultural oil labeled specifically for houseplants and follow application instructions on the product packaging. Horticulture oil suffocates both mites and eggs. Hancock recommends applying oil at dusk or on a cloudy day. (The oil on leaves in direct sun may cause leaves to burn.) Take care that the horticultural oil doesn’t go into the soil because it will suffocate and damage plant roots.
3. Be persistent.
Continue to treat. “Once and done doesn’t work” says Hancock, because spider mites reproduce so quickly. One treatment of horticultural oil will not solve the infestation. “Weekly follow ups are necessary, and I recommend them for at least three or four weeks,” says Hancock.
4. Control your environment.
Spider mites thrive best in warm, dry conditions. “They spread more rapidly in average room temperature or above and low relative humidity levels. This makes winter an opportune time for mite populations to explode, especially in the North,” says Hancock. Drop the temperature in the room and increase the humidity around your plants to slow down reproduction. Spider mites reproduce faster in a warm dry environment.
5. Consider beneficial insects.
If you have a large number of houseplants, or have a plant with lots of small leaves, consider beneficial insects for the job. Two of the more common beneficials are Chrsyopa carnea (green lacewing larvae) and Phytoseilus persimilis (predatory mites). Both are spider mite predators and can be ordered online. Opela calls beneficial insects “the lazy man’s method” of spider mite treatment. Phytoseiulus persimilis are formidable opponents, because “they can eat spider mites and reproduce faster than spider mites,” she says. “And they eat eggs and adults. They stab them and suck them dry,” she says. Phytoseiulus persimilis are blind so you need to place them near the mites on your plants. “If they feel webbing, they will go after the mites,” she says. These mite-eating mites come mixed with vermiculite. ”Sprinkle them on the plant. It’s a little messy,” admits Opela, but very effective. If you have a severe spider mite infestation or a lot of plants, it can also take a few weeks for your beneficials to build up enough of a population to offer good control.
Spider Mite Images: Frank Peairs, Colorado State University, Bugwood.org, Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org