Seeing silky webbing on the underside of your tomato leaves? Or you’ve found them on your houseplants and want to know how to get rid of them? Spider mites are a common pest on our favorite plants, but can be managed with a little effort and know-how.
What Are Spider Mites?
Spider mites are arachnids. They have eight legs as adults, just like spiders or ticks. About 1200 species of spider mites are identified. Extremely small, sometimes as tiny as 1/50th of an inch, they can look like little moving dots.
Spider mites damage plants by sucking out the good stuff inside a plant cell. Think back to biology class and remember things like cytoplasm and organelles. The yellowing of the leaf occurs because the mites will eat the chlorophyll as well as the other material in the cell.
Spider mites will infest most plants that a home gardener or market grower might raise. Tomatoes, lettuces, peas, beans, cucumbers, zucchini, winter squash, strawberries, blackberries, and fruit trees are all targets. They will also attack houseplants if conditions are ripe. They can be a particularly huge problem in a greenhouse, where natural predators aren’t present and conditions are favorable.
Spider mites are hard to see, and often you won’t notice them until you see the damage to your plants. Take a few minutes to look and inspect your garden plants or houseplants. Pay attention to the undersides of the leaves and let your eyes have a minute to focus.
Look for these signs:
Tiny yellow dots on leaves. As the infestation continues, many dots blend together, causing a leaf to look chlorotic. It is sometimes described as stippling or flecking.
Fine, silky webbing on the undersides of leaves, especially where the leaf attaches to the stem. This webbing is the reason for their name, spider mites.
Leaves change color from healthy green to yellow or bronze and eventually fall off.
A fuzz of little moving dots when you inspect the bottom of a leaf. Spider mites are hard to see on their own but often make colonies of hundreds.
If you aren’t sure, shake some of the dots off onto a white sheet of paper and see if they start to move around. They are hard to see with the naked eye, but a handheld magnifying glass or 10x hand lens will show you their actual forms.
What Causes an Infestation of Spider Mites?
Dry, dusty conditions and high heat are what spider mites love. As the temperature approaches the upper end of the thermometer, spider mites go into overdrive. They can reproduce several generations in a short time during high summer temperatures.
Spider mites overwinter in the soil but can also be blown to new areas by the wind. They can hitch a ride into the house on an unsuspecting gardener or pet, or that new houseplant you brought from the store. Spider mites in the home usually have ideal warm, dry conditions and a lack of natural predators. Prime conditions to take over your prized pothos.
A lack of natural predators caused by the overuse of broad-spectrum insecticides can also contribute to a population explosion outdoors. Spider mites are themselves the food of other insects, which may be killed off by the chemical spraying of pesticides.
Control and Prevention
If you have a spider mite infestation, it can be controlled. For home gardeners and market growers, several remedies are available.
Of course, prevention is better than fighting the eight-legged hordes later in the summer. Like any pest, their numbers will rapidly multiply when conditions are ideal.
Drought-stressed plants are more likely to attract spider mites. Keeping your garden well watered and your soil fertile will go a long way toward helping to keep your plants strong and able to shrug off a few mites.
Right plant, right place is the central pillar of Integrated Pest Management. If you have known dry areas of your property, use drought-tolerant plants for those hot, dry locations where mites might otherwise be a problem.
Keep houseplants well-watered and fertilized. Consider using a humidifier to keep your home from becoming too dry. Most houseplants like some humidity.
Treatment for Spider Mites
If you have detected spider mites that are still active and running about, it’s time to take action. A few options are available and depend partly on whether your problem is indoors or outdoors.
Spider Mites on Indoor Plants
If your houseplant is severely damaged, it may be best to remove it and quarantine it before trying the actions below.
Wash ‘em Off
Spider mites on houseplants can often be washed way. If your plant is of manageable size, take it to your sink or bathtub and use the sprayer attachment to give the underside of the plant a good washing. Don’t be timid. Spray them right off. You can gently rub the leaves while spraying to help knock them loose.
Soaps and Oils
Insecticidal soaps work well on houseplants. They are usually available at your local home and garden center and are inexpensive.
Apply them thoroughly to the entire plant, following the label directions, but take special care to wet the undersides of the leaves well. Three to four applications a day or two apart may be needed to finally get the mites under control.
Check out this excellent page on Less Toxic Insecticides from Clemson Cooperative Extension if you want to dive deeper into these products and their benefits.
Neem oil can also be used. While the insecticidal soap attacks the soft shell of the adult mites and dehydrates them, Neem oil suffocates them and interferes with their egg production. No more eggs means no more mites. Like before, make sure to coat the undersides of all the leaves. Don’t leave them any refuge!
Spider Mites on Outdoor Plants
Get the Hose
Spider mites in the garden can often be treated with your hose nozzle. A vigorous spray will knock many of them off, but you must spray the undersides of the leaves. Don’t be so zealous with the hose pressure that you break your plants.
Soaps and Oils
Insecticidal soaps work well in the garden too. Save your hands from constant squeezing of the spray bottle and save some time by mixing the soap in a pump-type spray bottle. Quarter gallon or half gallon handheld models are available, or larger 1-2 gallon types can be used if treatment area is larger.
These pump sprayers provide a better application rate and will be more effective. Both Neem oil solutions and insecticidal soaps may be applied in this way. Three to four applications will likely be necessary. These chemicals must contact the mite to kill it. Avoid spraying on water-stressed plants, as it can affect weak plants, too.
Use Good Bugs
Biological controls are also available for use in the garden and can provide longer-term control with less work. They are harder to find, and you will likely have to order them from a greenhouse or online. Search for “predatory mites,” and you’ll likely get the right ones.
Phytoseiulus persimilis is the most common predatory mite used. Neoseiulus fallacis is also used for lower densities but longer protection times. Predatory mites will attack all stages of the spider mites but may migrate away once their food is gone or eat each other.
More information on predatory control of mites can be found in the Mites Management Guidelines published by the University of California’s Integrated Pest Management program.
–Andy Wilcox is a Master Gardener with 25 years of gardening experience (mistakes) to benefit Almanac readers. He is the founder of That Gardener Writer Guy.