It’s been hot and dry for days. A fine webbing drapes across the leaves and flowers of your rose bush. The leaves are speckled, yellow, and sickly; some are turning bronze. The flower petals are brown and have a burnt look to them. A closer look shows tiny specks in the webbing. Oh no, spider mites!
There are over 1200 members of this species in the order Arancia, in a subfamily of mites called Tetranychidae, related to ticks, scorpions, daddy longlegs, and spiders. They attack all types of plants, both indoors and out, and they thrive in hot, dry weather, especially when temperatures are 85°F and above and the humidity is less than 60%.
Their two fangs pierce into a plant’s leaf surface and suck out the plant juices, leaving behind dead plant cells that quickly become small white or brown spots. These dead spots give the plant a speckled appearance.
Spider mites are very hard to see with the naked eye. You will need a magnifying glass loupe to see the two body parts and eight legs of this pest. Different species are different colors – they can be red, brown, yellow, or green. But you don’t have to see them to be aware of their presence – the damage they cause is very distinctive, and the web they build is the easiest way to identify these pests.
If you suspect these little pests, take a piece of white paper and hold it under a leaf. Give the leaf a shake or tap and look at the paper. If you see tiny moving dots, that’s spider mites. If you smash them with your finger, they will leave a green streak. If they are beneficial predatory mites (more on this later), the streak will be orange or yellow.
Their tiny size makes it easy for them to travel on the wind, using a bit of webbing to catch the air currents; this is called “ballooning.” They can also hitch a ride on clothing and animals – including the fur of your pets.
Spider mites can complete their development in 5 to 20 days, depending on the weather. Eggs are laid in webbing and on the underside of leaves and hatch in 3 days. The larva has three pairs of legs. There are two nymph stages before the adult stage. Nymphs and adults have eight legs.
Once they are adults, they reproduce quickly, and there can be several overlapping generations all living in a colony together. Depending on the species, they can overwinter as eggs or adult females. They settle into cracks and crevices, under tree bark, in leaf litter, and on weeds. Some of them are active early in the season – starting about the time magnolias are blooming. In warm climates, they reproduce and feed all year with no overwintering dormant stage.
The most destructive species is the two-spotted spider mite, so named because of the two spots on its back. These “spots” are food contents that show through its translucent body. Two-spotted spider mites thrive in hot, dry weather and are attracted to drought-stressed plants. They are damaging to hundreds of plant species, including houseplants, herbaceous garden plants (both annual and perennial), roses, vegetables, fruit trees, greenhouse crops, trees, shrubs, junipers, and other evergreens.
They feed on the underside of leaves and cause the foliage to take on a speckled, stippled appearance. As they continue to suck out plant juices, the leaves turn a speckled yellow, gray, or bronze. Leaves die and fall off. In severe infestations, they can kill the plant.
While spider mites can infest your houseplants, they cannot invade your house itself. They feed exclusively on plant juices, not carpeting, clothing, upholstery, bedding, or other household items, nor do they “bite” you. They can hitch a ride on your dog or cat, but they do not cause problems for your pets. If they are not on living plants, they will soon die from lack of food.
Other species feed on more specific plants, such as the spruce spider mite that feeds exclusively on evergreens and are more active in the cooler weather of spring and fall.
Controlling Spider Mites
Because they are so hard to see and appear from seemingly nowhere, it is easy for gardeners not to notice these pests until symptoms appear. Good scouting practices are an important first step in controlling these tiny pests.
Keeping your plants clean can help prevent an infestation from taking hold. A strong spray of water from the hose on your outdoor plants’ foliage, or a trip to the kitchen sink for a shower under the sprayer for your indoor plants, can keep leaves clean and dust free. Dry, dirty, dusty plants are a strong attractant.
These pests have several natural enemies – ladybugs, green lacewings, ghost ants (themselves a greenhouse pest), damsel bugs, and predatory thrips. If you see these insects on and around your plants, know that they are feeding on spider mites (and aphids and other pests, too!).
There are also predatory mites. These garden friends have longer front legs (you will need a strong magnifying glass to see that) and are more active and quick-moving than spider mites. Remember that when squished, they leave an orange or yellow streak behind. Some predatory mites are available for purchase online. Look for: Metaseiulus occidentalis, Phytoseiulus persimilis, and Phytoseiulus longpipes. Also, look for spider mite destroyers (yes, that is their name, not just their job), which is a species of a ladybug.
Use a miticide, not an insecticide, but keep in mind that they can become resistant to miticides quickly. Eggs are invulnerable to chemical controls, so be sure to know the life cycle of the specific spider mite you are trying to combat. Use chemical treatments every 7 to 10 days as generations overlap.
There are contact miticides that must touch the pest to be effective. There are also systemic miticides that are mixed with water and applied to the soil, and are then taken up by the plant. When the pests feed, they take in a lethal dose.
Using a broad-spectrum insecticide can actually cause an increase in spider mite populations because these products will kill the beneficial insects that prey on them. Insecticides do not kill spider mites!
Insecticidal soap, horticultural oils, and neem oil work to smother pests, and these can be an option for control. The product must cover the pest entirely for it to work. However, these can also kill the beneficial insects that will take care of the pests for you.
Many recipes are out there for homemade “natural” insecticides, many involving apple cider vinegar, cayenne pepper, dish soap, and other household staples. While some of these products may kill pests, they can also cause harm to plants. We recommend using commercial products that have been scientifically developed to deliver the least amount of active ingredient to be the most effective without damage to plants. You’ll protect your garden and save money in the long run.
Spider mites are attracted to drought-stressed plants, so keep your plants well hydrated. Watering with cold water may be helpful because these pests don’t like the cold. For indoor plants keep the humidity above 60%. Remember, spider mites thrive in hot, dry environments.
They also like dusty and dirty plants. So hose off your garden plants if it hasn’t rained in a while (do this in the morning to avoid conditions that invite fungal diseases). Clean your houseplants often, wiping leaf surfaces – both upper and lower – with a soft, damp cloth.
When you bring home a new plant, inspect it carefully and put it in quarantine for a few weeks. This is a good practice not just for spider mite infestations but also for other disease and insect problems.
Again, inspecting your plants often for pests is one of the best preventative measures you can employ. You may not be able to stop every plant problem but learning to recognize pests and diseases in the early stages and using good cultural practices helps keep your garden happy and healthy.