One day your plant looks great, the next day it’s mottled and covered in webbing. It’s a disaster!
There are many species of “spider mites” but for this we’ll be discussing the most common plant pest: The Two Spotted Spider Mite (Tetranychus urticae) and just using the term “spider mites” to address that one species. For other spider mite species, the solutions are simple: Stratiolaelaps in the soil and Fallacis on the leaves. (More on both of those below.)
Spider mites can quickly overcome plants during their preferred conditions. The rest of the time they exist in small numbers and are usually kept in check by some common natural predators that eat them, but won’t control them when a spider mite population is ready to explode. Almost always, when spider mites take over, some sort of human intervention is required to save your plant.
Here’s everything you need to know about these tricky pests.
- Mites are related to spiders, but are not spiders. They are tiny, eight-legged (and in this case) phloem (sap) feeders.
- Spider mites have a toxin in their probing mouth piece that irreversibly damages the plant cell it pierces. Spider mite damage is observed as these tiny spots or mottling of discolouration on plant tissue (usually lighter or bleached-looking.).
- Two spotted spider mites get their name because of two black spots on the backs of some mature spider mites. Their bodies are translucent, so the black spots are their poop.
- Spider mites create a webbing used for transportation, protection from predators and to keep them off the more humid plant surfaces.
- Spider mites prefer dry conditions. They want low humidity, air flow, and high temperatures. With humidity below 70% and temperatures above 24C their populations will grow extremely rapidly.
- Spider mites were seldom a concern as an indoor pest until growers stopped using overhead watering (which suppressed the mite populations.)
- Now, spider mites run rampant on indoor plants and decades of harsh chemicals and miticides have made them amongst the most chemically-resistant of all common plant pests. If there was a chemical that would effectively keep spider mites off all plants, I wouldn’t be in business.
How to control Two Spotted Spider Mites in your House:
Chances are you brought a plant in from outside, bought an infected plant or have one by an open window that now has spider mites on it.
Immediately assume all plants nearby have spider mites too.
Mist your plants.
You don’t run a risk of overwatering if you keep the soil on the dry side and mist your plants regularly. This misting inhibits spider mites movements, increases the survival of natural predators and (most importantly) can cause an internal bacteria in their gut to bloom and kill them. Yes, mist your plants.
Don’t get in the habit of thinking plants don’t like wet leaves. YOU brought them indoors, when they’d normally experience rain. Most plants want high humidity (as long as there is transpiration) and roots on the drier side for oxygen reasons. Grow happy plants and they have a better chance.
Move your plants away from the window.
Yes, they need light. But what tends to happen is the window creates a temperature differential and therefore air movement. When spider mites are an issue it is usually because cool air is being warmed by the sunny window, drying and causing humidity to plummet. While controlling for spider mite, move the plants to a more humid area or humidify with some other method.
Any pest that feeds on phloem (whitefly, aphids, scale..etc) benefit by the nutrients you’re pumping into the plant. Remember: plants don’t always need fertilizer, so drop it down. Read: All the uncomfortable facts about fertilizer.
Use a Trap Plant.
Buy beans. Buy a bush-bean like “Strike” or “Provider.” Put beans in a pot, watch them grow. Put them next to your house plants. Beans are so preferable to spider mite that they will cross a room to get to them. Commercial growers use beans in greenhouses and nurseries for this reason. In winter clean-out they just place beans in the middle of a greenhouse and then throw them in the compost when they get loaded with spider mites. When the plants stay clean, it’s time to bring in the actual crop. During growing season these same beans (new ones, grown every couple of weeks and replaced regularly) are used to scout: Check them every day. When you see spider mites, apply predators to the house. It’s easy. And for a few dollars worth of beans you’re saving a few hours of scouting time. (Other plants can be used, but beans are cheap and fast-growing.)
Apply a predatory mite:
Phytoseiulus persimilis (persimilis) is a great candidate. It is inexpensive and only eats two spotted spider mites (although it can eat whitefly eggs in controlled trials). You put them on your plants, they eat all the spider mites, then they go elsewhere. But you’ll still need to increase humidity and try to drop your temperature below 24C to help these beneficial mites get the upper hand.
Another option is Neoseiulus fallacis. It’s a premium product, and costs more, but it prefers two spotted spider mite and eats so many other pests that it usually stays with your plants permanently. So while you’ll spend more, it may actually cost you less in the long run.
Below: Fallacis are difficult to see: very small and translucent, as pictured with egg and nymph. Persimilis (large and singular) are bigger and red-orange. They are still about the same size as spider mite and can be difficult to see, as in the zoomed-out picture of persimilis and spider mites on a leaf. (The orange dots are the predators)
Apply the Spider Mite Destroyer, Stethorus punctillum.
Stethorus is best for commercial growers, or a home grower with a room dedicated to plants or a small greenhouse because Stethorus is a little black ladybug that flies. Open the lid and they will source out any species of spider mite and destroy them. It is the new industry standard, but will certainly cost more than the mites. Like fallacis, they are native to North America and tend to exist all season or even into subsequent years.
Notes on using beneficial insects and mites:
All of the above beneficial insects and mites are lab-reared in commercial insectaries. They are inspected for quality control and assurance. Do not buy wild-collected beneficial insects and mites. Read more, if you don’t know why.
If you are unable to increase humidity, reduce fertilizer, adjust temperatures or reduce wind speed, the beneficial insects and mites will still work, but you will need much more than the recommended rates to get control. Usually fan speed can simply be reduced to allow for flight, and for mites to move freely amongst the plants.
However, when you see webbing, it means the spider mite infestation is extreme and this should be dealt with by washing them off the leaves. Do not worry about washing away predators at that point unless you have applied and are seeing beneficials. Save your plant first!
No beneficial insects and mites are compatible with pesticides. Don’t believe anyone or any label that says so. Predators have little to no chemical resistance, and pests do. Anything you use will disproportionately reduce predator populations and in a short time you will have the pests return without the predators.
If washing any pest off your plant, never use soap.
Especially in the case of spider mites! Spider mites prefer some plants over others. What they absolutely love is soft plant tissue like that of plants that look matte instead of glossy. In some cases this is because the glossy look is part of a plants’ defence from sun, rain, pathogens and…pests. Soaps are sometimes effective pesticides but they also wash and destroy these protective layers on the plant tissue. This will INCREASE spider mite feeding. Do not use soaps. (And for the same reason, oils and acids).
Finally, do yourself a favour and put the in-soil predatory mite Stratiolaelaps scimitus into all potted plants, root balls, and garden beds. Spider mites overwinter (avoid freezing) by burrowing into the ground. Here, the predatory mite Stratiolaelaps will eat them. This is an important step in controlling spider mites that seem to return year after year. It’s also part of an organic clean-out of greenhouse spaces. Furthermore, you get the added benefit of flea control (yep, they have to have a soil stage), fungus gnat control, root weevil control and more.
Tips for Outdoors:
Spider Mites are seldom a problem outdoor because of the numerous natural predators. However, some plants or situations can lead to spider mite problems. For example, the hop industry and hemp industry have to control spider mites preventatively. Both plants are closely related, and have the perfect leaf texture for spider mites. Furthermore, because of concerns over mildew resulting in drip irrigation, and hot dry summer climates being preferable, the leaves are seldom washed. Also, in both cases, sometimes plants are trimmed to thin them out (for fear of mildew) which destroys the humid undergrowth area suitable for predators or where slow-generation predators have laid their eggs.
The easy solution is to hose your plants down, allow for rain (if it’s covered) and stop aggressively pruning. But when you really cannot do that, then it is important to interplant with those that may be suitable hosts for predators. Read more about that here.
As you can see, there are several ways to control spider mites, and in most cases it is to grow plants as close to “naturally” as possible. However, spider mites remain an economically important pest, and they certainly shouldn’t be overlooked.
Our greatest successes in spider mite control (as an industry) have been to inoculate large scale (thousands of acres) crops with the predatory mite Fallacis or the predatory beetle Stethorus. Both are regularly used in Hops, cannabis, arborvitae, raspberries, strawberries and more. Both work preventatively, and without any harsh chemical applied, will continue to work year after year.
On a small scale, these predators will work for you as well. If you’re looking to buy predators, check out our list of retailers and distributors here: Distributor Map/List