How to Get Rid of Spider Mites on Cannabis, From Veg to Flower

To get rid of spider mites on cannabis, hit them early: in veg you can spray insecticidal soap, organic pyrethrins, sulfur or targeted biological miticides, and in flower you stick to biological products only, plus hand-cleaning the undersides of leaves. Prevention does the rest: keep temperatures down, don't overdo the nitrogen and keep your grow clean. Below we break down how to identify spider mites, the damage they cause and the best strategies for each stage of the grow.

What are spider mites?

The two-spotted spider mite (Tetranychus urticae) is a tiny mite, about 0.5 mm long, from the Tetranychidae family. It spins webs on cannabis, which is why people often mistake it for a regular spider. Its color ranges from greenish-brown to bright red depending on how mature it is. It usually hides on the underside of leaves and uses its webbing for protection, which works both against natural predators and against some miticides.

Over the last few years, growers have seen a big jump in spider mites in cannabis grows. They used to die off over winter outdoors. But with indoor grows and greenhouses running year-round, plus warmer soil temperatures, they now survive every season, and keep building up resistance.

How spider mites attack cannabis plants

Spider mites don't take big bites out of leaves the way other pests do, like budworms, snails and slugs. Instead, they pierce the leaves over and over, covering them in tiny grayish dots as they suck out sap and cell tissue. In an advanced infestation, spider mites can even wrap your buds in webbing, causing serious harvest losses.

It's crucial to prevent spider mites or wipe them out the moment you see the first signs, because under the right conditions they spread fast and can take over whole plants in no time.

How to identify a spider mite infestation

• Despite their tiny size, spider mites are easy to recognize by their greenish-brown or bright red color and the slow way they crawl across the plant.
• You can find spider mites on both sides of the leaf: they suck sap on the top and lay eggs on the underside, just like whiteflies.
• Spider mites leave telltale tiny whitish or grayish specks on the leaves as they feed.
• If you run your finger along the underside of a leaf you suspect is infested and it leaves a thin reddish streak, there's a good chance you've got spider mites.
• In very advanced infestations, they also spin webs on leaves, stems and flowers.
• The best way to identify them is with a loupe or a microscope at around 100x, checking the undersides of the leaves. Spider mite larvae have 3 pairs of legs and adults have 4. They're easy to tell apart.

What damage do spider mites cause?

• Leaf damage. They can leave countless bite marks on the leaves, which look like tiny white dots to the naked eye. The damage happens as they feed and permanently discolors the leaf.
• They stress the plant out, so it drops leaves and stalls its growth or flowering, depending on how far along the infestation is.
• Smaller, lower-quality harvests. You get less weed and worse weed. Buds finish smaller and can even end up covered in webbing.
• They hurt the flavor of your buds: the bite marks can leave the plant open to rot and disease, and the mites also leave their droppings on leaves and flowers, which obviously doesn't help the taste.

How to prevent spider mites on cannabis plants

• Don't overdo nitrogen when feeding, since it attracts spider mites and other pests.
• Outdoors, keep your weed plants away from other plants, since lots of spider mites overwinter on rooftop or balcony plants.
• Indoors, make sure you've got proper ventilation and an exhaust fan so temperatures don't spike, because heat makes it much easier for them to take over. For more on airflow, check out our post on ventilation for indoor cannabis grows.
• High humidity doesn't favor spider mites, which is why they tend to go after underwatered or weakened plants.
• Keep your grow spotless, and try not to walk in wearing the clothes you've had on outside.
• Check your plants regularly for adult mites, eggs and so on. Look for eggs on the undersides of leaves, and remember they're usually laid in clusters.
• Use GHE's Bio Protect, or Green Hope's Bio Protect, to strengthen and protect your plants biologically.
• You can also use other biological preventives like Cannacure or Leaf Coat, which form a film over the leaf that makes it harder for pests to settle in and support growth by reducing stress.
• If you use homemade compost or add manure to your garden, make sure it's fully composted first and pest-free.
• If you just finished a grow that had spider mites and want to make sure none are left in your grow room or tent, you can use Solfac Automatic Forte (toxic: read the instructions carefully).

How to fight spider mites during the vegetative stage

Once you see clear signs of an infestation, your only option is to fight back as hard as you can and get rid of them as soon as possible. The good news: in veg you've got more chances to knock spider mites out fast. Plants are smaller at this stage, you can spray without worrying about damaging buds, and you can use both biological and chemical miticides.

Best biological sprays for spider mites in veg

• If spider mites have set up shop on your plants in veg, we recommend applying a potassium soap (insecticidal soap) like Mobet alongside or combined with organic pyrethrins like Spruzit, which also contains rapeseed oil that smothers spider mite eggs.
• You can also use other biological miticides made specifically for spider mites, like BAC's Plant Vitality.
• Another solid option for getting rid of spider mites is natural sulfur like Elosal, which also knocks out fungi such as powdery mildew.

Chemical insecticides during veg

If you grow indoors and recently had an infestation, you can make sure they're completely gone with Solfac Automatic Forte, an automatic fogger.

How to get rid of spider mites during flowering

During the flowering stage, stick to biological miticides only: chemical ones linger on the plant far too long, so applying them now could do real harm. Just like in veg, you can also bring in natural predators to wipe out the infestation.

Biological sprays you can use in flower

• Flowering is the worst possible time for an infestation to grow, so stay on top of your treatments and apply them correctly to avoid creating new problems.
• To fight spider mites, you can use effective biological miticides like Spruzit, which also kills eggs. You can combine it with Mobet potassium soap, too.
• Another one of the most popular miticides, thanks to how well it works, is Plant Vitality.
• Check your plants daily, especially the undersides of the leaves. To remove adults, larvae, protonymphs and eggs by hand, mix any of the 3 products above with water at the rate the manufacturer recommends, then wipe the eggs off the undersides of the leaves with a cotton pad soaked in that solution.