Spider Mites on Cannabis: The Bane of Growers Worldwide

If you haven’t dealt with spider mites yet, count yourself lucky. These things are the absolute worst. Sneaky, tiny, and relentless. Most pests at least have the decency to show themselves, but spider mites? They’re more like plant vampires, quietly draining your grow room behind the scenes, only revealing themselves when the damage is already done.

They don’t buzz around, they don’t leap, they don’t even really crawl in a way you can spot with the naked eye. At a glance, they just look like specks of dust. But they’re very much alive and hungry.

So What Are Spider Mites, Exactly?

Spider mites aren’t actually insects; they’re arachnids, which means they’ve got eight legs and a bit of an attitude problem. There are a few types that can show up in cannabis grows, but the one you’ll hear about the most is the Two-Spotted Spider Mite (Tetranychus urticae). It’s the most common and, unfortunately, also one of the most destructive.

You might also come across:

• Carmine spider mites: reddish, more common outdoors
• Strawberry spider mites: also a Tetranychus species, often found on, you guessed it, fruiting plants
• Broad mites and russet mites: not technically spider mites, but worth a mention because growers often confuse them when symptoms overlap

But the two-spotted mite? That’s public enemy number one.

You’ll know them by their signature: tiny yellow or white speckles on your leaves. That’s where they’ve been piercing individual cells and sucking them dry. Look closer, and you might spot the actual mites, pinhead-sized, pale with two dark spots on their backs. If you’re seeing webbing? Things have already escalated. That fine silk they spin is for protection and breeding, and by the time it’s visible, their numbers are out of control.

How Bad Are They, Really?

Pretty bad. Spider mites can go from a minor annoyance to a total crop-ruiner in no time. What makes them especially brutal is their reproductive speed, at warm temps, a single female can lay hundreds of eggs, and they hatch fast. Before you know it, you’ve got multiple generations living, feeding, and breeding right there on your leaves.

They thrive in hot, dry environments. So if your grow tent is running warm and your humidity’s on the low side? You’re practically inviting them in. And the thing is, they don’t just sit still, they move from plant to plant, spreading like wildfire. Left unchecked, they’ll cause:

• Faded, stippled leaves
• Reduced photosynthesis
• Leaf drop
• Slowed growth
• And in the worst cases, completely ruined buds

It’s not just about yield, it’s about plant health, terpene development, and overall stress levels. And trust me, stressed plants don’t produce their best work.

Catching Them Early (Before the Webs Come Out)

Honestly, you’ve got a small window before spider mites turn into a nightmare. The key is regular, hands-on plant inspection. And I don’t just mean glancing at them under the lights, get under the leaves. That’s where they start.

Signs to look for:

• Speckling or stippling on upper leaf surfaces
• Tiny moving dots under the leaf (get a loupe, they’re worth every cent)
• Fine webbing along leaf junctions or across whole colas if it’s gotten out of hand
• Dry, curled leaf edges, especially near the infestation zone

If you’re shaking a leaf and dust seems to be crawling, yeah, that’s mites.

So What Do You Do About It?

This is where things get tricky. There’s no silver bullet for spider mites, especially not once they’ve got a foothold. You’ll need to attack from multiple angles, and you’ve got to be persistent. Like, every few days, persistent.

A few methods that actually work (and when to use them):

Water pressure rinse

Just blasting them off with water helps a lot, especially outdoors or early on indoors. It’s not a cure, but it buys you time.

Insecticidal soap or horticultural oil

Works by smothering mites and their eggs, but you have to apply it thoroughly. Undersides of leaves, every time. Best in veg, avoid in flower unless absolutely necessary.

Neem oil

Some growers love it, others swear it makes things worse. It’s not very effective once you’re deep into an infestation, but it can help keep numbers down. Again, don’t use it in flower unless you’re okay with the smell and taste sticking around.

Spinosad and pyrethrin sprays

Organic-ish, decent knockdown, but mites can build resistance fast. Rotate these if you’re going to use them.

Predatory mites

Now we’re talking biological warfare. Phytoseiulus persimilis is the go-to. It loves eating spider mites and breeds quickly, but you’ve gotta match conditions to its needs (humidity matters). These work best as a follow-up after you’ve knocked populations down with sprays.

Heat treatments

Spider mites hate humidity and high temps, but your plants aren’t huge fans of extreme heat either. Some folks will crank up the RH to 70%+ and temps to the mid 30s (°C) for a short period to stress the mites. It’s risky, but it’s a trick that works in tight spaces if you know your limits.

And here’s something worth repeating: you’ve got to break the life cycle. Eggs, larvae, adults—all of them have to go. Otherwise, you’re just killing the visible ones while the next generation is already hatching.

The Long Game: Keeping Mites Away for Good

Prevention with spider mites is everything. Once you’ve had them, you won’t want to deal with them again.

A few habits that go a long way:

• Keep your space clean. No old plant matter, no dead leaves sitting in the corner.
• Quarantine new plants, always.
• Don’t bring outdoor plants or even clothes from your garden straight into the grow room.
• Keep your humidity up, especially during veg. Spider mites love dry air.
• Introduce predatory mites preemptively if you’re in a high-risk area (especially if you’ve had issues before).

And above all else, check your plants. Like, actually look at them. The undersides, the fans, the stalks. You don’t need to be paranoid, but you do need to stay sharp. One week of lazy inspection can lead to a month of cleanup.

Spider mites aren’t unbeatable, but they’re definitely one of the tougher pests to deal with once they’ve settled in. If you catch them early, you’ve got options. If you wait too long, well… you’re going to war. Either way, keep your tools sharp, your plants strong, and your eyes peeled.