Spider Mites on Cannabis: How to Spot Them Before They Take Over

Spider mites on a cannabis plant are usually the two-spotted spider mite, Tetranychus urticae, one of the most common and destructive pests in any grow. They're tiny arachnids, barely visible to the naked eye, that can do serious damage if you don't catch and control them early.

Spider mites feed on the sap in the leaves, leaving yellow speckles and an overall discoloration that weakens the plant and drags down the quality of your harvest.

Knowing what they look like and how they behave is the key to preventing infestations and dealing with them effectively.

The two-spotted spider mite (Tetranychus urticae)

The spider mite, or two-spotted spider mite (Tetranychus urticae), is a small but seriously destructive cannabis pest. These mites usually measure less than 1 millimeter and are recognizable by their red or yellowish color. Their size makes them hard to see with the naked eye, but the damage they leave behind gives them away.

What spider mites do to a cannabis plant

Spider mites mostly hang out on the undersides of leaves, where they feed by sucking out the contents of plant cells. That leaves tiny yellow spots on the leaf surface that can spread into wider discoloration. Over time, if the infestation goes unchecked, the mites spin fine webbing over leaves and buds, which makes them easier to identify.

What conditions help spider mites thrive

Spider mites love warm, dry conditions, which let them breed fast. That population boom can turn into a full-blown infestation in no time, making control a lot harder.

Signs of an infestation:

• Early stage: The first signs are tiny white or yellow pinprick spots. They cluster along the main veins of the leaves, causing discoloration that can fade to a light green or grayish tone.
• Middle stage: The leaves lose their color completely and may drop. Fine webbing becomes visible on the undersides of the leaves and even on the flowers.
• Late stage: A severe infestation covers the plants in webbing completely, which can kill the plant if you don't step in.

Catching and treating spider mites early is crucial to protect your cannabis plants and avoid major losses at harvest.

How to identify spider mites on cannabis

Spider mites (Tetranychus urticae) may be tiny, but identifying them is key to protecting your plants. Here's how to recognize them and what to look for.

What do spider mites on weed look like?

Spider mites range in color from greenish-brown to bright red. They tend to move slowly across the plant, which makes them easier to spot. In outdoor grows you may run into a similar-looking mite that's harmless to plants; it moves much faster than a spider mite and doesn't cause damage.

Where to look:

• Tops of the leaves: Spider mites feed on leaf sap, leaving small whitish or grayish dots that show they're active.
• Undersides of the leaves: This is where spider mites lay their eggs, much like whiteflies do.

Symptoms and warning signs:

1. Whitish or grayish dots: As they feed, spider mites leave small, telltale speckles on the leaves.
2. Reddish smear on your finger: If you run a finger along the underside of an infested leaf and it leaves a reddish streak, you're most likely dealing with spider mites.
3. Webbing: In advanced infestations, spider mites spin fine webs on leaves, stems and flowers.

How to confirm it:

• Use a loupe or microscope: The most reliable way to identify spider mites is with a magnifier or microscope of at least 100x. Check the undersides of the leaves. Spider mite larvae have 3 pairs of legs, while adults have 4 pairs.

Spider mite life cycle and habits

Spider mites are common in outdoor grows, indoor grows and greenhouses alike. They prefer warm weather, so they usually show up in spring and summer. They're tough and can overwinter on plants, weeds and wood. Thoroughly cleaning your containers and grow tools is crucial to keep them from carrying over from one season to the next. And don't reuse growing medium from an infested grow.

In peak summer, spider mites complete their life cycle in just over a week, and they're extremely prolific. Each female can lay 3 to 5 eggs a day, which makes infestations explode fast. When they run out of food, they gather at the leaf edges and ride the wind to new plants.

In grows with poor ventilation, underwatering or low humidity, and in spots where heat builds up, like glassed-in porches or sunrooms, spider mites take off. Good ventilation and smart watering are essential to keep them from spreading and protect your plants.

How to prevent spider mites in your cannabis grow

Keeping spider mites off your cannabis plants is crucial for a healthy, heavy harvest. Once they're established they're a pain to get rid of, so prevention is the name of the game. Here are the key ways to keep spider mites from showing up:

1. Watch your feeding: Avoid overdoing nitrogen in your fertilizers, since high nitrogen levels can attract spider mites and other pests. Keeping your nutrients balanced helps your plants stay healthy and less prone to infestations.
2. Plant placement: In outdoor grows, keep your cannabis away from other pest-prone plants, especially ones on patios or balconies where spider mites may have spent the winter. That cuts the risk of mites hopping over to your plants.
3. Ventilation and exhaust: For indoor grows, make sure you have proper ventilation and an exhaust fan. High temperatures help spider mites multiply, so good airflow goes a long way toward stopping them. For more on dialing in your airflow, check out our post on ventilation for indoor cannabis grows.
4. Keep humidity in range: Spider mites prefer dry environments and go after underwatered or weakened plants. Keep humidity and watering where they should be to discourage them from moving in.
5. Grow hygiene: Cleanliness is everything. Don't walk into your grow space in street clothes, so you don't bring in pests. And clean your tools and grow area regularly.
6. Regular inspections: Check your plants often for early signs of spider mites, like eggs and adults on the undersides of the leaves. They tend to lay eggs in clusters, so pay extra attention to those spots.
7. Biological products: Consider biological products like GHE's Bio Protect or Green Hope's Bio Protect to strengthen and protect your plants. These products help prevent pests naturally.
8. Preventive treatments: Use biological preventives like Cannacure or Leaf Coat, which form a protective film on the leaves, making it harder for the pest to settle in and reducing plant stress.
9. Compost and manure: If you use homemade compost or manure, make sure it's fully broken down and pest-free before it goes anywhere near your plants.
10. Post-grow disinfection: After a grow with spider mites, thoroughly disinfect the grow room or tent. You can use Solfac Automatic Forte (carefully, and following the label instructions) to make sure no mites are left behind.

Putting these practices to work will help keep your plants spider-mite-free and set you up for a successful, high-quality harvest.