You can see spider mites with a 30-50X hand-held microscope. Shown here: adults, juveniles and eggs. (Courtesy of Jorge Cervantes via "The Cannabis Encyclopedia")

How to get rid of spider mites on marijuana plants: Keep it clean

Cannabis Cultivation Q&A: A reader wants to know how to get rid of spider mites. These common garden pests can take over quickly, so be as fastidious as possible

Renowned grower Jorge Cervantes will answer readers’ questions and give advice on all things related to the cannabis plant. Got a question for Jorge? Email him at jorge@marijuanagrowing.com.

Editor’s Note: Laws for cultivating cannabis vary from state to state and city to city — before germinating any seeds or planting any clones, take care to learn what your local laws are.

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Question: I think my plants are infested with spider mites. How do I get rid of them?

Answer: The two-spotted spider mite is the most common pest mite to attack cannabis. Other pest mites include the hemp russet mite, broad mite, oriental mite, privet mite and more. This article focuses on the two-spotted one.

Spider mites are one of the most common pests in indoor gardens and greenhouses. Find them in dirty garden rooms and ill-kept greenhouses. Use a 10x to 30x handheld microscope to spot tiny mites (0.04-inch long) on the underside of leaves. Look closely and you can see the minute, translucent eggs.

Spider mites suck the life-giving sap from cannabis, causing loss of vigor and slow, stunted growth. The first sign most gardeners see, called stippling, appears on the tops of leaves as tiny white to yellow specks. By the time most gardeners see stippling on leaves, an infestation is well underway. Fertilized once for life, a female can lay 20 eggs a day. Eggs hatch in 3-4 days in warm weather.

Spider mites multiply super fast and adapt to pesticide controls quickly. Once the pest is discovered, using a full arsenal of methods and substances is the only way to make them go away.

Take precautions to exclude spider mites from your garden room or greenhouse. Spider mites can ride a current of air or hitch a ride on plants, pets and you. Keeping everything inside and around the garden clean is the best prevention. Keep tools clean. Dip dirty pruners in a can of rubbing alcohol to sterilize. Keep the garden area clean — tools, floor, walls, benches, pots, everything.

Stock your arsenal

There are many methods at your disposal to prevent and limit infestations. Preventative measures include:

• Dip (submerge) all new plants — container and all — including clones before they enter your garden area, in a separate container of miticide. Note: Many miticides can be toxic. Educate yourself about the options and proper use. Keep reading for more info.

• Temperatures below 60ºF and high humidity slow reproduction immensely.

• Wipe sticky Tanglefoot around container edges, the base of plant stems and at each end of drying lines.

• Remove foliage with more than 50 percent damage.

• Introduce predatory mites, available online and at a few garden centers.

Safe sprays

A good first step to battle mites is with water. Always spray first thing in the morning. Spray leaves, especially undersides, with a jet of cold water to blast the mites off foliage. For increased effectiveness, lower water pH to 3-4 (think acid rain) by adding nitric acid, phosphoric acid, citric acid or vinegar. Test the pH level and spray with gusto. Don’t worry, the minute amount of low-pH water does not affect soil pH.

When it comes to pesticide products, always use contact sprays, not systemic. Contact sprays stay on the outside of foliage and do not enter the system of a plant.

Contact sprays are safe to use on consumable crops, including cannabis. As a general rule, contact sprays must come into contact with target pests. Often, pests — including larvae and eggs — must be completely covered with the spray to be effective, and multiple applications are typically necessary.

Systemic products are for ornamental plants only and should not be used. These pesticides enter the vascular system of plants, such as through the roots, and travel up into the leaves and flowers, and can be toxic to humans and other mammals.

Insecticidal soap is one of the most common contact sprays. It’s a natural product readily available in garden centers. Soaps are virtually non-toxic to mammals and can be applied to food crops until the day of harvest. Insecticidal soaps containing potassium salts of fatty acids will keep light infestations of mites under control.

Homemade contact sprays include one or a mix of several of the following: Dr. Bronner’s soap, garlic, hot pepper, canola oil, citrus oil, liquid seaweed, and milk. Search online for recipes. Homemade formulations can cause injury to plants, so a small test application is recommended.

Products containing neem oil, pyrethrum or horticultural oil are stronger contact sprays. Neem and pyrethrum are derived from plants; horticultural oil can be petroleum- or plant-based. Always apply horticultural oils with extreme care and when temperatures are low.

Rotate sprays so mites do not develop immunity.

Get educated on other options by checking the lists of pesticides approved for use on cannabis by state agriculture departments in the legalized states of Oregon and Colorado. Take note of the active ingredients as well as the product trade names.

Important notes

Always follow ALL directions on the product label, including guidelines for personal protective equipment and disposal, and only use in accordance with the labeling. For example, if a product is listed only for outdoor use, do not use indoors.

And again: Apply only nonsystemic contact sprays approved for edibles — fruits and vegetables. Systemic compounds can be carcinogenic and highly toxic.

Your opinion is valuable:
Please tell us how you keep your garden spider mite free in the comment section below. Our readers want to know your tips and tricks!

See the “The Cannabis Encyclopedia,” available at amazon.com, for expanded control measures of spider mites, hemp russet mites and broad mites.