If you have never heard of the two-spotted spider mite, then congratulations. Spider mites are a common garden pest that every indoor grower will experience eventually. Before we discuss treatment and prevention of this bug, lets get to know the spider mite a bit better.
Common Name: Two-spotted Spider Mite
Scientific Name: Tetranychus urticae [Acari: Tetranychidae]
There is some debate regarding the taxonomic placement of this garden pest, I have used the name prefered by entomologists at U.C. Davis.
Affected crops: Too numerous to list. Let us focus on this pest as it applies to growing marijuana.
Spider mites are very small, typically a mere 1/50th inch in length (.4 mm). The spider mites body is separated into two parts: the mouth or gnathosoma, and the body or idiosoma. The mites body ranges from green to yellowish orange, the Two-spotted spider mite is characterized by two dark spots on the idiosoma. Closer inspection will reveal the mites body is semi transparent, and the coloration of their spots are deposits of stored waste. Spider mites have six legs during their larval stage but grow an additional set as they enter their two nymphal stages of development. This additional set of legs remains with the mite throughout its adult stage.
Adults reach maturity in 5-20 days, depending on temperature. A single female spider mite can lay as many as 20 eggs each day and up to 200 throughout her life. The eggs, which she deposits on the undersides of leaves are spherical, clear and about 0.14 mm in diameter. After reaching maturity, adult spider mites live an additional 2 to 4 weeks. Two spotted spider mites have many overlapping generations each year and can rapidly increase their numbers.
Spider Mites have sharp, slender mouthparts used for piercing the plants epidermis. This puncture serves the mite by delivering plant sap that constitutes the mite’s primary diet. Injury sustained to the plant in this manner commonly results in a localized collapse of the hosts mesophyll tissue. Within hours the affected area develops chlorotic spots, resulting in lost productivity for that section of leaf.
Look for small yellow or bronze spots along your plants leaves. Spider Mites prefer to conduct their business on the undersides of leaves but soon expand their territory as the mite colony grows. Within days or hours plants can become completely inundated by mites and covered in webs that provide transportation to different areas of the host plant.
Spider mite infestations can quickly grow out of control. The effects of uncontrolled spider mites can be so devastating that some growers abandon entire plants or indoor cycles. Minor problems during vegetative growth can quickly escalate into pandemic infestation a few weeks into flower.
By the third week of the flowering cycle, it is too late to use many popular sprays. One of the biggest concerns with spraying flowering marijuana plants is the potential for pesticide residue to affect the buds flavor or safety. Additionally, flowering marijuana plants tend to have greater phytotoxic reactions to sprays, which makes it easy to do more harm than good. If sprays must be used on flowering marijuana, reduce the light intensity by raising the lamps. After a day or so the lights may be returned to an appropriate height.
Considering the spider mite’s rapid reproduction, overlapping mite generations, and the ineffectiveness of sprays on eggs, regaining control can seem impossible. It is best to prevent infestation through a good integrated pest management program (IPM).
Effective Pesticides for Spider Mites
An entire rainbow of miticides are available at your local garden supply, or hardware store. Many of these are perfectly safe for use, while others should be avoided. When selecting pesticides be sure the product is approved for use on food crops. Below is a list of some safe and effective sprays for spider mite control.
|Product||Active Ingredient||Degrades After||Effective Against|
|Safer Insecticidal Soap||Potassium salts of fatty acids||7-10 days||Adults, nymphal, larval|
|Safer Bioneem||Neem seed extract||7-10 days||Nymphal, larval|
|Einstein Oil||Neem seed extract||7-10 days||Nymphal, larval|
|Dr. DoRights||Colloidal suspension of herbal oils||N/A||Adults, nymphal, larval|
|PyGanic||Pyrethrins||10 days on soil, 1-3 in air and water||Egg to adult|
|SMC (spider mite control)||Plant extracts, vegetable oil||N/A||Egg to adult|
Most of the products listed here are contact killers, so their effectiveness depends on spray coverage. Smaller plants can be dipped, to ensure even coverage but use special care with oils as they can trigger acute phytotoxic reaction. Because sprays that are considered to be ‘safe’ are generally ineffective against spider mite eggs, a follow up treatment should be applied 4-7 days after the first.
Neem oil is an extract from the seed of the neem tree. The Azadirachtin found in this extract interferes with mite and insects ability to molt, affected bugs are then unable to enter their next stage of development. Neem products are systemic, meaning they are able to enter the plants system and are circulated throughout the plant.
There are specialized miticide sprays that can effectively treat infestation in as little as two applications. These sprays may sound like an attractive method for treating spider mites, but they are not intended for use on marijuana, vegetables or any crop grown for human or animal consumption. The sprays listed below must never be used on marijuana:
These products are only approved by the Environmental Protection Agency (EPA) and/or the United Stated Department of Agriculture (USDA) for use on commercial ornamentals. To prevent misuse by amateur gardeners, many of these are not sold at your local garden supply store.
The only possible exception, while not technically legal, may be for use on marijuana crops not intended for consumption. The only example I can imagine would be a breeding program where seed is the only intended harvest.
Integrated Pest Management (IPM)
Spider mites adapt rapidly to adverse conditions; so control is best accomplished through a combination of methods.
Insecticidal and Miticidal Sprays
Avoid repeated applications of sprays that contain the same active ingredients. Spraying the same product repeatedly allows the mite population a chance to develop an immunity. Mites will almost always be present in different stages of development and insecticidal and miticidal sprays are rarely effective against mites in all stages of development. An application of spray might work to effectively kill all the adult and nymphal mites but spare mites in larval and egg stage. This means survivors, and survivors are bad for two reasons:
- Survivors will re-establish mite colonies.
- Mites which have survived one round of spray, can pass immunity to succeeding mite generations, thus weakening the effectiveness of the product.
To limit survivors, it is best to plan a rotating spray schedule of at least three products. This trifecta should include sprays that target mites throughout their various stages, from egg to adult.
In addition to sprays, spider mite colony suppression can be aided through responsible gardening practices
- Remove dead vegetation from the plants and grow room
- Keep floors and walls of the grow room clean and disinfect periodically
- Quarantine clones and plants from outside sources
- Regularly inspect plants for the presence of mites
- Filter intake air and exhaust vent.
Keeping your operation clean is the real secret to mite control. Remember that the gardener can become an outbreak vector when mites are introduced to the growroom on clothes, shoes or gardening tools.
There are several species of beneficial insects that can be used to control spider mites. If you choose to use a biological control to treat spider mites, be aware that some sprays are compatible with biologicals but others are not. Incompatible sprays will kill both the spider mites and the beneficial insects.
The most common type of beneficial insect used against spider mite populations are in fact, also mites. The species listed below are commonly referred to as predatory mites and have proven effective against spider mites under a variety of conditions:
Wide Temp Range, Moderate Humidity:
Temperature Range: 55 – 105+ F.
Humidity Range: 55 – 90%
Moderate Temp Range, High Humidity:
Temperature Range: 55 – 90 F.
Humidity Range: 60 – 90%
Widest Humidity and Temp Range:
Temperature Range: 55 – 105+ F.
Humidity Range: 45 – 90%
Ladybugs can also offer some assistance against spider mites but I don’t recommend them as the sole control method. Ladybugs will happily eat both predator and spider mites, so both should not be used together. My experience with ladybugs in grow rooms has always been a disaster. They seem to be overcome with a desire to fly into the light housings and die by the hundreds. Ladybugs are much better suited for greenhouses.
Predatory mites and ladybugs are available from a number of direct sources, or can be purchased from your local indoor garden supply store. A regular contributor here at MGHQ has compiled a great list of predatory mite suppliers on our forum. Feel free to order from any of these reputable vendors, or add some that you may know of.
Spider Mite Control in Marijuana Gardens,