You might be wondering what all those white spots are on your fan leaves. You might be concerned that your cannabis plants are yellowing and covered in silky webs. And if you look closely, you might see teeny tiny specks are on the underside of your leaves. If this sounds like your garden, then you might have a mite problem. Even if you don’t, you might soon.
Two-spotted Spider Mites
The two-spotted spider mite (Tetranychus urticae) is among the most common cannabis pests. Their damage is tremendous, but they are tiny. Spider mites are a bit smaller than dandruff flakes and barely visible to the naked eye. A magnifying glass reveals white, spherical eggs and eight-legged adults. There are two black spots on the upper back, hence the name.
Spider mites are plant vampires that colonize the underside of leaves. Adults suck out the chlorophyll and sap from plants causing small white dead spots. Photosynthesis slows due to the loss of living cells. The plant hemorrhages moisture from the wounds. Over time, the white spots increase, leaves yellow and the plant dies from a thousand cuts.
A single adult can lay hundreds of eggs within a few short weeks. Under the right conditions spider mite populations increase exponentially in size and scope. In a matter of days, eggs hatch and develop into adults that compete for space on the dying plant. Eventually, the population becomes so big that the colony looks for new plants to infest. They use silky web-like strands for wind dispersal. These filaments act like a sail that carries adult mites to their next victim.
Outdoor mite populations are kept in balance by the natural ecosystem. Heavy rainfall, predatory insects, fungal mite parasites and the annual winter die-off control the population. These conditions are not present indoors and in greenhouses. In fact, modern cannabis cultivation provides the perfect environment, year-round for spider mites! This leads many gardeners to use a lot of chemical miticides in an attempt to eradicate the pest.
Integrated Pest Management
Integrated Pest Management makes your garden inhospitable to spider mites. The goal is to prevent the problem and create a garden ecosystem that suppresses pests.
It’s easy to overlook the early signs of spider mite colonization. Symptoms become obvious once the infestation has progressed. By then you have a major problem at hand. That’s why early detection and constant monitoring are essential to IPM for spider mites.
Make a habit of inspecting your cannabis plants for damage whenever you are working in the garden. Watch for small white freckles on leaves, especially the bottom branches at first. Mites live on the underside leaves and eventually spin a silky web there. As infestation progresses, they produce a dense mass of webbing, eggs and adults at the top of the plant. Overall, the plant yellows and wilts from tissue damage.
Randomly sample leaves from each plant on a weekly basis. Use a 10-60X pocket microscope to inspect the underside of leaves. Look for white spherical eggs and eight-legged adult mites. Both are about 1/20th of an inch in diameter. Routine monitoring with a microscope is the most effective method to catch spider mite problems before they damage your crop.
Cultural Prevention & Treatment
The best way to fight spider mites is to never have them at all. Most mites hitch-hike their way from one garden to another. Avoid cross-contamination by:
Starting plants from seed rather than purchased clones.
Not accepting clones from other gardens.
Thoroughly inspecting and quarantining clones that you do accept.
Showering and changing clothes before tending to your plants.
Thoroughly cleaning used equipment purchases.
Batch harvesting so the room can be emptied and cleaned between harvests
Attend to young plants before older, potentially diseased plants.
Supplement plants with silica to strengthen cell walls and improve mite resistance.
Keep plants vigorous and healthy to improve overall immunity to disease.
Environmental Prevention & Treatment
Spider mites love hot weather. Metabolism and reproduction increase along with temperature. In cool conditions, spider mites take up to 21 days to complete their life cycle. When temperatures exceed 80°F, an egg can hatch and develop into an egg-laying adult in as little as 5 days. Warm temperatures mean mites can reproduce faster than the treatments meant to control them. Any treatment plan needs cool temperatures to be effective. Keep your thermostat set to 68°F or even a little lower, if you suspect spider mites.
The cold slows them down.
Low humidity and drought stress make plants susceptible to mites too. When plants are thirsty they are less able to defend themselves. Beneficial microbes protect plants when the relative humidity is 40-50%. When humidity drops, the fungi that prey on spider mites are not able to do their job. Excess humidity quickly causes fungal problems. Don’t let your humidity spike over 55% or you’ll have a host of fungal pathogens attacking your cannabis plants.
Physical/Mechanical Prevention & Treatment
Physical and mechanical methods obstruct and remove spider mites. Physical barriers prevent infestation and segregate problem areas. Mechanical methods remove established mite populations from the garden. This approach is a safe and cheap IPM.
Seal gaps and cracks in your indoor garden or treatment area.
Partition large warehouses into smaller compartments. This contains outbreaks and enables batch harvesting
Use intake fans to create positive air pressure in your garden structure.
Promptly remove dead plant material from the grow room.
Remove infested leaves.
Cull or quarantine infected plants
Spray the underside of plants with plain water to wash off eggs and adults.
Mop the floors and walls with soapy water every few days.
Thoroughly clean the grow room between harvests.
Wash all grow equipment before reuse.
Do not re-use grow medium without reconditioning it first.
Biological Prevention & Treatment
Beneficial insects and microbes are the natural enemies of spider mites. Biological methods are a highly effective preventive barrier and long-term solution for mites. Chemical methods kill the good along with the bad, so re-inoculate plants with beneficial organisms after using pesticides.
Organic living soils are full of probiotics that form a living “slime” that coats the entire plant. This biological barrier is hostile to spider mites and other pests. Non-organic growers can also benefit by using a a probiotic root inoculant with beneficial mycorrhizal fungus for a more pest resistant plant.
Ladybugs and other mite predators will hunt down and eat spider mites. These insects are commercially available. They need to be released into the garden every week to ensure continuous coverage. The most effective commercially available species are Galendromus occidentalis, Phytoseiulus persimilis, Mesoseiulus longipes and Neoseiulus californicus.
Organic and Synthetic Miticides
Traditional IPM recommends pesticides only when routine monitoring indicates that a problem exists. Chemical treatment is a last resort because it kills beneficial organisms and poses human health risks. Choose the safest pesticide and apply it in the minimal effective dosage.
Rotate between different pesticides to avoid resistance to any one chemical. Most pesticides will kill most, but not all of the population. This leaves the strong, pesticide resistant mites to reproduce. Alternating pesticides provides a one-two punch to take down spider mites.
The most effective and safe time to spray plants is during the clone phase. Complete coverage is easy to accomplish because rooted cuttings are small. Overall less pesticide is applied in comparison to mature plants. When treatment happens early, plants have time to recover and chemical residue can break down before harvest. Just make sure you treat plants when the lights are off. Many pesticides are photosensitive and may damage foliage if sprayed during the day.
Pesticide application should only happen in the veg phase. Never spray blooming cannabis plants with chemicals. Pesticide residue is a health risk for consumers and a legal liability in regulated markets like Colorado.
Retail cannabis in Colorado is tested for pesticide residue. The state also has a limited list of chemicals approved for use on cannabis. Oregon, Washington and Alaska have similar regulations. Check to see what local regulations apply to pesticide application in your garden.
Do research before exposing your plants, your consumers and yourself to agricultural chemicals. Read the label instructions and the Material Safety Data sheets. They contain important usage and safety instructions. Online gardening forums offer sometimes valuable user perspectives and reviews about cannabis pesticides. Protect yourself with a respirator, safety goggles, gloves and disposable chemical-resistant coveralls. Restrict access to the garden after spraying chemicals.
Maybe you’ve never had spider mites. Maybe you did, and you think you’ve won the battle. Spider mite IPM should never stop. Remain vigilant for signs of infestation. Use “many small hammers” to continuously discourage mites from living in your garden. Nip small problems in the bud so your buds can grow big!