The two-spotted spider mite, Tetranychus urticae, is a major challenge for cannabis cultivators. These tiny arachnids reproduce rapidly, especially in the warm, dry conditions often favored by the cannabis plant itself. An unchecked infestation can quickly compromise plant health, reduce overall yield, and even ruin entire crops, particularly during the critical flowering stage. Successful management depends entirely on prompt action and a multi-faceted approach that combines immediate knockdown treatments with long-term preventative strategies.
Identifying the Threat and Assessing Damage
Spider mites are difficult to spot without magnification, measuring less than one millimeter in length. They often appear as tiny, slow-moving specks on the undersides of leaves, with colors ranging from pale yellow to tan, sometimes displaying two dark spots on their bodies.
The earliest sign of an infestation is a condition called stippling, which presents as numerous tiny, pinprick-sized yellow or white dots on the upper surface of the leaves. As the population grows, the stippling becomes denser, causing the entire leaf to take on a yellowed or bronzed, unhealthy appearance.
A severe infestation is unmistakable due to the fine, silky webbing the mites produce across leaves, stems, and between branches. This webbing provides a protective barrier for the mites and their eggs, shielding them from environmental threats and topical treatments. If webbing is present, the population is well-established, signaling that an immediate, aggressive intervention is necessary to prevent total crop loss.
Immediate Steps for Physical and Horticultural Removal
Hosing down the plants with a powerful stream of room-temperature water is an effective, non-chemical way to knock off a large portion of the adult mites and nymphs. Focus the spray primarily on the undersides of the leaves, where the mites congregate and lay their eggs, ensuring thorough coverage of the entire canopy.
This physical washing should be performed just before the lights turn off in an indoor grow, or at dusk for outdoor plants, allowing the foliage time to dry completely in the dark. For plants nearing harvest, a gentle “bud wash” using a solution of water, hydrogen peroxide, and lemon juice can help remove mites and webbing from the flowers themselves.
Once the plants are dry, horticultural treatments are applied to kill the remaining mites and unhatched eggs. Insecticidal soaps and horticultural oils work by contact, with no residual activity once they dry on the plant surface. Horticultural oils, such as mineral or neem oil, work primarily by smothering the mites, blocking the spiracles they use to breathe. These oils are typically diluted to a 1% to 2% solution for application on actively growing plants.
Insecticidal soaps, which are derived from fatty acids, disrupt the mite’s cell membranes, causing rapid mortality. A common homemade solution involves mixing one tablespoon of mild, perfume-free soap and one tablespoon of vegetable oil as an emulsifier into one liter of water. Regardless of the product chosen, it is imperative to achieve 100% coverage, coating the tops and, especially, the undersides of all leaves until the product begins to drip off.
Because these treatments are contact killers and do not always eliminate the eggs, repeat application is mandatory to break the mite’s rapid life cycle. Mite eggs can hatch in as little as three days under warm conditions, so treatments should be repeated every three days for two to three weeks. Applying these oils and soaps during the evening or on a cloudy day is important, as application in high heat and direct light can lead to phytotoxicity, or leaf burn.
Long-Term Biological Control and Environmental Prevention
Biological control utilizes predatory mites that actively hunt and consume the pest mites. The specialist predator Phytoseiulus persimilis is highly effective for curative control, as it is voracious and can move easily through the webbing to reach its prey.
P. persimilis is a faster breeder than the two-spotted spider mite under optimal conditions, allowing it to quickly suppress a hot spot. For preventative control, the predatory mite Amblyseius californicus is often used, as it can survive on pollen or other food sources when pest mites are scarce. These beneficial mites thrive in conditions that are less favorable to the pest mites, specifically requiring a relative humidity of 60% or greater.
Spider mites flourish in the hot, dry air of an indoor garden or during summer droughts. Environmental management involves altering these conditions to inhibit their reproduction. Maintaining a relative humidity of 50% to 60% will slow the life cycle of the pest while simultaneously supporting the introduced predatory mites.
Ensuring excellent air circulation and ventilation is also important, as stagnant air allows mite colonies to build up undisturbed. Prevention begins with a strict quarantine protocol for any new plant material, such as clones, before they are introduced into the main growing area. New plants should be isolated for at least one week and inspected daily with a hand lens to catch any early signs of a hitchhiking pest. Consistent, routine inspection of the undersides of leaves remains the best defense against a new or recurring infestation.