Spider mites are one of the most common and also destructive pest insects that can affect a cannabis crop. Their short gestation period, teamed with large birth numbers and nearly indestructible eggs, makes eradicating a spider mite infestation a very difficult task for indoor cannabis growers. In fact, it only takes one fertilized female spider mite to start an entirely new population. That is just one of the reasons why it is nearly impossible to completely rid an indoor garden of spider mites once they have become established. However, growers who take preventative measures can reduce the likelihood of an infestation and gardeners who know the tell-tale signs can act quickly when spider mites do enter a garden space. After preventative measures, how quickly a grower takes action will be the greatest influence over how much damage is caused by a spider mite invasion.
Unfortunately, an indoor cannabis garden is a perfect environment for spider mites, so a cannabis grower may encounter these nasty, little beasts multiple times throughout his or her gardening career. Since it is quite likely that most cannabis growers will encounter a spider mite problem at some point, they should be equipped with an understanding of preventative measures, the early warning signs, and how to make a positive identification. This information will then help the cannabis grower determine the best treatment options so as to minimize the damage caused by the spider mite infestation.
The number one way spider mites gain access to an indoor cannabis garden is by the gardeners themselves. Growers naively accept cuttings from fellow gardeners in the search for new cannabis varieties. Unfortunately, when accepting cuttings from other gardeners, a cannabis grower can end up with much more than a new genetic. When acquiring a new cannabis strain (in clone form), it is a good idea for the grower to visually inspect it for mites and also “quarantine” it for a few days by placing it outside of the indoor garden. After a week, if it passes inspection, then a grower can introduce the new specimen into his or her indoor cannabis garden.
Spider mites will also enter an indoor garden by hitching a ride on an unsuspecting grower. If possible, a grower should avoid entering his or her garden immediately after doing yard work or other outdoor activities that would potentially expose him or her to spider mites. Cannabis growers should always change their clothes or avoid going into their gardens after visiting another indoor garden (even if that garden seems unaffected by spider mites).
Another simple preventative measure any cannabis horticulturist can take to prevent spider mites is to keep a clean growing environment. Dead leaves, discarded soil, and previously used planting containers are places for many types of pest insects to live and breed. A clean growing environment is beneficial in many regards and is one of the most simple and effective ways to prevent pest insects.
Although it is nearly impossible to have a hermetically sealed indoor garden, a grower should strive to seal his or her grow room as much as possible. Spider mites are drawn to the warm, moist conditions of an indoor grow room. By keeping an indoor garden relatively sealed, the grower limits the spider mite’s access to the environment. Caulk and/or expanding foam insulation are both effective ways to fill in the cracks and crannies of an indoor garden. It is also imperative to assess the grow room’s ventilation system. Unfiltered intake ports are easy access points for a number of pest insects. Mesh or a fine screen covering the ventilation openings of an indoor garden will greatly reduce the chance of spider mites and/or other pest insects from entering. An intake filter, such as a HEPA filter, can be an invaluable investment for avoiding a spider mite invasion.
As previously mentioned, a well-meaning gardener who shares genetics may, in fact, may also be sharing their spider mite problem. Spider mites are very small and are not easy to spot with the naked eye. Just because a plant looks “clean” doesn’t necessarily mean it is free of pest insects. Often, a plant may be free of adult pest insects, but may contain eggs or larvae buried in the medium or tissue of the plant. Perhaps the best spider mite preventative measure an indoor gardener can take is to start his or her cannabis garden from seed. Although this will mean the grower will have to wait a little longer before the first harvest, it also means the grower will start with a spider mite-free cannabis garden. Starting from seed is also the best way to recover from a devastating spider mite infestation. After losing significant yield while battling spider mites, many growers will wisely choose to “start over”. In other words, they remove all of the plants and treat the grow room and all of its equipment with a bleach-water solution. In my opinion, when starting over, it is best to do so from seed. The last thing a grower wants to do after dealing with a devastating spider mite infestation is to accept a cutting from someone and contract the same problem.
The most obvious initial signs of a spider mite problem are leaves that appear misted with yellow spray paint. This “spray paint” effect is caused by the spider mites sucking precious nutrients from the undersides of the leaves. Another definitive spider mite sign, which appears once they have become firmly established, is spider mite webbing. This looks like a tightly woven spider web. These webs can be found on leaves or in the intersections of branches. In extreme cases, cannabis flowers may be fully encased in the webbing. The spider mites themselves are very small and difficult to see with the naked eye. With a pocket magnifier, a grower can usually make them out. They look like a tiny tick and can be red, brown, green, or spotted in color.
There are many different treatment options available for combating spider mites. When dealing with spider mites in the flowering stage of cannabis, it is strongly advised to avoid any chemical miticides. Most chemical miticides can be extremely dangerous and even the ones designed for use on edible plants are not necessarily safe for cannabis. Agricultural regulating agencies rate the safety of a product for its known consumption methods, namely eating; however, the main mode of consumption for most cannabis users is combustion (smoking). Until there is more acceptance of cannabis as an agricultural crop, it is better to be safe than sorry and just avoid the use of chemical miticides.
Beneficial insects are the safest pest insect control when it comes to the health of both the grower and the cannabis product’s end-user. Beneficial insects leave no residue or potentially harmful substances on the plant, which means they can be safely used during the cannabis plant’s flowering/ripening stage. There are many different beneficial insects available that will effectively combat spider mites, including spider mite predators (phytoseiulus persimilis, neoseiulus californicus, mesoseiulus longipes), spider mite destroyers (stethorus punctillum), predatory midge (feltiella acarisuga), and ladybugs (coccinellidae).
There are many chemical and organic insecticides available that are designed specifically for mites (miticides). Above all else, diligence is the key to eliminating a spider mite infestation. Miticides (insecticides created specifically for mites) are generally the best option for complete eradication. To achieve complete eradication, a cannabis grower should treat the garden every 3-4 days for at least a month. This rigorous treatment will kill any new spider mites that may hatch (hatching occurs every 3-4 days). Although some miticides may claim to destroy the eggs, spider mite eggs are extremely durable and seem to hatch no matter what is sprayed on them.
Although I, personally, never use chemical miticides regardless of the stage of growth, many cannabis growers employ chemical miticides during the vegetative stage. Some chemical miticides can cause serious health issues to, not only the end consumer, but also the grower during the application process. This is why it is so important to follow the application instructions explicitly. Some chemical miticides commonly used by cannabis growers are Avid (active ingredient abamectin), Akari (active ingredient fenpyroximate), Floramite (active ingredient bifenazate), Hexygon (active ingredient hexythiazox), Judo (active ingredient spiromesifen), Ovation (active ingredient clofentezine), Pylon (active ingredient chlorfenapyr), Sanmite (active ingredient pyridaben), Shuttle (active ingredient acequinocyl), TetraSan (active ingredient etoxazole), and Promite (active ingredient fenbutatin-oxide). The difference between the various chemical miticides is the mode of action the active ingredient takes. In other words, how it specifically affects the spider mites.
Just because an insecticide is natural or organic it does not necessarily mean it is completely safe to use. As with chemical miticides, cannabis growers should always follow the manufacturer’s application instructions and research the toxicity of each substance before use. Again, cannabis growers should avoid using all pesticides/miticides during the flowering stage, whether they are organic or not. Natural and organic mite treatment options include pyrethrins (a natural derivative from chrysanthemums), azadirachtin (a compound found in neem oil), castile soap (commonly referred to as insecticidal soap), and various plant oil extracts (garlic, clove, cayenne pepper, rosemary, cinnamon, and thyme). Like chemical miticides, natural miticides differ by the mode of action it takes against the mites.
An indoor cannabis garden with an environment conducive to accelerated plant growth is also an ideal breeding ground for the dreaded spider mite. This is why it is so important for cannabis growers to implement any and all preventative measures available. If a spider mite infestation is contracted, most hobby cannabis cultivators are best off controlling the problem until harvest and then starting over from seed. Large scale cannabis operations may not have the option of a restart and must choose the most effective treatment option. Some foreknowledge of spider mites and their behavior will only be beneficial for quick and positive identification. Once spider mites are in the garden, the cultivator must weigh their options and act quickly. Beneficial insects offer the safest solution for controlling spider mites, while chemical miticides (when used correctly) can achieve a higher level of eradication. All in all, cannabis growers should do everything they can to protect their plants from spider mites, but priority should also be placed on the products and methods which protect themselves and the consumers of the crop.
Eric Hopper is a Professional Marijuana Grower Senior Editor. He can be contacted at firstname.lastname@example.org.