How to Prevent and Get Rid of Spider, Broad and Russet Mites on Your Cannabis
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Spider mite webbing. Photo by W. Cranshaw, CSU, Bugwood.org
Mites
The term “mite” encompasses thousands of different arthropod species, most of them well under one millimeter in length — roughly the size of the head of a pin — and the largest mite common to cannabis is roughly half that size.
Mites are vectors for infections, but their main damage is the sucking out of nutrients from the plants’ leaves, which interferes with photosynthesis. Draining liquids and nutrients from the leaves slows down the plants’ growth. This, along with vectors they can carry, and excrement and secretions lead to telltale damage in new growth.
Three types of mites account for almost all infestations of cannabis
Spider mites congregating on a cannabis plant.
Broad mites, Russet Mites and Two-Spotted Spider Mites
They range from difficult to impossible to spot without magnification and reproduce rapidly. This means, with the smaller species particularly, the damage is what you see first and by then there are already multiple generations of mites inhabiting and destroying your plant. It’s more or less impossible to control a full-blown mite population bloom, so it’s especially crucial to maintain preventative conditions and regularly inspect plants so action can be taken as early as possible if mites do appear.
When it comes to greenhouse grows the primary source of mites is infected clones, so in addition to proper sanitation, great care must be taken when sourcing new genetics. Biocontrol of a mite infestation needs to be established before flowering, otherwise the reduced light times will cause a clustering response in the mites, rendering predatory mites all but useless. If mites are detected early enough, before too much damage has been done to the affected plants, their numbers can be effectively thinned by spraying plants with water, but additional control methods are required to fully eliminate them.
Broad Mites
Broad mite. Photo by W. Cranshaw, CSU, Bugwood.org
Actual size 0.1-0.2mm
How common are broad mites and what do they look like?
The broad mite or, Polyphagotarsonemus Latus, is a new pest in cannabis gardens and sightings are becoming more common.
The most notable thing about the broad mite’s appearance is its diminutive size. They are so small that a 60x loop or stronger is recommended to properly identify them. With the naked eye you will only notice a large infestation, clusters of egg sites and the telltale broad mite damage.
Up close the broad mite has a structure and appearance similar to larger mite species. Its coloration can vary, but often it looks like a pale yellow or clear dewdrop with tiny legs.
Broad mite. Photo by W. Cranshaw, CSU, Bugwood.org
Broad mites have a large cephelathorax, two pairs of small front legs, one pair in the middle of its body, and a wispy pair of back legs; which are more pronounced in the smaller males. Under a microscope they have a medium-size head with a definitive mandible structure. Broad mite eggs are translucent, round and with white spots. These are actually tuffs of spine-like hair, but appear as spots. They are 0.08mm in diameter.
Broad mites: How to look for them and what they do to the cannabis plant
Broad mite. Photo by W. Cranshaw, CSU, Bugwood.org
Broad mites lay eggs on the undersides of leaves. They prefer newer growth and the crevices of the cannabis plants. Always inspect the damaged growth areas with magnification, paying close attention to the ribbing of the veins on the underside of the leaves.
Broad mites cause two types of damage that are great clues for identifying them. The first is often “stipling,” which is a pattern of yellow dots on the leaves. These are the tiny feeding sites from the infestation. Often stipling is inconspicuous, but the leaves turn darker. As time passes after the initial wounding the sites become yellow to gray or even necrotic. These yellow speckled leaves as well as a twisting and/or yellowing of new growth are the second signs of damage. This is a result of the mites feeding. If you see these two signs in a garden, use a 60x loop to look for broad mites.
Mites are vectors for infections, but their main damage is the sucking out of nutrients from the plants’ leaves, which interferes with photosynthesis. Draining liquids and nutrients from the leaves slows down the plants’ growth. This, along with vectors they can carry, and excrement and secretions lead to telltale damage in new growth.
How to prevent, control and target Broad mites
Neoseiulus Californicus from Biotactics
Ways to prevention Broad mites
Prevention of broad mites can be tricky. It’s difficult to surmise where they came from in many instances. Often they come in the wind, infected plant material, or are deposited by animals. One of the most likely sources of infection is importing infected clones. For this reason, many growers start plants only from seed. A good integrated pest management program is always recommended. A periodic spray with an herbal pesticide is a good start.
Ways to target Broad mites
To target Spider Mites, Clover Mites, Broad Mites and Cyclamen Mites, Neoseiulus Californicus from Biotactics, is a hearty and effective Type II predator that will lay one egg for every 4-6 units of food (spider mite adult/ nymph/ egg). Effective on almost every plant from roses to strawberries to cannabis and alfalfa.
Ways to control Broad mites
• Citric acid • Herbal oils: • Cinnamon • Clove • Peppermint • Rosemary • Thyme
Other ways to control Broad mites
A product called Nuke Em produced by flying skull is effective at eradicating broad mites. It contains potassium sorbate and yeasts.
Debug, a product described in aphid treatment and control, can be used to combat broad mites. Neem oil can be effective, but this concentrated formulation is much more effective.
Azamax and other azadirachtin-concentrated amendments are also effective and allowed in most regulated states.
Russet mites
Russet mite. Photo by W. Cranshaw, CSU, Bugwood.org
Aculops cannabicola first appeared in Serbia and Hungary in the 1960s. This cannabis specific mite is still not common but is spreading quickly through the country.
What do Russet mites look like?
They are smaller than conventional spider mites but larger than the broad mite so a 30x or larger magnification may be necessary to fully identify an infestation. They are built slightly differently than the spider or broad mites as well. Having an elongated body, often described as carrot shaped. They have a clear to milky color makes them almost look like the larvae of some other pest. Russet mites have only four legs instead of eight like other mites.
Where Russet mites are found and what they do to the cannabis plant
Russet mite damage on cannabis plant. Russet mite. Photo by W. Cranshaw, CSU, Bugwood.org
Like the other mites, they will be located on the undersides of leaves. Russet mites work their way up plants, preferring the younger soft terminal leaves, so they are found on leaves just above damaged leaves and stems.
Russet mites suck precious sap and nutrients from the leaves and petioles of the plant. This causes a type of stippling that often turns orange or yellow “russet- ing.” Soon the infected leaves die and drop off from the damage. As the colony progresses up the plant it inevitably weakens and dies.
All mite populations have the potential to increase numbers quickly when environmental conditions are favorable. For the three species of mites listed here that mostly pertains to rises in temperature. As air temperature reaches favorable conditions for cannabis growth, it also increases the mites’ rates of metabolism and reproduction. A space that peaks at 85°F decreases the mites’ time to sexual maturity as well as their gestation time. They do not multiply quickly compared to other mites.
Sometimes a clean-out and a replacement crop is better than fighting them as they multiply logarithmically.
How to prevent, target and control Russet mites
Russet mites in various stages of growth. Photo by W. Cranshaw, CSU, Bugwood.org
Ways to prevent and target Russet mites
• Inspect clones entering the garden.
• Quarantine plants for several weeks before introducing them to the nursery. Sometimes mites can lay dormant for a bit or colonies can be so small they go unnoticed.
• Never take clones from plants that have been infected even if they have been treated and seem fine. This can serve to minimize the spread of these very tiny and hard-to-control mites.
• Have predatory mites present. Amblyseius swirskii and Amblyseius cucumeris are two predators that will prevent high populations of russet mites.
• Maintaining a tidy garden and observing clean gardening habits won’t protect you from this particular pest as it is mostly introduced. There are large concerns over the future of this now international traveler and commercial hemp crops worldwide.
Ways to control Russet mites
Galendromus Occidentalis.
To treat Spider Mites and Russet Mites, apply Galendromus Occidentalis during the vegetative bloom stage. This Western Predatory mite can withstand hot arid summers and can be applied with crop dusters and drones for precise application.
The russet mite works its way up plants from the bottom. So remove damaged leaves. Quarantine and treat the infected plant.
The damage mites can cause in a hot grow room can easily be so substantial during the first three weeks of flowering that starting over in a clean room with new plants maybe an economically better option. Russet mites have been reported to be particularly resistant to neem and other horticultural oils so this throw-’em-out and start over mentality may prove doubly in order instead of a pitched chemical battle.
In hemp fields the introduction of predatory mites can prove effective. Using biological control measures outdoors can have its issues, but if it’s timed right and multiple applications of predatory mites are used in intervals of four to seven days, the damage can be minimized enough to have a successful yield. Russet mites remove cell content from the leaves with piercing, sucking mouth parts.
Azadiractin Citric acid
Herbal oils: Cinnamon, Clove, Geranium, Peppermint, Rosemary, Thyme Potassium salts of fatty acids
Sulfur: Micronized sulfur applied as directed.
Spider Mites
Spider mite. Photo by Biotactics.
Actual size: .02” • 0.5 mm
How common are Spider mites and what do they look like?
Spider mites are very common. They are the most serious pests in the cannabis garden.
Spider mites are barely visible with the naked eye since they are only 0.06 inch (1.5 mm) long. They are arachnids (relatives of spiders), and like other arachnids, they have four pairs of legs and no antennae. Like spiders, they have two body segments. Their colors range from red, brown and black to yellow and green de- pending on the species and the time of year. Spider mites are so tiny though that most of these details are visible only with a magnifier. Spider mites make silk, hence the name spider mite.
The two-spotted spider mite, the spider mite most likely to attack cannabis gardens, has two dark spots visible on its back when it is an adult.
How to spot Spider mites and what they do to the cannabis plant
Cannabis plant sick with spider mites. Photo by Kristen Angelo.
They live on the plants, mostly on the underside the leaves, but can be found on the buds. They can also be found moving along their silvery webbing, from leaf to leaf and even plant to plant.
Spider mites pierce the surface of the leaves and then suck the liquids from cells. These punctures appear on the leaves as tiny yellow/brown spots surrounded by yellowing leaf.
Identify infestation by tiny spots on the leaves. They can be seen as colored dots on the leaf undersides. As the population grows they produce webbing that the mites use as a protective shield from predators, a nursery for their eggs and a pedestrian bridge between branches or plants.
Spider mites pierce cells and suck their liquids. They are more of a threat than most pests because of their high rate of reproduction.
Spider mites are by far the most fearsome of all plant pests. They suck plant juices, weakening the plants. Spider mites multiply quickly. They are most active in warmer climates than cold ones
Reproduction rate and life cycle: Newly hatched mites are 3:1 female: male, and each female lays up to 200 eggs, 1 to 5 per day, as an adult. This life cycle can repeat as often as every eight days in warm, dry conditions such as a grow room. Spider mites spread through human transport as well as by wind in out- door gardens.
Because of their rate of reproduction and the short time from egg to sexual maturity, a spider mite population can explode with shocking speed.
How to prevent, target and control Spider mites
Spider mite damage on cannabis. Photo by W. Cranshaw, CSU, Bugwood.org.
Ways to prevent and target Spider mites
Almost all spider mite infestations enter the garden on an infested plant, through the ventilation system or by gardeners who carry the hitchhikers into the garden. Use a fine dust filter (at least 300 microns) in the ventilation system, and never enter the grow space wearing clothing that has recently been outdoors, especially in a garden. Best practice is to keep pets out as well.
Neem oil is often used as a preventive, but always look out for webbing and for the yellow-brown spots mites leave when feeding. Infected mother plants transmit mites on their clones, so it is especially important to watch for mites in a mother room. When you spot mite symptoms take action immediately.
Growing from seed in an indoor environment is a great start to prevention in an indoor grow space.
Don’t introduce plants from other spaces or quarantine new plants for several weeks.
Ways to control Spider mites
Spider mites thrive in dry climates. Increasing humidity in the vegetative and early flowering stages can slow population increase.
• Insecticidal soap kills many of the mites, lowering the population and the damage, but does not eliminate the population.
• Pyrethrum is effective against some mite populations, but others have developed immunity to it.
• Cinnamon-clove tea
• Herbal pesticides
Use predatory mites
There are many varieties of predator mites. Get those best suited to the environment of the garden. Apply predator mites at the earliest sign of infestation. Most predator species reproduce faster than spider mites, but if the mites get a good head start, the predator population can never catch up. Even in optimal conditions, control with predator mites is very difficult. Four effective species are Phytoseiulus persimilis, Amblyseius swirskii, Amblyseius fallacis and N. californicus.
Persimilis
Persimilis mite. Photo by Biotactics.
Persimilis is the first predatory mite ever used as a pest management product, to attack Two Spotted Spider Mites and Pacific Spider Mites in both Vegetative and Bloom State. These Type I predators only have one prey and can eat 35 food unites (a spider mite egg or adult) per day while laying 4-7 of their own eggs. They are extremely ravenous but need 70% relative humidity for their eggs to hatch, making them a great predator for hot spots with larger infestation problems. Find them at Biotactics.
Mesoseiulus Longipes
Mesoseiulus Longipes. Photo by Biotactics.
Targeting Spotted Spider Mites and Pacific Mites, Mesoseiulus Longipes is the rarest predatory mite species in the world and is perfect for high elevation growing as well as dry desert areas, and is available from Biotactics.
Neoseiulus Fallacis
Neoseiulus Fallacis. Photo by Biotactics.
Attack Spider mites, Clover Mites, Broad Mites, and Bank’s Grass Mites in the vegetative state or bloom state with Neoseiulus Fallacis, a cousin of Californicus. From Biotactics, this is hearty and aggressive predatory mite and will overwinter in the soil and cover crop in hibernation and is used on tens of thousand of acres of coastal California strawberry farms and min farms in Southern Oregon.
Here are several other predatory mites feed on spider mites
• Feltiella acarisuga, the predatory spider mite fly
• Stethorus punctillum. the spider mite lady beetle
Other ways to control Spider mites
• Green lacewings • Minute pirate bugs • Beauveria bassiana (beneficial fungi) • Capsaicin • Carbon dioxide • Cinnamon oil • Coriander oil • Fish oil • Garlic • Herbal oils • Horticultural oil • Insecticidal soaps • Limonene • Neem oil • Saccharopolyspora spinosa (beneficial bacteria) • Sesame oil
In warm indoor gardens, things can get out of hand quickly. If the infestation is noticed on a new crop, it may be more advantageous to cull that crop and clean the grow room with a light bleach solution. Additional measures include waiting 10 or more days and re-cleaning in case of a substantial outbreak. When treating plants, it is always advisable to remove lower leaves that contain large amounts of eggs. This along with repetitive spraying may be necessary.
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