The hemp russet mite (Aculops cannabicola) is a cannabis pest so small that you cannot see it without magnification. It feeds on the outer cells of leaves and stems. As those cells die, the tissue turns bronze or rusty, with a faint oily shine. By the time you notice that colour, the population is usually already large.
Russet mites also spread much faster than their size suggests. Moving air lifts them from plant to plant. Pruning shears, gloves and clothing carry them too. This guide shows how to confirm the mite, how to tell it from broad mites and spider mites, and how to remove it from a grow.
Most facts below come from pest guides published by four US universities: Colorado State University, North Carolina State University (NC State), the University of Florida (UF) and Utah State University. All four run extension services, the departments that turn university research into practical advice for farmers. Paragraphs marked “Translator’s note” add conditions in Thailand and Southeast Asia.
What the hemp russet mite is
The hemp russet mite is an eriophyid mite (family Eriophyidae). Eriophyid mites are tiny plant-feeding mites, and their feeding often twists or deforms the plant. Colorado State describes this species as highly host specific. That means it feeds and breeds on one kind of plant only: cannabis, including hemp.
NC State adds that researchers tried to establish the mite on related plants in the Cannabaceae family and failed. The practical meaning is simple. If you find russet mites in your grow, they arrived on cannabis. Most often they came in on a clone or on a mother plant.
What to look for, according to these extension sources:
- Size. NC State gives females as 170 to 210 microns long and males as 160 to 165 microns. A micron is one-thousandth of a millimetre, so a female is about 0.2 mm.
- Legs. Only two pairs, close to the head. Most adult mites have four pairs.
- Shape and colour. Pale and elongated, narrowing sharply toward the rear, like a tiny carrot.
- No webbing. If you see webbing, you are dealing with spider mites, not russet mites.
- Movement. The mite is too small to walk far. Mites gather at leaf edges, and air currents carry them away.
Translator’s note: if you buy clones or cuttings from another farm, you can bring in russet mites. The same purchase can also bring hop latent viroid (HLVd) or a plant that is not the variety you paid for. Before you buy, ask to see the mother plant and check its stems under magnification. Starting from seed avoids this risk, as the prevention section explains.
What magnification you actually need
Many growers go wrong at this step. Colorado State says you cannot see hemp russet mites without a lens of at least 15 to 20x. UF’s hemp pest page gives the same 15 to 20x minimum for both russet mites and broad mites. Utah State goes further. It says eriophyid mites are not visible with an ordinary magnifying lens and are best identified with a dissecting microscope. This is a low-power microscope with two eyepieces that shows a leaf or stem in three dimensions; it is also sold as a “stereo microscope”.
So a 10x loupe cannot confirm russet mites. Use one of these instead:
- a 30x or 60x jeweller’s loupe;
- a clip-on macro lens for your phone (see the forum guide to sharp pest photos with cheap equipment);
- an inexpensive stereo microscope.
NC State also describes a wash method. Its purpose is to make the mites easier to count, so you can see whether numbers are rising or falling after treatment.
- Collect a sample of leaves.
- Wash them in soapy water or in 70% ethanol.
- Pour the wash into a dark container, or look at it over a dark surface.
- Count the pale mites, which stand out against the dark background.
Translator’s note: if you use the 70% alcohol sold for first aid, check that the label says ethanol, because that is what the method describes. Wash the same number of leaves from the same part of the plant each time. Then your counts can be compared week to week.
What to do: get a lens of at least 15 to 20x, ideally a 30x or 60x loupe or a stereo microscope, before you need it. Keep it in the grow room so checks become routine.
Signs on cannabis
A small population causes no visible damage. That is why the mite is so often found late. As numbers climb, you will see these signs:
- Leaf edges curl upward. Leaves become dull, brittle and smaller than normal.
- Leaves and stems take on a bronze, grey or rusty colour. This “russeting” gives the mite its name. Utah State notes that feeding on stems can turn the stems bronze.
- NC State reports that the mites build up first on stems and petioles (leaf stalks). At high numbers they move to leaf undersides. Buds then stay small and stunted.
- Colorado State describes a dark, powder-like layer on heavily infested leaves and stems. This layer is made of the mites’ cast-off skins. Buds and new leaves become distorted.
- Damage often starts low on the plant and moves upward. Utah State advises looking for bronzing on lower leaves and stems in hot, dry conditions.
These symptoms look like disease, nutrient trouble or heat stress. For that reason NC State states that you need to look under a microscope to confirm an infestation.
Translator’s note: Thai rooms often run at 30–35 °C, and real heat damage can look similar (see the forum guide to heat, light, water and salt damage). The only reliable way to separate the two is to look for the mites themselves. In Thailand’s hot season, follow Utah State’s advice and pay special attention to lower stems and leaves.
What to do: once a week, check the lower stems, leaf stalks and lower leaves of a few plants in each room under magnification. Write down what you find. The purpose is to catch the mite while numbers are still low and the problem is still cheap to fix.
How to tell russet mites from broad mites and spider mites
Spider mites have their own detailed guide on the forum. The table below only helps you separate the three groups while you have a plant under the lens.
| Feature | Hemp russet mite | Broad mite | Spider mite |
|---|---|---|---|
| Approximate size | about 0.1 mm (UF); 0.16–0.21 mm (NC State) | about 0.15 mm (UF) | about 0.3 to 1 mm (UF) |
| Magnification to see | 15 to 20x minimum; a microscope is best | 10x hand lens or more (Utah State); UF gives 15 to 20x | Several times the size of a grain of salt (UF); a lens helps |
| Legs on adults | 2 pairs | 4 pairs | 4 pairs |
| Webbing | None | None | Yes, at higher populations |
| Typical damage | Leaves curling upward; dull bronze or rusty leaves and stems; brittle foliage; stunted buds | Twisted, hardened new leaves and buds; stunted plants | Fine yellow speckling, then bleached or bronzed leaves |
| Where to look | Stems, leaf stalks, leaf undersides, leaf edges | Shaded new growth, crevices | Leaf undersides |
The pattern of damage also helps. Utah State describes broad mite injury as warped, hardened leaves and flower buds. Russet mite injury usually appears as slow bronzing and curling that climbs from the lower plant. It lacks the dramatic twisting of new growth.
What to do: before you choose a treatment, decide which of the three you have. Count the legs, look for webbing, and note where on the plant the damage started. Each mite needs a different response, especially when you buy predators.
Life cycle
The biology of this mite has not been studied in much depth. Colorado State says the whole life cycle can take as little as one week under favourable conditions, especially high humidity and high temperature. There are several overlapping generations each year. In other words, eggs, young mites and adults are all on the plant at the same time.
NC State gives a range from about 7 to 10 days up to 30 days, depending on conditions. Outdoors, it is not clear how the mites survive between crops. Utah State says they appear to overwinter in crop debris. Indoors they survive all year wherever there is a steady supply of plants. This is why perpetual grows, where new clones are produced all the time, are so hard to clean.
Translator’s note: Thailand and most of Southeast Asia have no winter to slow the mite down. Rooms and outdoor plots stay warm all year, and the rainy season brings high humidity. Expect the fast end of the cycle, around one week, rather than the slow end. Because the mite lives only on cannabis, any leftover plant material or self-seeded cannabis plants near the plot can carry it into the next crop.
What to do: after every harvest, remove and destroy all plant debris from the room or field. Pull any stray cannabis plants nearby. This breaks the chain of host plants the mite needs to survive.
Prevention
- Start clean. NC State says starting from seed is the best way to prevent infestation, because the mites cannot survive on seed. If you take cuttings, inspect both the cutting and the mother plant under magnification. The purpose is to stop the mite before it enters the room.
- Quarantine new plants. Keep incoming clones in a separate area. Check them under a microscope before they join the main room. This way an infested clone damages only itself.
- Clean tools between plants. Colorado State and Utah State both stress cleaning any tools used on infested plants. Shears, gloves and trays all carry mites.
- Control traffic. Limit the movement of people and equipment from infested areas to clean ones. Each day, work in the clean rooms first and the infested rooms last.
- Remove debris. Infested plant material left in the room keeps the population alive.
Translator’s note: in a small home grow, a separate tent or a closed room can serve as the quarantine area. One check can miss a small population, so check the new plants again before you move them. These steps fit inside a wider integrated pest management plan, which combines prevention, monitoring and control.
What to do: set up a quarantine space before your next batch of clones arrives, and decide which room you will enter first each day.
Treatment and biological control
Oils and soaps. NC State notes that two kinds of oil have been used: highly refined mineral oils, and seed oils such as neem, soybean, cottonseed and canola. Oils kill mites by coating and suffocating them. They have no residual activity, which means they stop working once the spray dries. So the spray must cover everything, including stems and leaf undersides, and you must repeat it. Check which products are registered where you grow, and whether they are allowed at your plant’s stage of growth. Many products may only be used before flowering.
Translator’s note: in Thailand, use only pesticides registered with the Department of Agriculture (กรมวิชาการเกษตร), and use them exactly as the label says. The forum guide on what you may legally spray under Thai GACP explains how to check registration and the label. Anything sprayed on flowers can end up in the dried product and in a laboratory residue test. For licensed growers and exporters, that can mean a batch that cannot be sold.
Predatory mites. Colorado State reports that the predatory mite Amblyseius andersoni is known to feed on hemp russet mites. Two standard spider mite predators, Phytoseiulus persimilis and Amblyseius californicus, do not control russet mites. Buying the wrong predator is a common and expensive mistake.
Translator’s note: we could not confirm whether Amblyseius andersoni is sold in Thailand. Ask local suppliers before you plan on it. If it is not available, do not buy a spider mite predator as a substitute. Instead, rely on clean starting material, oils registered for your crop stage, and the clean-out described below.
Remove and reset. Colorado State describes established infestations in continuous production as extremely difficult to eliminate. It recommends removing the plants from the area. With no host plants present, the remaining mites die within about a month. A full clean-out with a host-free gap often costs less than months of spraying.
Translator’s note: a clean-out in a Thai grow room could follow these steps.
- Bag and remove every plant and all plant debris.
- Clean the room, tools, trays and pots.
- Keep the room free of cannabis for at least one month, the period Colorado State gives.
- Restart from seed, or from clones checked under magnification in quarantine.
What to do: if the mite is established in a perpetual grow, compare the cost of a one-month empty room with the cost of repeated spraying and lost yield. Then plan the clean-out between two harvests.
Common mistakes
- Checking with a 10x loupe, seeing nothing and ruling mites out. The mite is too small for that lens.
- Treating it as a deficiency. Dull, curling, bronzed lower leaves get blamed on low magnesium or potassium. Confirm with magnification first, then compare with the forum guide to nutrient problems on cannabis leaves.
- Releasing spider mite predators. They do not solve this problem.
- Spraying only the tops of leaves. The mites sit on stems, leaf stalks and leaf undersides.
- Keeping an infested mother plant. Every clone taken from it carries the problem forward.
Local rules
Some places regulate this pest. UF warns that finding it in Florida can lead to quarantine. Check the current local rules and product registrations before you act.
Translator’s note: we could not confirm whether Thai plant quarantine rules list the hemp russet mite. What does apply in Thailand is the pesticide rule above: registered products only, used per label.
What to do: if you think you have russet mites, post your photos in the Plant Infirmary board. Include a shot taken through at least 15 to 20x magnification, plus where on the plant the damage started. You can also ask questions or share your own experience with this mite directly below this topic.
Sources
- Colorado State University Extension: information on the hemp russet mite.
- North Carolina State University (NC State) Extension: information on the hemp russet mite, including size, life cycle, the wash method and oil treatments.
- University of Florida (UF): hemp pest page covering russet, broad and spider mites.
- Utah State University Extension: information on eriophyid mites and broad mites on hemp.