Russet Mites On Cannabis: Signs, Damage, And Control
Quick answer: Russet mites on cannabis are hemp russet mites (Aculops cannabicola), tiny pale mites too small to see without strong magnification. Their feeding can turn leaves dull, bronzed or rusty, curl leaf edges, stunt growth and leave fewer, smaller buds. Confirm them under a microscope, inspect incoming cuttings and young plants, and remove infested plants and debris.
What Are Russet Mites?
The russet mite that attacks cannabis is the hemp russet mite, Aculops cannabicola, an eriophyid mite. Its known host range is limited to Cannabis sativa: in lab studies it failed to establish on related plants in the Cannabaceae family.
It’s tiny, about 110 to 210 microns long, or roughly twice the width of a human hair. The body is pale yellowish to white and worm- or cigar-shaped, with two pairs of legs near the head. Russet mites don’t spin webbing.
Its life cycle hasn’t been studied closely. NC State Extension says it’s believed to last anywhere from 7–10 days to 30 days, depending on environmental conditions. Utah State University Extension notes that generations overlap. In greenhouses, russet mites can survive year-round with a constant supply of new plants.
Signs of Russet Mite Damage on Cannabis
Utah State University Extension notes there’s no visible damage while mite populations are low, so russet mites usually go unnoticed until plants show symptoms. Look for:
- Dull, grayish, bronzed or rusty-looking leaves
- Leaf edges curling upward or downward
- Fine spotting, brittle foliage and smaller leaves
- Bronzing on stems and leaf stalks
- Stunted growth, especially at the growing tips, which can kill young seedlings and new clones
- Fewer, smaller buds
- A brown or tan powdery film of cast skins on leaves and stems in heavy infestations
Utah State University Extension recommends looking for bronzing on lower leaves and stems, especially in hot, dry conditions. As numbers grow, mites spread from the undersides of leaves to the upper surfaces, leaf stalks, stems and flower buds.
Russet Mites in Flower
In outdoor hemp in Kentucky, high russet mite populations were seen from August to October, while buds develop. Severe injury to flower buds can reduce flower size.
How to Confirm Russet Mites
Russet mite injury can look like disease or other stress, so confirming the mites under magnification matters. Utah State University Extension suggests a strong hand lens or a microscope for scouting and notes the mites are best identified under a dissecting microscope.
- Collect damaged leaves along with nearby leaves that still look healthy.
- Check the leaves directly under a microscope, or wash them in soapy water or 70% ethanol.
- Look for mites in the wash, holding it in a dark container or over a dark surface to make them easier to see.
Your county Extension office or a plant diagnostic lab can also help confirm russet mites.
What Russet Mite Damage Can Be Mistaken For
Utah State University Extension lists problems that can look like russet mite damage: spider mites, thrips and physiological conditions that cause leaf curling and yellowing.
- Spider mites spin webbing and have four pairs of legs. Russet mites have two pairs and no webbing. The spider mite and thrips sections in protecting marijuana plants from pests and bugs cover those pests.
- Broad mites are another mite that’s very hard to see. They’re covered in common cannabis plant problems.
- Nutrient, water and climate stress can also curl and yellow leaves. If no mites turn up under the microscope, check the causes in problems that cause stress in cannabis plants.
How Russet Mites Spread
Russet mites are too small to travel far on their own. They crawl to leaf edges, where wind can carry them to other plants. Cuttings and young plants are another route: the mites are likely carried on them as eggs or active mites, and warm, humid propagation conditions help them multiply. People and supplies moving from infested areas to clean ones can spread them too. The mites also appear to overwinter in crop debris.
Inspect Cuttings and Young Plants
Thoroughly inspect any cuttings, and make sure the mother plant they came from isn’t infested. Sample young plants before transplanting. Because there’s no visible damage while populations are low, a clean-looking plant isn’t proof that it’s mite-free, so check under magnification.
How to Control Russet Mites on Cannabis
Cultural control centers on sanitation and keeping infested material from spreading.
Remove Infested Plants and Debris
Prune or remove infested plants, and remove and destroy infested plant debris rather than leaving it in the grow.
Keep Tools, People and Supplies Clean
Clean tools after working on infested plants and before touching healthy ones, and limit the movement of people and supplies between infested and clean areas.
Break the Cycle Between Grows
Russet mites can survive year-round in greenhouses with a constant supply of new plants, and they appear to overwinter in crop debris. Clearing out plants and debris between runs keeps them from carrying straight into the next crop.
Predatory Mites
NC State Extension notes that the predatory mite Amblyseius andersoni feeds on other russet mites, though how well it works against hemp russet mite is still unclear. Our overview of beneficial insects for cannabis growers covers predatory mites in general.
Frequently Asked Questions
Can You See Russet Mites With the Naked Eye?
No. Hemp russet mites are about 110 to 210 microns long, roughly twice the width of a human hair, and pale in color. They’re usually noticed only after damage appears. A dissecting microscope is the most reliable way to see them, and a strong hand lens can help with scouting.
What Does Russet Mite Damage Look Like on Cannabis?
Leaves turn dull, grayish, bronzed or rusty, and leaf edges may curl up or down. Foliage can turn brittle, leaves stay smaller, stems can bronze and buds may be fewer and smaller. Heavy infestations leave a brown, powdery film of cast skins on leaves and stems.
Do Russet Mites Make Webs?
No. Hemp russet mites don’t spin webbing, which is one way to tell them apart from spider mites. They also have only two pairs of legs, while spider mites have four. Because they’re so small, confirm suspected russet mites under a microscope rather than relying on the lack of webbing alone.
Can Russet Mites Live on Other Plants?
Cannabis is the only known host. In lab studies, the mite failed to establish on related plants in the Cannabaceae family. In greenhouses, it can survive year-round with a constant supply of new cannabis plants, so inspect incoming cuttings and young plants before adding them to the grow.
How Long Is the Russet Mite Life Cycle?
It hasn’t been studied closely. NC State Extension says the hemp russet mite is believed to have a life cycle of anywhere from 7–10 days to 30 days, depending on environmental conditions. Utah State University Extension notes that generations overlap and that the mites appear to overwinter in crop debris.
Are Russet Mites a Problem During Flowering?
They can be. In outdoor hemp in Kentucky, high russet mite populations were seen from August to October, while buds develop, and severe injury to flower buds can reduce flower size. Keep scouting through bloom, checking damaged leaves and nearby leaves that still look healthy.
Sources
