Spider mites on weed give themselves away with three signs: pale speckles on the upper side of the leaves, mites about 0.5 mm long with their eggs on the underside and, in heavy infestations, fine webbing between leaves and buds. They thrive above all in warm, dry air, roughly above 27 °C and below 50% humidity, and at 30 °C a generation takes only about a week. You get rid of them by acting at once: remove affected leaves, correct the climate, release predatory mites and, before buds start to form, use contact treatments such as potassium soap.
In brief
- Check the undersides of the leaves every week with a 10x or stronger loupe.
- Warm, dry air favours them: keep temperature and humidity within the values for each stage.
- Quarantine every cutting or new plant for 1–2 weeks before it goes into the tent.
- No sprays on the buds once they start to form (weeks 2–3 of flowering): after that, only leaf removal and predatory mites.
- After an infestation, empty and clean the tent, pots and accessories before the next cycle.
How to spot spider mites
The name usually refers to Tetranychus urticae, the two-spotted spider mite. It is not an insect but a mite: about half a millimetre long, yellowish green with two dark spots on its sides, while females getting ready to overwinter turn orange-red. It lives on the underside of leaves, where it pierces the cells and sucks out their contents.
- Pale or yellowish speckles on the upper side of the leaf, scattered at first, then so dense that the leaf looks dull and dusty.
- Moving dots and tiny round, pale eggs under the leaf, easy to see with a loupe.
- Leaves that yellow, dry out and drop, starting from the worst-hit areas.
- In advanced cases, fine webbing on shoot tips and buds, with mites travelling along the threads.
Quick test: shake a suspect leaf over a sheet of white paper. If dots start walking, they are mites. Check different plants and both low and high leaves, because the first outbreaks are often localised.
Why they appear: heat, dry air and new plants
Mites almost always come from outside: on bought cuttings or plants, on clothes and tools used in the garden or on houseplants, sometimes on pets. An enclosed space such as a grow tent keeps many pests out, but once inside, spider mites find constant warmth, dry air and no natural enemies.
| Stage | Temperature, lights on | Relative humidity | Note on spider mites |
|---|---|---|---|
| Vegetative stage | 22–28 °C | 55–65% | at this humidity mites multiply more slowly than in dry air |
| Flowering, first 5–6 weeks | 20–26 °C | 45–55% | avoid heat peaks above 28 °C |
| Flowering, last 2–3 weeks | 20–24 °C | 40–50% | do not raise humidity: it increases the risk of bud mould |
How to measure and adjust humidity without overdoing it is explained in the guide to humidity in a grow tent.
Prevention
- Quarantine: keep every new cutting or plant separate for 1–2 weeks and check it with a loupe before it goes into the tent. Skipping this is one of the most common growing mistakes.
- Hygiene: do not go into the tent straight after working in the garden; pick up fallen leaves every day.
- Climate: keep temperature and humidity within the values in the table. In winter, with the heating on, a humidifier stops the air getting too dry during the vegetative stage.
- Air circulation: air should move across the whole canopy, with no warm, still corners.
- Inspection: once a week, turn over at least 5–10 leaves per plant, high and low.
The same hygiene rules also protect against other common tent pests such as fungus gnats.
Getting rid of them in the vegetative stage
The sooner you act, the easier it is. Combine several methods in this order:
- Cut off the worst-hit leaves and seal them in a bag straight away, without shaking them over other plants.
- Rinse small plants with a lukewarm shower aimed at the leaf undersides, with the pot covered.
- Treat with potassium soap (potassium salts of fatty acids): it only works on contact, so wet the undersides thoroughly. Repeat at the intervals on the label, because eggs survive the first application.
- Alternatively, use a product based on azadirachtin, the active ingredient of neem, in the vegetative stage only.
- Release predatory mites, observing the waiting time given by the supplier after each treatment.
Only use plant protection products authorised in Switzerland and follow the label for permitted crops, doses and intervals. Spray with the lights off or shortly before the dark period, never on warm leaves under the lamp, and test the product on a single plant first.
Predatory mites in a tent
Phytoseiulus persimilis feeds almost exclusively on spider mites and is the fastest against outbreaks: it works well between 20 and 27 °C with humidity above roughly 60%. Neoseiulus californicus (also known as Amblyseius californicus) copes better with heat and dry air, survives longer with little prey and is also suitable for preventive releases. They arrive alive in bottles or sachets: spread them on the affected leaves straight away and do not use insecticides or miticides in the following days.
What to do in late flowering
Once the buds start to form, that is from weeks 2–3 of flowering, do not spray anything on the buds: it leaves residues and odours and brings moisture between the calyxes, raising the risk of bud mould. These measures remain:
- remove the worst-hit large leaves and check every 2–3 days;
- lower the lights-on temperature below 26 °C without letting humidity rise above 50%;
- release Neoseiulus californicus, which tolerates the dry air of late flowering better;
- if 1–2 weeks remain and the webbing reaches the buds, consider harvesting early and discard buds covered in webbing.
After harvest: cleaning the tent
Females can survive for weeks without plants, hidden in folds, zips, pots and plant debris, and start again with the next cycle. Before you start over:
- Dispose of plants, fallen leaves and used growing medium in sealed bags.
- Vacuum the floor, corners and zips of the tent.
- Wash walls, pots, saucers and fans with hot water and detergent, then wipe with a surface disinfectant according to the instructions.
- Leave the tent empty, open and dry for a few days.
- In the next cycle, check the leaves with a loupe twice a week for the first month.
Frequently asked questions
How can I tell if I have spider mites?
Look for pale speckles on the upper side of the leaves and, with a 10x loupe, mites and eggs on the underside. Webbing only appears once the infestation is already advanced.
Can I use neem oil during flowering?
It is not advisable: on buds it leaves residues and odour, and the moisture increases the risk of mould. Use neem-based products only in the vegetative stage, if authorised in Switzerland and according to the label.
Do spider mites come back after harvest?
Yes, if the tent is not cleaned: females hide in folds, pots and debris. Empty, vacuum and wash everything, then watch the next cycle closely.
Do predatory mites work in a tent?
Yes, if you release them early, with a suitable climate and no insecticides in the days before. Phytoseiulus persimilis needs humidity above roughly 60%, while Neoseiulus californicus copes better with heat and dry air.
Humidifiers, fans and accessories for the tent climate are in the humidification and air and climate collections.
Updated on 13 October 2026.