Learning how to prevent spider mites is the best way to protect an indoor grow, because by the time you can see these tiny pests, a colony is usually already well established. A regular barrier spray, good grow room hygiene and weekly leaf checks will stop most infestations before they start.
This guide covers how to identify spider mites and their eggs, why they multiply so quickly, how they get into your grow room, step-by-step prevention, how to treat an infestation and how to fix common spraying problems.
What are spider mites?
Spider mites are tiny sap-sucking arachnids, related to spiders and ticks rather than insects. The most common species in Australian grow rooms is the two-spotted spider mite (Tetranychus urticae), also known as red spider mite.
Adult females are only about 0.5mm long and males are smaller, at around 0.3mm. Adults have eight legs and are usually yellowish-green with a dark spot on each side of the body, although their colour can range from orange and reddish-brown to dark green depending on the plant they're feeding on.
To the naked eye they look like tiny moving dots. A 10x hand lens makes them much easier to identify.
Do spider mites lay eggs?
Yes. Spider mites lay round, transparent eggs about 0.14mm across, which turn pale yellow-white as they get closer to hatching. All life stages are usually found together on the underside of leaves, often near the veins and under fine silk webbing.
Why prevention beats treatment
Spider mites breed fast, so a handful of mites can become a damaging colony within a few weeks. Here's why waiting until you see them is risky:
- Eggs hatch in 3 to 10 days, depending on temperature
- Larvae and nymphs mature in 4 to12 days
- A female can lay up to 6 eggs a day and 70 or more over her lifetime
- Adult females can live for about a month
- In hot, favourable conditions a full generation can be completed in under a week
- Their webbing helps shield them from sprays and predators
Populations build fastest in hot, dry conditions, and mites in protected environments like grow rooms and greenhouses keep breeding through winter instead of going dormant.
⚠️ Signs of spider mite damage
Spider mites feed by piercing leaf cells a
nd sucking out the contents. The first sign is usually stippling — clusters of tiny pale yellow or white dots on the leaf surface.
As feeding continues, leaves can turn yellow or bronzed, shrivel and drop, which reduces the leaf area your plant has for photosynthesis. Fine webbing on leaves and stems is a sign the infestation is already heavy.
To check for mites, turn leaves over and look along the veins with a hand lens, or shake a branch over a sheet of white paper and watch for tiny moving specks.
How do spider mites get into an indoor grow room?
Spider mites can't fly, but they're very good hitchhikers. They're carried passively on the wind and on people, plants and tools, so they can ride into your grow room on your clothes, shoes or bags, or arrive on a new clone or houseplant.
What you need
A simple prevention kit covers most home grows:
- Mr 24/7 Plant Barrier Treatment (a botanical miticide made with neem, garlic and eucalyptus oils)
- A good-quality pump spray bottle that sprays at any angle, so you can reach the undersides of leaves
- A 10x hand lens or magnifier for checking leaves
- A set of clean clothes kept just for the grow room
💡 A-Grade Tip: Add a 30cm lance extension to your sprayer. It makes it much easier to coat the undersides of leaves in a dense canopy, which is exactly where spider mites hide and lay their eggs.
🌿 How to prevent spider mites, step by step
1. Inspect and isolate new plants
Check every new cutting or plant with a hand lens before it goes into your grow room. If you can, keep new arrivals separate for a week or two and check them again before moving them in.
2. Keep the outdoors out
Change into clean clothes before tending your grow room, especially if you've been in the garden. Keep tools used outside separate from your indoor tools.
3. Keep plants well watered and the room dust-free
Water-stressed plants suffer more damage from spider mites, and dusty conditions favour outbreaks. Keep your plants consistently watered and wipe down dusty surfaces.
4. Check leaves weekly
Turn over a few leaves on each plant every week, focusing on the lower canopy and any leaves showing stippling. Catching mites early means fewer sprays.
5. Apply a regular barrier spray
The A-Grade team spray Mr 24/7 weekly through the vegetative stage as a preventative barrier. Mix 5mL with 2L of lukewarm water and coat the tops and undersides of the leaves. This product also allows preventative use every few weeks during high-risk warm, dry periods.
How to treat a spider mite infestation
Sometimes prevention isn't enough. If you find mites, act quickly.
1. Confirm mites are present
Stippling can have other causes, so confirm you're seeing live mites, eggs or webbing before you treat.
2. Mix and spray thoroughly
Mix Mr 24/7 at 5mL per 2L of lukewarm water. Spray to runoff, covering the tops and undersides of leaves and in between branches, because contact sprays only work on the mites they reach.
3. Repeat every 3 to 4 days
For heavy infestations, reapply every 3 to 4 days to hit newly hatched mites before they start laying. Use the treatment for up to a week, then check your plants and assess the results.
Follow these foliar spraying rules every time:
- Don't spray in direct sun or with the grow lights on — spray just before lights off or at a cooler time of day
- Spray under leaves and in between branches
- Turn off ventilation for 30 to 60 minutes after spraying so the solution can settle on the leaves
- Don't spray once edible flowers or fruit have begun to form, and don't harvest within 7 days of applying Mr 24/7
💡 A-Grade Tip: Avoid broad-spectrum insecticides when you're dealing with spider mites. They often kill the mites' natural predators, and some can actually trigger a population spike. Stick with a selective miticide like Mr 24/7 or browse our foliar sprays.
Predatory mites such as Phytoseiulus persimilis are another option and are sold by commercial biocontrol suppliers in Australia. Don't use them alongside Mr 24/7, as the spray can harm beneficial mites.
Common spider mite spraying problems and fixes
| Problem | Likely cause | Status | Fix |
|---|---|---|---|
| Mites return a week after spraying | Eggs hatched after the first spray | Common | Reapply every 3 to 4 days to break the breeding cycle |
| Mites survive on the lower canopy | Undersides of leaves missed | Common | Use a 360° sprayer or lance extension and spray to runoff |
| Leaf burn after spraying | Sprayed under lights or in heat | Avoidable | Spray just before lights off or at a cooler time of day |
| Infestation keeps coming back | Mites carried in on clothes or new plants | Common | Change clothes before entering and inspect new plants |
| Mite numbers spike after spraying | Broad-spectrum insecticide killed predators | Avoidable | Switch to a selective miticide |
Summary
Spider mites are tiny, breed quickly and are easily carried into indoor grows, so prevention is far easier than treatment. Inspect new plants, keep outdoor clothes out of the grow room, check leaves weekly and use a regular barrier spray. If mites do take hold, spray thoroughly every 3 to 4 days and focus on the undersides of the leaves.
Browse A-Grade Hydroponics' pest and disease control range to get set up, or get in touch with the team if you've got questions about your own grow.
References
- Agriculture Victoria, Two-spotted mite, 2024. https://agriculture.vic.gov.au/biosecurity/pest-insects-and-mites/priority-pest-insects-and-mites/twospotted-mite
- University of California Statewide IPM Program, Pest Notes: Spider Mites (UC ANR Publication 7405), 2011. https://ipm.ucanr.edu/home-and-landscape/spider-mites/
- University of Connecticut Integrated Pest Management Program, Biological Control of Two-Spotted Spider Mites, 2019. https://ipm.cahnr.uconn.edu/wp-content/uploads/sites/3216/2022/12/2019biologicalcontrolofspidermitesfinal2-1.pdf
