Spider Mite Predators (Phytoseiulus)
Using predatory mites like Phytoseiulus persimilis to control two-spotted spider mites in cannabis grows without pesticides.
Phytoseiulus persimilis genuinely works against two-spotted spider mites — it's one of the oldest and best-documented biocontrol agents in greenhouse agriculture. But it isn't a cure-all. It only eats Tetranychus-type mites, it dies when its prey runs out, and it hates low humidity and miticide residues. Released early at the right ratios it can save a crop; dumped on a heavy infestation in a 30% RH flower room, it will fail and you'll blame the bug instead of the conditions.
What it is
Phytoseiulus persimilis is a small, fast-moving predatory mite native to subtropical regions and now reared commercially worldwide as a biological control agent. It is an obligate predator of spider mites in the genus Tetranychus — most relevant to cannabis growers, the two-spotted spider mite (T. urticae) [1][2].
Adults are roughly 0.5 mm long, pear-shaped, and bright orange-red, which distinguishes them from the pale yellow-green pest mites they hunt. They have no resting stage and reproduce faster than their prey under warm, humid conditions, which is why they can crash spider mite populations when conditions are right [2][3].
Phytoseiulus is the most-studied predatory mite in commercial agriculture, with documented efficacy in greenhouse tomato, cucumber, strawberry, and ornamentals going back to the 1960s Strong evidence[1][3].
Why growers use it
Spider mites are arguably the worst arthropod pest in indoor cannabis. They reproduce explosively in warm, dry rooms, develop resistance to miticides quickly, and webbing on flower is unsellable [4]. Most synthetic miticides are not labeled for cannabis in regulated markets, and many state programs restrict pesticide residues on harvested flower [5].
Biocontrol with Phytoseiulus sidesteps both problems: no residue, no resistance pressure on the predator, and it works inside webbing where contact sprays struggle to reach Strong evidence[1][3]. Released early enough, it can eliminate a localized infestation in 2–3 weeks under good conditions [2].
Limitations are real and worth stating up front:
- It only eats Tetranychus spider mites. It will not control russet mites, broad mites, hemp russet mites, thrips, or aphids [1][2].
- It needs relative humidity above roughly 60% for egg survival; below ~40% RH eggs desiccate and populations collapse Strong evidence[2][3].
- It does poorly above ~30 °C (86 °F) sustained leaf-surface temperature [2].
- It is sensitive to pesticide and even some fungicide residues for weeks after application [6].
When to start
Two strategies, both legitimate:
Curative release. Begin as soon as you find the first stippling or live mites on a scouting pass. Earlier is dramatically better — Phytoseiulus overwhelms small, localized hot spots far more reliably than established infestations with heavy webbing Strong evidence[2][3].
Preventive release. Some growers release small numbers during veg as insurance. This is less efficient because Phytoseiulus starves without prey, so it requires repeated reintroductions. For preventive coverage, slower-feeding generalist predators like Neoseiulus californicus or Amblyseius andersoni are usually a better fit, sometimes paired with Phytoseiulus once prey is detected [3][7].
Avoid releasing in late flower if your buyer or testing lab has concerns about live arthropods on harvested material. Practically, the predators will leave drying flower, but check your jurisdiction's rules.
How to do it: step by step
1. Confirm the pest. Use a 10x–30x hand lens. Two-spotted spider mites are pale with two dark spots; russet and broad mites are too small to see with a 10x lens and require a different predator (Amblyseius swirskii or Neoseiulus cucumeris). Releasing Phytoseiulus on a russet mite problem wastes money [1][2].
2. Order from a reputable insectary. Major suppliers include Koppert, Biobest, Applied Bio-nomics, and Beneficial Insectary. Request overnight shipping and inspect on arrival — predators should be visibly moving on the carrier material [3].
3. Set the environment. Target 65–75% RH and 20–27 °C (68–80 °F) at canopy. Mist foliage lightly before release if your room runs dry; Phytoseiulus eggs need humidity Strong evidence[2].
4. Calculate dose. Typical curative rates are 2–10 predators per square foot (20–100/m²) on infested plants, or roughly 1 predator per 5–10 spider mites observed [3][7]. For hot spots, concentrate releases there. Bulk carrier (vermiculite or corn grit) is sprinkled directly on leaves; sachets are hung on stems.
5. Release in the evening or with lights dimmed. Predators are light-sensitive and disperse better in lower light [2].
6. Scout weekly. Count spider mite and predator numbers on the same leaves each week. You should see predator populations climb and mite populations crash within 2–3 weeks under good conditions. If you don't, recheck humidity, look for pesticide residues, and consider a second release [3].
7. Plan for the crash. Once Phytoseiulus eliminates its prey, it starves. This is expected. If reintroduction is needed for a new outbreak, you must order more — they do not persist between cycles [2].
Common mistakes
- Releasing into a dry flower room. Below 40% RH the predators fail. This is the single most common reason growers say "biocontrol doesn't work" Strong evidence[2].
- Recent miticide or pyrethroid use. Residues from abamectin, bifenthrin, and many "organic" pyrethrins kill predators on contact for days to weeks. Sulfur sprays also harm them [6]. Wait at least 1–2 weeks and rinse foliage where possible.
- Mismatching the predator to the pest. Phytoseiulus does not control russet mites, broad mites, or thrips. Identify the pest before ordering [1].
- Releasing too late. Once webbing is heavy, predators have trouble moving through it and prey numbers outrun them. Combine with a hand-removal pass on the worst leaves before release [3].
- Storing too long. Predators are perishable. Release within 24–48 hours of arrival; store cool (10–15 °C) and dark if delayed [2].
- Believing the dose-and-done myth. Severe infestations usually require two releases 1–2 weeks apart Weak / limited[3].
Related techniques
Phytoseiulus works best as one piece of an integrated pest management (IPM) program. Pair it with:
- Generalist predators like Neoseiulus californicus for low-prey persistence [3][7].
- Sticky cards for monitoring fungus gnats, thrips, and winged pests (they won't catch spider mites but tell you what else is in the room).
- Sanitation: quarantining clones, foot baths, dedicated room clothing — most mite outbreaks trace back to incoming plant material [4].
- Environmental control: keeping RH above 50% in veg makes life harder for spider mites and easier for their predators [2].
See also: Integrated Pest Management Basics, Spider Mites, Russet Mites.
Sources
- Peer-reviewed McMurtry, J. A., & Croft, B. A. (1997). Life-styles of phytoseiid mites and their roles in biological control. Annual Review of Entomology, 42, 291–321.
- Peer-reviewed Sabelis, M. W. (1985). Reproductive strategies of Phytoseiulus persimilis and other phytoseiid mites. In: Helle, W. & Sabelis, M. W. (eds.), Spider Mites: Their Biology, Natural Enemies and Control, Vol. 1B. Elsevier.
- Government van Lenteren, J. C. (2012). The state of commercial augmentative biological control: plenty of natural enemies, but a frustrating lack of uptake. BioControl, 57(1), 1–20. (Published with support of IOBC.)
- Peer-reviewed Cranshaw, W., Schreiner, M., Britt, K., Kuhar, T. P., McPartland, J., & Grant, J. (2019). Developing Insect Pest Management Systems for Hemp in the United States: A Work in Progress. Journal of Integrated Pest Management, 10(1), 26.
- Government Colorado Department of Agriculture. Pesticide Use in Marijuana Production – Criteria and Allowed Active Ingredients List.
- Peer-reviewed Bostanian, N. J., Thistlewood, H. M. A., Hardman, J. M., Laurin, M.-C., & Racette, G. (2009). Effect of seven new orchard pesticides on Galendromus occidentalis and Phytoseiulus persimilis in laboratory bioassays. Crop Protection, 28(7), 561–566.
- Practitioner Koppert Biological Systems. Spidex (Phytoseiulus persimilis) product information and release guidelines.
How this page was made
Generation history
Drafting assistance and fact-check automation are used, with a human operator spot-checking on a weekly basis. See how articles are made.