So you've found spider mites, eh? If that's bumming you out, count your blessings it isn't thrips. On the scale of plant pests these are fairly easy to treat, and far easier to cure than others.
You've got options, and when you're stressed and want this gone fast, options are the last thing you need. The two options are persimilis and californicus, predatory mites that hunt spider mites, and they suit opposite situations. So here's the short version.
Webbing, in a humid room. Persimilis.
Stippling, in a dry room. Californicus.
A mix of the two. Change one of them, or double up on californicus.
Stippling is the fine pale speckling mites leave on the upper side of a leaf.
The two, at a glance
| P. persimilis | N. californicus | |
|---|---|---|
| Eats | Spider mites and nothing else | Spider mites first, plus other mites and some thrips |
| Speed | The fastest measured | Slower |
| Without food, with water | Adult females last about 6 days | About 18 days |
| Once the mites are gone | Starves or disperses | Persists three to five times longer |
| Buy it for | Webbing, and a heavy infestation | Light, early, or one that keeps coming back |
| A dry room, under about 60% humidity | Loses most of its eggs | The better bet — not immune |
First, how bad is it?
Take thirty seconds and actually look at the plant, because this is the half of the decision you can see.
Webbing strung between leaves and along the stems, foliage going pale and dusty, mites moving when you tilt a leaf to the light — that is a heavy population and you need speed. Fine pale speckling on the upper leaf surface, nothing woven anywhere, and you had to hunt to find any of it — that is early. Or it is the tail end of something you treated a month ago and it is creeping back. Either way speed is not what you are short of.
That is not me hedging. Californicus does its best work against low to moderate numbers: release it (put it on the plant) too early and it wanders off looking for something to eat, too late and it cannot knock down what is already there. Against a heavy population it is the weaker of the two, and the people who wrote the fact sheet say so.
Webbing, and leaves going
Persimilis. Buy the fastest thing there is, and plan on releasing twice.
Speckling, or it keeps coming back
Californicus. Slower off the mark, still on the plant in a month.
Persimilis: fast, then gone
Phytoseiulus persimilis eats spider mites. Not prefers — that is the only thing it can complete a life cycle on, and specifically the ones that spin heavy webbing. So webbing on your plant is good news for this particular purchase. It means you are buying the right animal.
It is also the fastest predatory mite anybody has measured, at five to twenty spider mites a day each. Egg to adult takes five days in a warm room and twenty-five in a cold one, which in a warm room is roughly twice the speed the pest is breeding at. That is how a webbed plant comes clean in about two weeks.
It starves once it wins
An empty plant is the receipt, not the complaint.
There is nothing else on its menu, so when the spider mites are gone persimilis dies or disperses. An adult female lasts about six days without food.
I know that reads like the product failed. It is the opposite, and it is what a specialist is. But it does mean persimilis is a treatment with an end date rather than a guard — so budget the second release when you buy the first, and if mites turn up again in six weeks, nothing is waiting for them.
Californicus: slower, sticks around
Neoseiulus californicus gets called a generalist (a predator that eats many kinds of prey) and it is not quite one. The taxonomists who sort these mites into lifestyles looked at moving it into the generalist group and decided against it, because you nearly always find it where spider mites are spinning heavy webbing. It will take other mite species and the occasional thrips. It is a spider mite predator first.
What it has is staying power, and the numbers are not close. Once the prey runs out it hangs on three to five times longer than persimilis. Given water and nothing to eat, a female lasts eighteen days against persimilis's six. Counted from her first egg, fifty-eight days against nineteen.
It also works across a wider band of temperature — more than 97% of eggs hatch and more than 81% of larvae reach adulthood anywhere between 59 and 95 °F. Above 95 it unravels: at 99.5 °F females still lay but no eggs hatch, and at 104 °F they stop laying altogether.
Pollen keeps them alive. It does not start a colony.
Californicus from a bottle will not build a population on pollen.
It survives on pollen, so a bottle released onto clean plants will not leave you a standing guard. In the study that tested it directly, adults did fine on pollen — but mites raised on pollen from the egg up never reproduced at all.
Cornell puts it in five words: it will not persist on pollen. That is why the treatment bottle is for plants with spider mites on them. Prevention on clean plants is what the californicus sachets are for.
Dry room? Californicus
This is the half of the decision you cannot see, and it is the reason the summary up top pairs humidity with what is on the leaf.
Both of these mites are drought-sensitive, and it has been measured properly: half their eggs die somewhere between 66 and 70% relative humidity, the number on a humidity gauge. Both species, not just one.
Persimilis is the worse of the two. Held at a steady 60% — a number most people would call perfectly reasonable — fewer than one egg in five hatched. Five separate strains, same answer, and no genetic difference between them, so there is no dry-adapted persimilis out there to go shopping for.
A heated room in winter runs 30 to 45%. That is not near the line. It is well under it.
So I am not overselling californicus: it is the better bet in dry air, not a dry-air specialist. In the study that ranked four species for humidity tolerance, 12% of its eggs died at 75 to 80% humidity and 68 °F, against 16% of persimilis's, and persimilis came last of the four. Better. Still on the wrong side of 66%.
In one 50% humidity trial, no predator saved the cucumbers
This shows up on real plants, not only in a chamber.
One trial ran persimilis against two dry-adapted californicus strains on whole plants at 50% humidity. On strawberry everything worked and persimilis worked fastest. On pepper persimilis led for two weeks and then fell apart, finishing behind both californicus strains. On cucumber nothing worked — no predator suppressed the mites and the plants burnt out inside four weeks.
And the trial that tested the actual recommendation, rather than the mechanism — sweet pepper, hot dry climate, two seasons — came out for californicus, calling it the superior spider mite predator at high temperatures and low humidity. Somebody went and checked. It is not a preference.
When persimilis is still worth it
It is much the faster animal, so there is one case where it still earns the order: webbing you need gone in two weeks, on a plant you can keep cool and damp for those two weeks. In the experiment that held humidity steady at 40% and at 80%, persimilis controlled spider mites at both — more slowly at 40%, but it controlled them, at 70 °F.
Warmth is what shuts that door. Same experiment, 81 °F at 40% humidity: not enough control, and egg survival dropped hard. Push past 86 °F in dry air and the predators give up on the top of the plant and retreat down into the canopy, leaving the mites exactly where you can see them. Dry and cool you can work with. Dry and warm you cannot.
Making persimilis work in dry air
Every number above is for humidity held dead steady, and your house is not a chamber. The same 2019 study swung the humidity up and down instead — which is what happens in a real room — and more than 73% of eggs hatched, even when the damp stretch covered only 13% of the time the egg took to develop. A companion paper found the mothers adjust: hold a female in dry air for twenty-four hours and she starts laying eggs built for it.
So water the plant the day you release. Tent it for a few hours. Let the room drop overnight. Humidity that moves is worth more than humidity you get high and hold there.
Both of those studies only tested persimilis, so none of it makes persimilis the better buy in a dry room. It is how you get more out of persimilis once you have decided on it.
Why I am not telling you about leaf-level humidity
You may have read that the air right at the leaf is more humid than the room.
It may well be true and it would explain a lot. But nobody has measured it alongside how the predators actually perform — the one paper that raises it raises it as a possibility it did not test. What has been measured is the fluctuation above, so that is what I am resting this on.
When neither one is right
Two situations where you should not buy either of these bottles.
You are not certain what you have. No webbing, and the new growth is puckered or bronzed rather than speckled? That is more likely broad mites or thrips, and neither mite on this page will help you. Persimilis especially — it only eats spider mites, so getting the identification wrong means buying an animal that will not touch your problem. Confirm the webbing first.
You want cover on plants that are clean right now. That is prevention, and prevention comes in sachets: the californicus sachets for a hot or dry room, or Amblyseius andersoni or one of the blends. Andersoni is a third predatory mite. Neither bottle does this job — persimilis starves once the spider mites are gone, and californicus from a bottle will not build from pollen.
And they are not either/or. Several studies back running them together, which is the thinking behind a blend: the fast one for the crash, the durable one for what comes after.
Field notes
Look before you buy. Webbing and leaves going means persimilis. Speckling, early, or recurring means californicus. That one look sorts it better than any spec on a label.
Buy persimilis twice. It is built to run out. Budget the second release with the first, so you are not reading an empty plant three weeks later as a failure.
Dry room, californicus. Half of persimilis's eggs die between 66 and 70% humidity, and under a steady 60% fewer than one in five hatched. A heated room in winter is well below that.
Dry room and webbing already? Pick your poison. Get the humidity up for a couple of weeks and use persimilis, or double up on californicus and accept it will take longer.
If you are running persimilis dry anyway, water the plant the day you release. Humidity that moves beats humidity held high. A damp pot and a cool night are both on your side.
Bottles are for treatment, sachets are for prevention. The californicus bottle is for plants that have spider mites; it will not reproduce on pollen alone. For clean plants, use the californicus sachets.
On tomatoes, use more. Californicus does not care for plants with dense sticky hairs, and Cornell's fact sheet names tomato and calls for higher release rates. Raise the rate.
Questions people actually ask
My house is 40% humidity. Is persimilis a waste of money?
At 40% I would buy californicus. Half of persimilis's eggs die somewhere between 66 and 70% humidity, under a steady 60% fewer than one in five hatched across five strains, and the field trial run in hot dry conditions came out for californicus. Persimilis is still much the faster animal, so if you have heavy webbing and you can keep the plant cool and damp for a couple of weeks, it can do that job. Otherwise buy the one that survives your room.
Which one works faster?
Persimilis, and not by a little. It has the highest consumption rate of any predatory mite anybody has measured, and in the one whole-plant comparison at ordinary room humidity it cleared spider mites faster than either californicus strain tested. Nobody has published a number for how much faster, so I am not going to invent one.
Can I use both at once?
Yes, and several studies back it. They do eat each other's eggs and larvae in laboratory arenas — and one result worth knowing is that on detached bean leaves with plenty of spider mites present, californicus displaced persimilis in every combination tested. That is a leaf-sized arena with nowhere to go. No whole-plant or greenhouse trial has reproduced it, and it is not what we see on plants. The pairing is the fast one for the crash and the durable one for afterwards.
My predators vanished after the spider mites died. Did something go wrong?
No. For persimilis that is the ending you paid for. It only develops on spider mites, so when they are gone it starves or disperses, and adult females last about six days without food. Californicus in the same spot lasts three to five times longer.
Is californicus a generalist?
Not formally. The standard classification considered moving it into the generalist group and kept it with the selective spider mite predators, because you nearly always find it where spider mites are webbing heavily. Wider menu than persimilis, yes. A mite that lives on anything, no.
Does either of them eat broad mites or thrips?
Californicus will take some other mite species and the occasional thrips. Persimilis will not — spider mites only. If you are not sure which pest you have, identify it before you order rather than after.
References
- Schmidt-Jeffris, R. Neoseiulus californicus – Predatory Mite. Cornell University Integrated Pest Management. cals.cornell.edu
- Phytoseiulus persimilis. Cornell University Biological Control: A Guide to Natural Enemies in North America. biocontrol.entomology.cornell.edu
- Phytoseiulus Predatory Mites. UC Statewide IPM Program, Natural Enemies Gallery. ipm.ucanr.edu
- Cloyd, R.A. (2024). Phytoseiulus persimilis: Biological Control Agent of the Twospotted Spider Mite. MF3665, Kansas State University Research and Extension. ksre.ksu.edu
- McMurtry, J.A., de Moraes, G.J. & Famah Sourassou, N. (2013). Revision of the lifestyles of phytoseiid mites (Acari: Phytoseiidae) and implications for biological control strategies. Systematic and Applied Acarology 18(4): 297–320. doi.org
- Stenseth, C. (1979). Effect of temperature and humidity on the development of Phytoseiulus persimilis and its ability to regulate populations of Tetranychus urticae. Entomophaga 24(3): 311–317. doi.org
- De Courcy Williams, M.E., Kravar-Garde, L., Fenlon, J.S. & Sunderland, K.D. (2004). Phytoseiid mites in protected crops: the effect of humidity and food availability on egg hatch and adult life span of Iphiseius degenerans, Neoseiulus cucumeris, N. californicus and Phytoseiulus persimilis. Experimental & Applied Acarology 32(1–2): 1–13. doi.org
- Gotoh, T., Yamaguchi, K. & Mori, K. (2004). Effect of temperature on life history of the predatory mite Amblyseius (Neoseiulus) californicus (Acari: Phytoseiidae). Experimental & Applied Acarology 32(1–2): 15–30. doi.org
- Walzer, A., Blümel, S. & Schausberger, P. (2001). Population dynamics of interacting predatory mites, Phytoseiulus persimilis and Neoseiulus californicus, held on detached bean leaves. Experimental & Applied Acarology 25(9): 731–743. doi.org
- Palevsky, E., Walzer, A., Gal, S. & Schausberger, P. (2008). Evaluation of dry-adapted strains of the predatory mite Neoseiulus californicus for spider mite control on cucumber, strawberry and pepper. Experimental and Applied Acarology 45(1–2): 15–27. doi.org
- Weintraub, P. & Palevsky, E. (2008). Evaluation of the predatory mite, Neoseiulus californicus, for spider mite control on greenhouse sweet pepper under hot arid field conditions. Experimental and Applied Acarology 45(1–2): 29–37. doi.org
- Le Hesran, S., Groot, T., Knapp, M., Bukovinszky, T., Forestier, T. & Dicke, M. (2019). Phenotypic variation in egg survival in the predatory mite Phytoseiulus persimilis under dry conditions. Biological Control 130: 88–94. doi.org
- Le Hesran, S., Groot, T., Knapp, M., Bukovinszky, T., Nugroho, J.E., Beretta, G. et al. (2020). Maternal effect determines drought resistance of eggs in the predatory mite Phytoseiulus persimilis. Oecologia 192(1): 29–41. doi.org
- Vacacela Ajila, H.E., Colares, F., Lemos, F., Marques, P.H., Franklin, E.C., Santos do Vale, W., Oliveira, E.E., Venzon, M. & Pallini, A. (2019). Supplementary food for Neoseiulus californicus boosts biological control of Tetranychus urticae on strawberry. Pest Management Science 75(7): 1986–1992. doi.org
- Messelink, G. & Leman, A. (2020). Are low humidity levels a limiting factor for spider mite control by phytoseiid predators under fluctuating climatic conditions? IOBC-WPRS Bulletin 149: 101–102. edepot.wur.nl
Heavy and webbed, or light and early — they are different purchases.
