I've mentioned before that there are only seven primary houseplant pests. In descending order of seriousness (in my admittedly subjective opinion), they are:
4. spider mites,
6. fungus gnats, and
The first three on the list (mealybugs, scale and whitefly) can be very serious problems: they are very contagious, they multiply quickly, and they can cause a lot of damage. The last three (aphids, fungus gnats, and thrips) can be problems under certain circumstances, but indoors, you can either get rid of them pretty easily (aphids), they don't really cause much damage in the first place (fungus gnats1), or you're unlikely to run into them (thrips).
Spider mites occupy that special middle rung, where they're very contagious and multiply quickly, and they can cause real damage, but at the same time they're not necessarily that big of a deal. The severity of a spider mite problem depends hugely on A) what plant is affected, and B) what the general growing conditions of the plant happen to be: change the plant or the conditions, and you have an entirely new ballgame.
So let's start with the basics. What is a spider mite?
Spider mites are not insects. What they are is arachnids, which means they're more related to spiders, daddy-longlegs and ticks than they are to flies and beetles. (Forgive the pedantry, but this sort of thing actually matters sometimes.2) And if you look at them closely (if you can look at them closely), this is actually pretty obvious, especially the resemblance to ticks. So there's that. Spider mites are all in the family Tetranychidae, and there are about 1600 species in the family, wikiposedly, though most of these are not abundant enough or hardy enough to pose a problem for cultivated plants.
The most common species of spider mite is Tetranychus urticae, usually called the two-spotted spider mite. It can be various colors,3 but most of the time when I've seen them, they've been red, beige, or brown. There are very, very few good pictures of them on-line, for the simple reason that they're very tiny, plus they move around, so it's difficult to get a good, well-focused shot, and if you do get a good shot, you're usually so close in to the plant that it's difficult to tell what parts of the picture are the mites and what parts are the plant. Though in cases where the background is all the same color, it's easier:
Fortunately, one doesn't need to see the mites themselves to know they're there. The usual first sign of an infestation is that the plant just starts to look kind of tired. The leaves in particular get a dusty or bronzed look to them, and close inspection of the leaves will probably show both a little actual dust and dirt, plus a bunch of very tiny yellow or gray spots, which are what give the dusty look. The spots are plant cells that have collapsed and died due to the spider mites' feeding. The mites pierce the cell walls with their mouthparts, suck out the liquid, and then move on when sated. The cells are unable to repair the damage, and consequently die, leaving a tiny gray or tan spot. Multiply times ten gazillion, and you have faded-looking leaves.
If you suspect spider mites from looking at the top of the leaves, looking underneath will generally confirm it. In very advanced cases, you'll see some webbing underneath, usually visible because it has "dirt" sticking to it. The "dirt" may be actual dirt that's been splashed up during watering of the plant, in which case it's no big deal, or it's mite droppings, which isn't a big deal either but it's kinda gross. Once you've seen both a few times, it's pretty easy to tell the difference. (As a rule, actual dirt will crumble and fall at the slightest touch, and embedded droppings will stick until you wipe them off with a finger.) Should also note that I know this mostly because I do basically all my watering at home and at work from overhead, which is a splashier process than using a watering can like a civilized person would, and leads to more dirt hanging from the undersides of leaves than you probably have on your plants.) In very very bad cases, especially if it's hot, you'll see the mites themselves running around.
Spider mites are, at least in theory, capable of living off of any plant, but in practice they're limited to those with fairly thin, easily-penetrated leaves, so your cacti and succulents are safe. Even among thin-leaved plants, they have some strong preferences for particular kinds.
It's impossible to come up with a comprehensive list of which plants are the bad ones for spider mites. That said, there are a handful of plants that I, customers, or people I know refuse to attempt anymore, because of spider mites, so I guess they're good candidates for the worst of the worst. Those unlucky few are:
Aspidistra elatior (cast-iron plant),4
Calathea spp., including Calathea ornata,
Codiaeum variegatum (croton),
Hedera helix (English ivy),
Schefflera actinophylla ,
Schefflera arboricola , and
Schefflera elegantissima (Dizygotheca elegantissima, false aralia).
I've got a list of other mite-prone plants. The list is long, and hard to make interesting, so I'll stick it in a footnote rather than ask everybody to read through it all.5
Ordinarily you'll find that mite problems are cyclical, recurring things, rather than a full-scale battle all year long. Mites have certain environmental conditions they prefer, and if you can make things less to their liking, they'll slow way down or even sometimes disappear entirely. This is the good news, if there is good news.
Mites prefer hot, dry conditions and stagnant air. Change a part of this equation, and you can accomplish a lot. If your home must be dry and stagnant, try cooling it down. If it must be hot and dry, at least get some air moving. That kind of thing. Another option, if you have a space available, is to move your plants outside for the summer: in a lot of areas, outdoors may be hot, but at least there will be humidity and air movement.
Most spider mite problems indoors start in the fall and last through the early spring, when homes are being heated, the humidity level is low, and moving plants outside isn't really an option. I had relatively good luck at home last winter using a humidifier (I still had mites, but it was a lot less of an issue than it had been the winter before.). Misting and pebble trays are better than nothing, though (in my opinion) not by much.
One can also try poison, of course. In the greenhouse at work, we spray miticides weekly, but for all the good it does us we could be spraying hopes and wishes; I think we're selecting for miticide-resistant mites, not keeping a pest population under control. And I have said so. And I am being roundly ignored.6 Last winter, we had a particularly bad plague sweep through the larger crotons (small ones were fine), most of which lived through it, but of course then they looked like crap, and they're only just now beginning to come back. Last fall, it was the banana (Musa) trees and the Alocasia 'Polly.' During the spring, it was some of the annuals (particularly Dahlia, Vinca, Angelonia, Nemesia, Alternanthera, Catharanthus roseus).7 At the moment we're pretty close to clear, though we've been preoccupied by the flood, too, so maybe there's a problem I'm just not yet aware of. I should probably check the bananas.
Spider mites are known for being able to develop resistance to miticides, so if you do elect to go with poison, my advice (which I do not, myself, follow, not that that makes it bad advice) would be to buy two or three types, containing different active ingredients, and use them in rotation. Using the same product repeatedly may still work: it's just that it's much harder for pests to adapt to multiple pesticides in rapid succession than it is for them to adapt to one: they have to hit a moving target, so to speak.
Other things to try, besides fans, humidifiers, poisons, and air conditioning, include moving affected plants (and any other plants, while you're at it) to the shower to water. It's not that watering from high overhead is any better than watering with a can, necessarily, but water droplets can dislodge the occasional mite, and every mite that gets knocked off is one more mite that isn't going to be laying eggs on your plant.8 Some insecticidal soaps also work on mites, though you're probably money ahead to skip the specially formulated "insecticidal" soaps and go with a solution of dishwashing liquid in water, or liquid hand soap in water. The insecticidal soap we sell at work is, if I read the label correctly, more or less repackaged, diluted liquid hand soap, i.e. potassium salts of fatty acids.
There are also oils one could try: we have a product at work that's formulated from cottonseed oil, clove oil, and garlic extract, that I've used sometimes at work. I'm not sure if it's all that effective, really - it wasn't terribly helpful with the bananas last summer - but it may also not have been used as regularly as directed. This sort of product seems to be intended to just annoy the mites into leaving: if they can't walk around or start new webs, because everything's covered in oil, I suppose they might eventually die off as a result, but it seems like it'd be hard to cover every available surface, and the oil does go away after a certain amount of time has passed.
Neem oil, on the other hand, has an actual poison in it, yet is safe to use in the house (more or less. I mean, don't go putting it on your salad or anything.). You still might not want to, as it has a weird smell: to me it smells sort of like peanut butter and garlic and socks. (Your perception may vary.) It has a chemical in it which disrupts the normal function of mites in a number of ways. I've only tried it once on a plant with mites, and the plant recovered (plus it was a Dizygotheca elegantissima, even), but I couldn't say that the neem was what won the battle.
So in conclusion: you are probably going to have spider mite problems sooner or later. (The saying was originally "death, taxes and spider mites.") One should be particularly vigilant with the plants that are known to be especially attractive to mites. If you find mites on a plant, it's not necessarily the end of the world: you have a lot of treatment options. The one thing you can't do is be complacent about watching for them or procrastinating on treatment if you see them, because infestations can flare up in very little time.
Photo credits: all my own.
Pre-emptive defense against charges of plagiarism: I just want to say for the record that this article has certain similarities to the corresponding article at Water Roots, including the assertion that some plants are more mite-prone than others and the metaphor of warfare. This is not because I plagiarized from said article; it's just that, you know, some plants really are more mite-prone than others, and the only metaphor that really works is warfare.
1 I know, I know, fungus gnats killed your sycamore tree. Fine. Whatever. Bad fungus gnats.
2 Among other things, mites not being insects is an indicator that the chemicals that will kill insects (like mealybugs, whiteflies, etc.) will not necessarily kill spider mites. It also, obviously, suggests that if you use a miticide in the greenhouse, you run a strong likelihood of killing any spiders, ticks, or daddy-longlegs that happen to be around too. It's hard to get sentimental about ticks, but how anybody could want the daddy-longlegs dead is beyond me.
3 According to the stuff I've found in the research for the post, the color of spider mites depends variously on what and how much they've been eating, the time of year, and the point of the life cycle a given individual happens to be in. Green, brown, tan, beige, translucent, red, red-brown, and probably others I can't think of right now are all options. The "two spots" in the common name may or may not be present.
4 I think under normal circumstances Aspidistra elatior is usually okay, because people don't usually bring lots of plants in and out of their home all the time. We have trouble with the ones at work because new plants are always being brought in, any one of which might have a mite or two on it, plus we always have a few lingering somewhere in the greenhouse, and the plants get rearranged a lot. So, however good a job we do at keeping the Aspidistras clean, inevitably there's going to be a new population springing up. It's been enough to put me off of buying one, though the A. lurida 'Milky Way' I had previously is still okay, knock wood. I do not know whether A. lurida is supposed to be better with respect to mites than A. elatior, but so far it does kind of look that way.
5 Other plants which are rumored to be exceptionally mite-prone, or which I've actually experienced having mites on: Anthurium andraeanum, Araucaria heterophylla, Aspidistra lurida 'Milky Way,' Caryota mitis, Chamaedorea elegans, Chamaedorea seifrizii, Citrus spp., Cissus rhombifolia, Cordyline fruticosa, Dieffenbachia spp., Dracaena marginata, Euphorbia pulcherrima, Ficus benjamina, Ficus elastica, Gardenia jasminoides, Maranta leuconeura erythroneura, Musa spp., Philodendron bipennifolium.
6 (Not that I'm bitter.)
7 Lest you think we're terrible people who are selling buggy plants to customers knowingly and on purpose: if I see mites on a plant, I'll say so to the customer and look for another plant of the same variety that's better, or suggest a different variety. I think this is generally the policy for everybody else, too. That said, during busy days, it's not always possible for an employee to inspect every plant that goes out the door, and different employees vary in their familiarity with pest problems, and mistakes do get made, so you should always always always do your own inspection too. Or at the very least you can ask the cashier to look when you're buying, if s/he doesn't do so spontaneously. I doubt that this matters a whole lot when it comes to annuals, because once people plant them outside and they've got some air movement, I think the spider mites generally go away anyway; it's only an issue because we have to keep some of them in the greenhouse, and the greenhouse gets very hot and dry sometimes:
Also, while I'm here: although Alternanthera is in the list of things we had trouble with in the greenhouse, spider mites don't seem to go for my Alternantheras at home any more than they do for the rest of my plants. Not sure how to explain this, but it's possible that the variety at work is just an extra-delicious one.
8 I've seen the claim made that spider mites are "born pregnant;" this is not the case. They lay eggs. The person making the claim was probably thinking of aphids, some of which are born pregnant. Not sure about thrips, but whiteflies, spider mites, scale, mealybugs, and fungus gnats all lay eggs.