Monday, June 23, 2008

The War With the Spider Mites

I've mentioned before that there are only seven primary houseplant pests. In descending order of seriousness (in my admittedly subjective opinion), they are:

1. mealybugs,
2. scale,
3. whitefly,
4. spider mites,
5. aphids,
6. fungus gnats, and
7. thrips.

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.

Spider mites on a Ficus elastica.

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:

Spider mites on a Catharanthus roseus flower.

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.

Mite damage to an Aspidistra elatior from work.

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.

Another Catharanthus picture.

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:
Outdoor temperature: 85F/29C. Greenhouse temperature: 104F/40C. Greenhouse humidity: 60%. Heat index in greenhouse: 145F/63C.

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.


cruz4him said...

I had a co-worker with a beautiful english ivy that was unfortunately overrun with spider mites and lost much of it's luster. She was about to throw it out when I begged for the chance to rescue it. I cut off all the vines hanging lower than 1-inch off the bottom of the pot and cut them into manageable pieces for rooting. I repotted the mother plant in a pot with many holes for drainage, then for 2 weeks, I washed both plant and cuttings thoroughly in the office's kitchen sink every 3 days or so, making the effort to make sure the undersides of the leaves and the stems were wiped clean during the washings, too, using my bare hands. That was a year ago and both mom and kids are lush and beautiful again, earning me the reputation as some kind of plant miracle worker! It really was worth the effort for this particular plant, although I have to admit I only had a total of 6 plants to look after at the time.

Mr. Green Genes said...

How perfectly timed, this post! I went in the garden today after reading the post and checked on my lemon tree, a past victim of spider mites... and I found it tormented once again by the little red devils!

They had disappeared since some time, but they're back. I sprayed them with some chemical stuff I bought this winter. Supposedly it is a systemic thing, but evidently the mites have developed resistance...

I am wondering- does the fact that a plant is in bloom facilitate the little pests in some way? I have the feeling that the two times my lemon tree has been under attack, it has happened after an extensive (in number of flowers and in time) flowering period. Or maybe the plant flowers when it is hot and dry (as much as it can be in England, anyway).

Anyway thanks for the involuntary help you gave!

sheila said...

Nice essay. Good review for my pesticide applicator exam that I am finally getting around to taking today.

Although in my personal plant collection, I would put spider mites as public enemy number 1. Followed closely by whiteflies. I don't think I water enough to keep mealybugs very happy.

Am said...

Very often I can't find the pests causing the damage.

The only ones I can see are the mealy bugs, and even then sometimes I can only find their sticky cottony residue and not the bugs themselves.

Aphids I can spot - if I'm lucky - too, but have no clue how to remove due to their small size. Read somewhere that some oil spray can be used but haven't purchased it yet.

Just had to cut away 3 long stalks of lush beautiful purple dendrobiums today after I sprayed them the day before cos they were severely infected with flower thrips, the infestation of which happened unbelievably quickly. I check my plants every other day, and they managed to elude me till all of a sudden, I spotted masses of long slender black bugs entering and exiting each bloom, and the flowers fading. Sigh.

mr_subjunctive said...

Mr. Green Genes --

Sorry this is late. Sometimes Blogger tells me there's a comment, and sometimes it doesn't, and sometimes I have time to reply, and sometimes I don't.

I suspect it's probably got more to do with the mites and the flowers needing similar conditions, though I don't know that for certain. Flowering is sometimes stressful for plants, so I wouldn't rule that out, but I doubt you could make mites go away by knocking off the flower buds.


The husband got a Hedera helix a couple winters ago even though I told him it was a bad idea, and it naturally developed a bad mite problem more or less immediately. I didn't cut anything off; I just washed it a lot, and I think the washing (even when I was trying to keep the water off the soil) got it too wet and stressed it out more. Hedera is one of the genera I just don't try anymore, having lost that one and a H. canariensis both to mites.

I'm impressed that you managed to make it work. I don't suppose you want a couple five-foot-tall Cereus peruvianus with mealybugs, would you? 'Cause I could probably make that happen.


Yeah, sometimes it's that way for me too. Usually I have to go by the droppings and webs.

I've also finally seen some thrips lately, on some new arrivals, though they don't seem to pose long-term problems: I don't know if they just don't like most of the food we're offering, or if they're susceptible to the pesticides we use, or what. It's hard for me to believe it could be the pesticides, since I'm pretty sure the place we get them from spray them with pesticides too.

traci_dlc said...

LOL! I was all set to say "Bring it on!" when I google Cereus peruvianus and discovered what exactly it was! Sorry, I leave the TLC for the plants that give me some back. To this day, as much as I'd love some, I refuse to buy cactus as I'm always afraid that I'll trip and fall down on it! I'm the kind of klutz that actually stepped on a rake and received an egg-size bump on my forehead as a result so it's not so farfetched...

cruz4him said...

Oops, used wrong acct - traci_dlc and cruz4him are one and the same... just so you know...

p.s. I impressed myself with saving the ivy! It was my very first plant and it has jumpstarted a gardening kick that now has my cubicle surrounded by plants and earned me the nickname "plant lady" at the office!

Kerri said...

"we may as well be spraying hopes and wishes" almost knocked me out of my chair! It's almost worth my spider mite problem to have read that! :D

JeffB said...

I'm curious, though, what you would do if you did start having problems indoors with thrips. I'm experiencing just such a problem and I'm having a tough time beating them down. Last summer I had thrips on about 5% of my indoor plants (like you I have... many), and while they're mostly dormant a few tribes of intrepid thrips have remained to mock me throughout the winter. I've tried just about everything: insectidal soap, neem, alcohol, rain flushes, winter exile, color strips, etc. None have proven effective in the long term. I've even taken to using systemic insecticide on some of the more prized plants, but that does not seem to be working either and I'd rather not turn my home -- inside or out -- into a superfund site. So any advice you might be able to offer, potentially even in a future blog post, would be greatly appreciated.

mr_subjunctive said...


Honestly, I think you've already tried everything I would be able to come up with. If neem and systemics haven't worked for you, then I don't know what to say.

I would maybe suggest that using systemics on all the plants at once, rather than just "the more prized plants," because otherwise you're just leaving a reservoir for re-infestation. Beyond that, though, I don't know of any magic bullets for thrips.

One site I found said that imidacloprid (the most common systemic pesticide in my experience) is not useful for thrips, so you should check the label of your systemic to make sure thrips are listed, if you haven't already.

If you're mixing your own neem in water, be advised that it does break down relatively quickly in water. You can't mix up a batch, use some, and then spray the rest a few days later, because in a few days it will no longer be effective.

More frequent applications of the other remedies might help, too, though if you have a lot of plants there's obviously a point at which that becomes more trouble than it's worth. I don't know specifically what the life cycle of thrips is like, but one source said that they lay eggs inside the plant, and most of the methods of control (particularly insecticidal soaps and pyrethrins) require direct contact with the insect to kill it.

Sorry I can't be more helpful.

JeffB said...

I appreciate the quick reply. I'll double check the label on the systemic. Now that you mention it, I think it was kinda non-committal about thrips. Maybe I'll just have a mass neem spraydown one day this spring. I'll keep looking and report back if I find success with anything. Now if I can just keep those mealybugs off my sans! Thanks!

foxhead128 said...

Thanks for this post. I recently obtained a Chamaedorea elegans and heard that they're prone to spider mites, so I needed to know how to deal with them.

Cathome said...

I grow datura metel this summer and I've just read that it's prone to spider mites. And - surprise! - I've found them. This is the first time I get spider mites at home. It's a tragedy for me, because for the last 2 years I've been fighting them at work in vain (thanks to someone who decided to present me a rose bouquet seasoned with spider mites!)

Cathome said...

Over the past months I've tried almost every acaricide I've been able to get. And even I haven't these bastards for about 2 months. But today I found them again. I think it's impossible to get rid of them forever :(((

Jenny Maurer said...

After a long cold Winter, we decided to take our Ficus outside and clean it off on this nice breezy, warm Spring Day after we realized that the sticky leaves we were seeing were due to , what we found to be, Spider Mites. I took a spray bottle with water and proceded to get as many singles leaves as possible and wipe them off as I sprayed. Bare handed. About an hour into the project, I found some itching spots on my wrists with what looked like spider bites! Sure enough, these little buggers DO BITE and will leave an itchy welt. I happened to have some Theives spray and that seemed to help almost immediately. I also found out that they like to travel... found a few more spots on my shins as well and I was wearing blue jeans! So, word to the wise: use gloves and wash up frequently after your contact with these little critters. Otherwise you'll feel creepy, crawly all day...

mr_subjunctive said...

Jenny Maurer:

Sticky leaves wouldn't be from spider mites; they'd be from scale or mealybugs. It is, of course, possible to have spider mites and scale at the same time.

Spider mites can't bite through human skin, but I believe you about the itchy welts: some people are allergic to Ficus benjamina sap (it makes me itch; other people have more severe reactions); if you were wiping off a lot of leaves, you were probably also touching a lot of sap. Can't explain the spots on your shins (were you wearing shorts? Were you doing this on your lawn, where chiggers might have gotten you?), but whatever you experienced, it wasn't from spider mite bites.