Fungus Gnats: Like Puppies That Try To Fly Up Your Nose

One of the things that eventually drove me away from the Garden Web House Plant forum (GWHPF) was the way people posted the same questions there over and over. It wasn't the same person asking the same questions, obviously, but, you know, you see anybody asking something you've answered thirty times already for other people, and you start to wonder why people think it's okay to ask you to type out an answer for their problem for fifteen minutes, but it's too much trouble for them to spend three minutes searching the archives.¹ Which leads to irritation.

And then my irritation would bleed through and I'd write subtly hostile, cranky responses, which would just convince the person asking the question that everybody in the Garden Web forums was hostile and mean.² I suspect I started writing PATSP partly so I could have a place to link to for questions I'd seen a million times already, rather than spending half an hour answering them all over again from scratch.

By far, the most frequently re-asked question at the GWHPF went something like, HELP!!!1!!!1!! I just bought this plant and I don't know what it is but there are all these fruit flies flying around it all the time and this is the worst thing that has ever happened to me so please please please can somebody tell me what to do to make them all die because they are ruining my entire life!!1!!!!!1eleven!! (And yes, some of them were actually that dramatic. You would be amazed.)

So.


First, don't panic. Or, if you'd already started panicking, please stop, 'cause it's annoying to read anything with that many exclamation points.

Second: they're not "fruit flies," (Drosophila) they're fungus gnats (Bradysia). I realize you probably don't see a difference or care, but you have to know your enemy in order to defeat them.

Where do they come from? Most likely, they came in with a recently-purchased plant. If you haven't brought in a new plant, but you've repotted something recently, then your potting soil probably contains gnat eggs. Miracle Gro potting mixes in particular are consistently about 67% gnat eggs by weight,³ but gnats are a risk you take with any high-peat mix.

Why? I dunno. They just like peat, I guess.

What do they want? The adults lay eggs on wet soil, the eggs hatch into larvae, the larvae eat (mostly decomposing organic matter, not necessarily fungus per se), grow, pupate, and then they're transformed into beautiful . . . fungus gnats.⁴ Adults don't eat,⁵ so they don't live very long: they're focused on mating and laying eggs. Since the survival of the eggs depends a lot on moisture, adults are very interested in things that strike them as potentially wet. Which means that sometimes, they will try to fly up your nose or in your mouth. Also one will occasionally find them drowned in unattended cups of coffee, or hovering near sinks.

Are they hurting my plants? Probably not. The larvae are capable of feeding on the fine root hairs of plants, but even if they do, this would only harm plants that were very young or very weak. Living tissue is probably not their first choice anyway.

How do I make them go away? You have many options here, with varying levels of ease, expense, and effectiveness.

Personally, I would recommend either watering less often or doing a soil change. If the soil the larvae are in goes completely dry, they die, and then you don't have a next generation of adults. It may take a while, especially if you have a lot of plants, bring home new plants often, or have a lot of plants that will die if they get too dry, but if you're consistent and patient, it should happen.

Letting plants dry between waterings is what I personally do. I bring in new plants often enough that we usually have a few fungus gnats around at any given moment, but considering the number of plants here, a few is as good as none. And honestly, I kind of like them.⁶

Soil change can be thorough (shake off every bit of soil from the roots and replace with all-new soil from a brand-new bag) or partial (shake off some of the soil and replace it with a faster-drying mix, like potting soil cut with perlite, coarse sand, aquarium gravel, aquatic soil, etc.). Thorough soil replacement may be more stressful for the plant, but it should stop the gnats immediately, assuming the replacement mix is clean. Partial replacement is easier on the plant, but won't fix the problem as fast. Some of the old soil will still be there; it'll just be drying out faster.

Another option: sticky traps. I don't like sticky traps. They sound good in theory -- they're just pieces of yellow cardboard with adhesive on them. Gnats land on the cardboard and then can't fly away again, so gradually you wind up with fewer and fewer gnats flying around.

In my personal experience with sticky traps at work, mostly the traps stick to leaves and hands, not to bugs, and when they do function as advertised, you then get to look at a brilliant yellow piece of cardboard that's covered in dead insects, which . . . makes a statement, certainly, but I don't think it's going to catch on in New York.

Soil drenches are another option. If you're going to go this route, a product based on Bacillus thuringiensis ssp. israelensis, or "BT," is probably the way to go. Some brand names are Gnatrol and Knock Out Gnats; several "mosquito dunks" also use BT and can be used instead, if you don't find gnat-specific preparations.⁷ BT is a bacterium that produces a protein which screws up the larvae's digestive systems and kills them. It's organic and safe around people or pets. One may have to re-treat once or twice before the gnats completely go away.

Several people at Garden Web swear by the potato method, which is: slice a potato in half. Lay the cut end on top of the soil. The fungus gnat larvae will find the potato, get excited, and start chowing down. After a few hours, you can pick the potato back up, with all the larvae now on it, and throw it away. Or you can slice off a section and put it back on the soil again. I'm not sure this is sufficient to eliminate a gnat infestation all on its own, but it might speed up the process if you decide to reduce watering, and the more larvae you can get rid of, the fewer adults you have to see.

A simple and fairly cheap solution for small houseplant collections: spread a thin layer (maybe 1/4 inch or 6 mm) of coarse sand on top of the soil in your houseplants. The adults can't get through the sand to lay eggs, and the larvae can't get through the sand to become adults, so the problem disappears in one generation. Plants so treated will stay wet longer, since water will have a harder time evaporating from the soil surface, but I suppose one could always scrape the sand back off again after two or three weeks, if it's really a problem.⁸


Actual pesticides like pyrethrins, imidacloprid, etc. are not, I think, necessary, nor will most of them even be effective, either because of their targets (if the label doesn't specifically say it will kill fungus gnats, it probably won't) or their mode of application (pyrethrins have to make direct contact with an organism to kill, and break down quickly -- they're useless for a fungus gnat larva an inch below the soil). There might be a role for a soil drench of neem oil with dishwashing liquid, if you're going to insist on a pesticide, but I think the other methods will probably work just as well as poison, and some of them are very cheap and easy. I mean, technically, watering less often should save you money. Just not very much of it.

That's pretty much everything I know about fungus gnats and what to do with them. If there's something I haven't covered, or if there's something I did address that you think I got wrong, feel free to leave a question or comment. Alternate methods of dealing with fungus gnats are welcome too.

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¹ Of course, the people posting questions sort of are being responsible and doing their own research, in a way -- if you have a plant problem, it makes sense to go to a plant-related forum to ask somebody about it. And if you've never been to GW before, you might not be able to appreciate how big it is, you might not be able to locate the search field, you might try to search and wind up getting gibberish back, if GW's code is being glitchy.
Really I was the one with the problem. I wasn't responsible for answering fungus gnat questions, and if I didn't want to answer them, then I could have just not answered them, rather than making it all about me and getting upset.
² Which, last I checked, was mostly true, actually, but it wasn't always that way, and it certainly doesn't have to be. The dynamics of online communities is perhaps a topic for another time.
That, plus feeling obligated to answer people's questions even though I wasn't, made it sort of a bad place for me to be, overall, and I'm glad I'm done with posting there. I do go back every few months to look around. Sometimes I look to see if I can find a blog topic, though I don't remember that ever actually working out.
³ (Exaggeration for comic effect; I'm sure it's less than 20%.)
⁴ Not every ugly duckling becomes a swan, yo.
⁵ (Pretty sure I read this somewhere at some point.) (UPDATE: Aralia left a comment with a link saying that this isn't true; adult fungus gnats do eat. So never mind.)
⁶ Yes, it's alarming to inhale one accidentally, or have one flying around your face for a few seconds like it's trying to figure out how to get in, but it doesn't actually hurt you. I mean, if you inhale a gnat, you're definitely getting the better end of that experience.
Around here, they come and go -- we are presently having a high-fungus-gnat period, which is why this post -- but I kind of think they're cute. Like puppies! Puppies that try to fly up your nose!
⁷ The difference, as I understand it, is that mosquito BT is made to dissolve slowly, over a long period, to kill mosquitoes that attempt to lay eggs in whatever body of water the "dunk" is sitting in, while gnat BT is usually either made to be dusted directly on the surface of the soil or made to dissolve more or less immediately in water. I'm fuzzy on this, though, having never used either product, and I've only occasionally seen the mosquito dunks.
⁸ This might be the place to note that decorative moss or stones on top of the soil will also keep the soil wetter longer. If you have such items on your soil, you should take them off at least until the fungus gnat problem is resolved. (I would actually advise not putting them back on again ever, but that's just me.)

Materials and Techniques: Selenicereus chrysocardium cuttings

This post might be a little premature, but even if we're not quite at the endpoint yet, enough has happened that I think a post can be built around it. So here we go.

I decided a while ago that I wanted to propagate some of my Selenicereus chrysocardium cuttings, both as insurance against a potential Selenicereus disaster and also to have on hand to trade or sell, should the situation arise. I wasn't sure quite how to do this, so I . . . basically just guessed.

The first step was to decide how many cuttings to take, and of what size. When we'd done this at work, we just cut the ends off of some long "fronds" (actually stems) and stuck them in soil. Which worked. I couldn't really do that, because my plant is only just so large, and and I didn't want to hack it to pieces just for propagation. So what I did was, I tried to make multiple cuttings from a single stem, which I did like so: I cut the -- I guess you'd call it the "midrib?" -- in multiple places, dividing the stem into multiple pieces which all contain some of the midrib.¹ The pink lines in the below photo illustrate where I cut.


I planted them in vermiculite, in a plastic salad-mix container. Why vermiculite? Well, I was concerned about fungal problems if I planted in potting mix, and vermiculite is a sterile medium. It also tends to be really good for rooting things, even things that are ordinarily difficult. This may not have been completely necessary either: like I said, when we did this at work, we just used soil directly and it all turned out fine. But I wanted to be careful. Thus:

I think it looks sort of like a rib cage.
Very little has happened since then. I initially left the top of the container off, for fear of fungal problems. After it'd dried out a few times because I forgot to water it,² I tried putting the top on. This turned out to be a bad idea; the next time I opened it up to check, I had botrytis growing on the cut ends.

Botrytis, sometimes also called gray mold, is a grayish fungus that attacks many, many types of houseplants, and I saw it all the time in the greenhouse at work, usually on leaves that had died and fallen on top of wet soil. It's unusual in a home environment because it's unusual for a home to be humid enough to keep an infection going. I removed the cover from the plastic container, sprayed the cuttings with neem oil, and haven't seen any Botrytis since.

Notice that the botrytis is only located where the plant was cut; it's not ordinarily able to attack uninjured tissue.
Extreme close-up to show the structure. But also because extreme close-ups are cool. (I still want a microscope so bad, y'all.)
I'm a little impatient for something visible to happen -- it seems like I should have at least one of these producing new foliage by now -- but I know they're doing something, because they don't pull out of the vermiculite easily like they used to, so there must be roots in there, and I've actually seen one root so far:


So it should only be a matter of time before I get foliage. And then I can pot them into separate pots, and they will thrive and grow and life will be perfect forever. Or that's my plan, anyway.

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¹ This may not be necessary, strictly speaking; I am unsure whether one can propagate from pieces which don't contain the "midrib," never having tried it. I think they did try it at work after I left, but I don't know how that turned out. So I was basically hedging my bets here -- I know that you can at least do it the way I'm describing, and if it turns out that a person can also propagate from smaller pieces of the plant, then that's great. Let me know if you know.
² Wet vermiculite looks exactly the same as dry vermiculite, so I can't just look at the plants and know whether they need water. Which means that sometimes they dry out. Fortunately, Selenicereus chrysocardium handles drying out quite well, so long as it's for a reasonable amount of time.

Unfinished business: fern babies, Anthurium babies

I am extremely sorry to have to report that the baby Cyrtomium falcatum and Asplenium nidus ferns I started forever ago, as well as half the Anthurium andraeanum seedlings I started half a forever ago, are not going to make it. I'm even more sorry to report that the reason why they didn't make it is really stupid. I got greedy.

But first, a quick recap.

At some point I didn't bother to write down, I brought home some fern spores from work. I put them on wet vermiculite, and in November 2008 I had the beginnings of baby ferns.


Eight months later, in July 2009, they'd developed to the point where there were signs of actual fronds developing, which was pretty exciting.


Meanwhile, I had some seeds form on an Anthurium andraeanum 'Pandola' that I'd bought in spring 2009, and I planted half of them with some Begonia leaf sections I was trying to propagate, and the other half, I planted with the baby ferns. And they developed and grew and all was good. So at this point we've got some developing Cyrtomium, Asplenium, and Anthurium, all in the same container of moist vermiculite.


And then I did something really, really stupid.

I had bought, somewhere in winter or spring 2008, a bunch of seeds of various tropical plants. I was going to plant them, and raise many, many baby plants, and it was going to be fun and awesome and stuff. Unfortunately, I planted the seeds in a plug tray, which I then failed to keep watered, so the seeds didn't sprout.

But I'd remained hopeful, and didn't throw the seeds out, because who knew, maybe they'd still be good for something. Which I apparently found a convincing argument until sometime during the summer of 2009, when I decided that there was no point in hanging on to plug trays full of very dry dirt anymore; if they'd been viable to begin with, surely they no longer were.

Which in retrospect was a really good decision.

However! As I was dumping them out, I noticed the Cordyline fruticosa seeds, which were big enough to be easily picked out of the soil and recovered. And I thought: hey. Maybe I could still plant these. What could it hurt?

The answer turned out to be, everything. It could hurt everything. Because, see, the seeds, when planted in the vermiculite, also introduced some odd little bugs, which I'd never seen before. I also kind of haven't seen them since, or at the time, because they are/were very tiny and semi-transparent and moved around a lot. This is the best picture I was able to get of them:


And this is the scale we're talking about:


So. Trapped in a more or less airtight plastic container with nothing to eat but developing seedlings and . . . um . . . let's call them sporelings, these bugs slowly went about the business of eating what was there to eat, apparently from the roots up. So the tops of the ferns withered, then browned, then collapsed. Patches of the ferns went black and slimy, and the Anthuriums turned sort of a weird bronze color (though they hung in there for quite a while, way longer than the ferns). I dithered for a long time about whether or not to throw the whole thing out, and eventually decided to toss 'em . . . right at about the same time I noticed that the Cordyline seeds were beginning to sprout. (So at least we know they can still be viable after sitting around for a couple years, even if they're totally dry for a six-month stretch there in the middle.) But the Cordylines were also bronze, and kind of stunted-looking, even for seedlings, so out the whole thing went. (No pictures. Too painful.)

So there's the whole tragic tale. I've started over again, with fresh vermiculite and fresh spores (obligingly produced by my own personal Cyrtomium last November, almost like it knew I was going to need them -- maybe I should have gone with "Psychic" for the "person," when I wrote the Cyrtomium falcatum profile), and so maybe in early 2012 I'll have some news on that.

The moral of the story, clearly, is: don't put nonsterile seeds into sterile media if you really value the stuff that's already growing in the sterile media. And insects are bad.

One could also make a case for part of the moral being that ferns are slow, frustrating, and not worth trying to raise from spores, but I'm not going there just yet.

The reader will be relieved to hear that the other half of the Anthurium seedlings, the ones with the Begonias, are still intact. I moved them into a 3-inch pot a week or two ago, because the Begonias were getting too big for the container and I couldn't transplant one without also doing the other, and so far they're doing okay, though I do worry a little.

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.

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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.

Mealybugs and You

There are really a pretty limited number of pests that attack houseplants, when it comes down to it. Mealybugs, spider mites, scale, whiteflies, aphids, fungus gnats, and, if we're feeling really charitable and want to make them feel important, thrips. None of these are good for your plant, but some of them are considerably worse than others. And this guy:



is pretty much as bad as it gets.

There are a number of species of mealybug,¹ and all of them are evil. The most significant, as indoor pests go, are the citrus mealybug (Planococcus citri) and the long-tailed mealybug (Pseudococcus longispinus), though there are others, including root mealybugs (Rhizoecus spp.). Root mealybugs are the vilest creature ever dreamed up in the fires of hell, and are too distressing for me to even discuss in detail here. If your plant has root mealybugs, not only should you give up on saving it immediately (you can try to take cuttings, maybe, if it's that kind of plant), but you should put it and the soil in which it was planted into a trash bag, seal the trash bag well, bury the trash bag, carve a granite monument to warn future generations of the dangers buried in that spot, and then salt the earth above the burial site so that no living thing will ever grow there again.²

But what we're talking about is the more ordinary (still evil!) foliage mealybug. These are usually light gray or white (occasionally kind of barely lavender or pink), and if they're off by themselves, they're perfectly oval-shaped. More often, though, they appear sort of ridged, like in the picture below,³ and have a sort of fringe around the outside of the oval. They may also be partly hidden under a fluffy, cottony-looking layer of wax, which is almost always pure white. The long-tailed mealybugs will have, guess what, long tails as well. All mealybugs tend to congregate in more cramped areas: around a cactus areole⁴ like on the picture, or in a leaf axil. ⁵

Pay attention to that "water" droplet. It'll come up again later.

Here's a typical day in the life of a mealybug:

You wake up in the morning and, if you are located on a plant, you begin to suck the sap out of it. If you are not located on a plant, you try to move until you are located on a plant. Depending on the sort of day you're having, you might also lay 300-600 eggs, mate 9-13 times, or take part in a three-way. Then you go to sleep and/or die. The end.⁶

The sap-sucking may or may not be a big deal to the plant: it depends on how severe the infestation, and how healthy the plant is otherwise. Some plants are also better sources of nutrition, apparently, too: mealybugs are an especially big problem for Hoya species (especially the 'Hindu Rope' variety of H. carnosa) and cacti, and at work we've also had to deal with them on Saxifraga stolonifera, Streptocarpus cvv., and Echeveria. No plant is immune, however, and there's no such thing as a tolerable level of mealybugs, either: they're always capable of stunting or distorting new growth, causing reduced flowering and dropped buds, weakening a plant to other attacks, and creating a mold problem.

Mold?

Yes, mold. Here's the deal: plant sap has carbohydrates (=sugar) in it, mostly, and plant sap is the only thing mealybugs eat. However, all animals, mealybugs included, need protein as well, which is present in much smaller amounts. This is why life as a mealybug is so boring: you pretty much have to eat all day long in order to get enough protein to function, and all "functioning" gets you is the ability to stay in one place and eat all day long. But wait, there's a problem – if you eat all day long, you're going to get pretty full, right? Unless you do something? So mealybugs excrete whatever they don't need, on a pretty much constant basis. Since the sap is mainly water and carbohydrate (sugar), the waste is also mostly water and sugar. This is, I'm almost positive, the source of the "water droplet" in the picture. What likes sugar water besides teenagers and hummingbirds? Well, plenty of things, but mold is way up there too.



In a backhanded way, the mold is kind of a good thing for a houseplant grower, because it may get your attention when the bugs themselves do not. The bugs can look like dust, or lint, or a little chunk of perlite. The mold, though, is black and sooty, and there's no way you're going to mistake that for anything innocent.

By this point you might be saying, Okay, Mr. S., Jesus, I get it, they're bad. When do you get to the part about how to make them go away?

Patience, patience. Calm down. It'll be over soon.

So anyway. The common name of "mealybug" is apparently a reference to the waxy coating, which gives the insects a sort of "rolled in flour" look. Make the flour = meal connection, and there you have it. I mention this mostly because it's new to me (I've always wondered about the name), but it also happens to lead into our next topic, which is, why they're so goddamned difficult.

The coating doesn't just look waxy: it actually is wax. Wax, unfortunately, is very good at repelling water. What this means is, most insecticidal sprays aren't going to work, because most sprays have to contact the pest's body in order to do any damage. Two ways to deal with an obstacle like this: around, or through.

To go around, one can use a systemic insecticide. These are insecticides which are introduced into the soil and taken up by the roots. They don't harm the plant, but they do change it from snack to poison. Since the mealybugs have to eat more or less around the clock, once the poison is in the plant's system, the bugs begin to die. It sounds elegant in theory, though I suspect that there's a catch somewhere. Still, we did have a pretty dramatic decrease in our whitefly problem within a few weeks of treating the worst-off plants with Marathon (imidacloprid).⁷ So systemics have impressed me before, and I just bought some to use on my cacti, so we'll see if they're as impressive at home.

To go through, we have lower-tech solutions: you can dissolve away (or at least soften up) the wax with rubbing alcohol, and then hit the plant with a regular spray pesticide after that. You can also go through by taking some Q-Tips and squishing the miserable little bastards by brute force: it's emotionally satisfying, but not especially effective, because you won't be able to see all of them. Rubbing alcohol might be useful just in loosening the grip of the bugs on the plant: a rubbing alcohol misting, followed by a forceful but not insanely strong shower of water, might get rid of enough bugs to be useful, but I wouldn't rely on that by itself. (I've tried. Doesn't work.)

A strategy that worked in the past on the smaller Cereus peruvianus, but which has not worked so far on the larger ones, has been to add mealybug-squishing to my daily routine for a few weeks. Every day, or at least every few days, I'd get out the rubbing alcohol and Q-Tips and go over the entire surface of every affected cactus, squishing every actual critter I saw and giving a good firm rubdown of everything else. After a month or so of this, the problem seemed to be more or less over (two weeks isn't enough, on the other hand: I tried that too, and there were still enough residual eggs and crawlers that they mounted a comeback. I may have an asymptomatic mealybug carrier somewhere in the group of plants, in which case you can expect a post titled Typhoid Mary sometime soon. Assuming that I can figure out which one it is.), though recent inspections of the small Cereus peruvianus have turned up new activity anyway. (I really did seem to manage to eradicate them from four out of the six affected small cacti, with nothing more than Q-Tips and rubbing alcohol, which is proof that the concept is sound. Wouldn't necesarily recommend it, because it's a time-consuming way to go, but it's at least more than theoretically possible.)

With the big cacti, or with any leafier plants that have more nooks and crannies, you need a different strategy. I've now spent a few nights going over the big cacti, areole by areole, with a fistful of Q-Tips and a quarter of a bottle of rubbing alcohol, and that's great stuff, but I think I'm not going to win this one without some systemics besides. And probably another few rounds with the rubbing alcohol. I started with the Bonide preparation of imidacloprid last Thursday; we'll see if I can shake the damn things that way.

Don't be ashamed to give up on a plant if you need to. It's better to throw a plant out at the first sign of trouble than it is to do a half-assed job of eradication and find them spreading to all your other plants. With my Cereus, especially the five-foot ones, I feel like I kind of don't have a choice but to fight, since I've had them so long and there's considerable sentimental attachment there. But battling pests can easily become a chore that will suck all the enjoyment out of the hobby, and with mealybugs, most other indoor gardeners will understand if you just don't want to go to the trouble.

Something to consider: if the plant is a recent acquisition, you might consider telling the store (nicely!) where you bought it that they sold you a bad plant. They're not necessarily going to offer you a new one, or hand you a complimentary bottle of pesticide, but it's only neighborly to let them know, in case they're not aware of the problem. And it's also a way to gauge what kind of business they are, and decide if it's really a place where you want to be spending your money. I'm not actually sure which way my own employer would jump, in this situation, though I could see the boss being okay with taking the plant back until the mealybugs were under control: we haven't done that before, to my knowledge, but it seems like the kind of thing we would do.

Two other final notes: never never never never never knowingly buy a plant with mealybugs, thinking that you'll be able to get rid of them easily. I have done this myself (with a small Hoya), and it did work out okay, but at that time I hadn't actually dealt with trying to get mealybugs off of anything, and looking back at that purchase now, I shudder to think of the risk I was taking. Just don't do it. Even if it's the most awesome plant ever. Even if it's only one mealybug. Just say no, and walk away.

Readers looking for another perspective on mealybugs may find Water Roots' discussion of them interesting. The perspective isn't so different that we don't both agree that they're evil, but she has a much more specific eradication procedure.

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Photo credits: all me, alas.

¹ (which is actually a type of scale insect, just with a waxy instead of hard covering. Or at least that's what the scientists say, if you want to believe them.)
² Kidding, but – not. Root mealybugs are the sorts of things indoor gardeners have nightmares about. They're just as bad as regular mealybugs, with the added obstacles of being more mobile and essentially invisible, because they live in the soil most of the time. It really is probably more in your interest to take a few cuttings and then destroy the plant and soil: the only exception to this would be for a very valuable plant that has a relatively light infestation, in which case as much isolation, systemic insecticides, and much bleaching of tools and workspace are all but mandatory. And even then, you should seriously, seriously consider whether and for how long you want to be battling mealys. The good news is, they're not all that common: I have yet to have a root mealybug infestation on any of my plants.
³ In case anybody's wondering: yes, this is a mealybug infestation on my beloved 5-foot Cereus peruvianus, both of them, at the same time, which is why I'm doing this post now and how I'm able to get pictures. It's really very depressing.
⁴ Areole: the small bumps from which clusters of cactus spines emerge. They get a special name because they are, for all practical purposes, very short "branches," from which the thorns (which are very pointy "leaves") emerge. Only plants in the Cactaceae family have areoles, and everything with areoles is in the Cactaceae. At least, that's my understanding of the term, and Wikipedia backs me up on that.
⁵ Axil: point where a leaf's petiole attaches to its stem.
⁶ (What – you were expecting a couple hours to work on your novel? You're a mealybug.)
⁷ Imidacloprid is also sold as "Merit," and it's one of three active ingredients in Bayer 3-in-1 spray, and the active ingredient in some flea control products for cats and dogs as well. It's thought to be pretty safe, and protection against pests lasts for about a month after application, though there is a granular preparation from Bonide that's supposed to last twice that.

Work-related: whitefly

I've talked previously about how we got our poinsettias in two different batches; the majority came from a company in Pella (IA) that appears to be a major greenhouse supplier in the area, and then we had a second batch, about 1/3 the size of the first, from a different grower in Centerville.

Along with the Centerville plants came whiteflies. A few of the Pella plants may also have had a whitefly or two on them, and we'd had some in the greenhouse already, that came in with some of the things that had been outside, but we'd undertaken a pretty serious effort to get rid of the stuff that came in from outside (nearly killing a few Hibiscus in the process, though they did all survive in the end), so: one, it was frustrating as hell to be bringing in new whiteflies and unable to stop them, and two, I'm pretty sure Centerville is responsible.

This is what they look like: this one is photographed on the back of a poinsettia (Euphorbia pulcherrima) leaf. The picture, if you open it in a new window, shows considerably more detail than I've ever been able to see with just my eye; normally I just see a tiny white rectangular shape and start leaping to conclusions.


Now for a confession: whiteflies scare the crap out of me, more so than any other bug, even though they're not necessarily as damaging or as difficult to get rid of as some other bugs.¹ I had a small invasion of whitefly in late winter this year, brought in on a plant that was huge, and beautiful, and (most importantly) free, which meant I didn't inspect it as closely as I might have. (UPDATE: I have since determined that the bugs in question were not whitefly, but were some other, as yet unidentified, thing that has a particularly strong affinity for Sansevierias.)

(The trojan Sansevieria.)
The infestation never got as bad as it could have, because I went into full alert mode and started hosing down all the plants with rubbing alcohol², and I hand-defoliated a few to remove pupae. Eventually the crisis passed. But whiteflies still freak me out, because unlike spider mites or mealybugs, they fly.

Happily, my observations at work have shown me that they're not that mobile. In theory, yes, they could fly around your home, laying eggs on every plant as they go, but they're not that ambitious, and flying seems to tire them out quickly: the ones at work rarely spend more than two seconds in the air at a time. In fact, if you turn a leaf upside down and find one there, it will probably be too lazy to fly away,³ which is how I managed the photo above.

They also have strong preferences for which plants they like and don't like. Hibiscus is a big favorite, as are poinsettias. We've also seen a lot of them on sweet potato vines (Ipomoea batatas), various herbs (don't recall which ones specifically) and ornamental kale (Brassica oleracea). My own infestation centered mainly on two plants: the Sansevieria they arrived on and a couple Euphorbia trigona that they enjoyed laying eggs on.

Supposedly the favorite color of the whitefly is yellow. I don't know about this personally. In my experience with the (white, yellow, green or red) poinsettias, eggs and adult flies are usually on the underside of the green leaves. If disturbed, they do often settle back down on a yellow or pale green leaf or bract, but I think that's because the points are just starting to bloom, so yellow and pale green are the colors of the topmost leaves/bracts, and the whiteflies are just trying to land as soon as they can. So I'm not convinced. They don't go out of their way to land on the yellow sticky traps we've got laying around for them.

As far as whitefly control goes, the most effective by far seems to be Marathon (imidacloprid), which the Pella group of plants had already gotten before they came to us, and which we've added to the Centerville batch but probably too late. Imidacloprid is a derivative of nicotine which can be absorbed by a plant's roots and taken into its tissues, effectively poisoning all parts of the plant (this type of insecticide is called a systemic insecticide). It comes with its own problems,⁴ but it does seem to work. The alternative, walking among the points, turning over leaves, and squishing flies with my fingers, is also effective, more entertaining, and less likely to result in pests becoming resistant, but it's also much slower and doesn't affect eggs, larvae or pupae.

Imidacloprid is approved for indoor amateur use (and it's also in a lot of treatments for fleas on cats and dogs), usually as a spray. We sell a few different formulations of it at work. Yellow sticky traps might also be helpful in small infestations, if you can place them close to the affected plants without actually touching the affected plants, though I'm skeptical, as I've indicated. Yellow sticky traps do have going for them that they're not toxic (at least, not if used as directed - I wouldn't lick one). But, as with all pests, the better approach is to not get them in the first place. Check your plants before you buy.

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Photo credit: me.

¹ (If I were more rational, I would be more alarmed by mealybugs.)
² Exaggerated for comic effect. Do not use hoses to apply rubbing alcohol to your own plants.
³ (I sort of imagine that the one in the picture is looking back at me with an annoyed expression on its face, saying something like, "Do you mind?")
⁴ It could be responsible for some or all of the recent decline in honeybee populations. Though the jury is still very much out on that, and there seem to be as many theories about the bees as there are people looking into the situation.