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
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.
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.
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.