Episode 102: spider mites
Red spider mites suck the sap from the undersides of leaves. Photograph: Gilles San Martin on Flickr.
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Transcript
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Jane: Hello and welcome to episode 102 of On The Ledge podcast, I am your host, Jane Perrone, and I am hoping this is going to be a spine-tingling episode. Well, if not a spine-tingling episode then maybe a skin-tingling episode, because this week we are talking about the king of the houseplant pests, the tiny king, because you can't see it with your naked eye, but nonetheless, this red spider mite has to be one of the most annoying and hard to treat pests on your indoor plants, particularly those of the Maranta group, the Calatheas, Ctenanthes, Stromanthes, and so on.
And in this episode, I'm going to talk to two expert entomologists about the life cycle and habits of these crazy creatures, explain what signs to look for when you think your plants has been infected, and of course, how to treat them. And, I'm answering questions about easy trailing plants for cat filled households.
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Jane: Congratulations this week to Nikky Knijfin South Africa, who was the winner of the Mother grow light competition that we ran in episode 100. Well-done Nicky! Your grow light is spinning its way to you now and I really hope you enjoy using it and get lots of pleasure from it. Nicky was so excited when I emailed with the news, so, well done to you and thank you to everybody who entered. I got some lovely messages along with your entries, which is really nice, and it was great to hear from you all. And thanks for all the lovely feedback on episode 101 on spider plants. Yes, I know, spider plants to spider mites, from the sublime to the ridiculous. Ha! That's what we like to do here on On The Ledge podcast.
Welcome to Lisa, Lea and Rod who are all legends this week. That's because they've signed up for the $5 Tier of my Patreon, and not only can they download an exclusive digital artwork, they can also listen to 30 episodes of an Extra Leaf, the additional mini podcasts that I put out for Patreon subscribers at $5 a month or more, and this week they can enjoy me talking about three books that have inspired me, all with a planty theme although not directly about house plants, and they can hear an extended interview with one of the entomologists in today's show about the crazy world of mites. And I'll also be adding an Extra Leaf episode on my ventures with Caladiums in the next few days.
So, if you are not a Patreon subscriber and you want to be, check out the show notes on janeperrone.com to find out how. Yes, it is in dollars but you can use any form of currency you would like to pay for it, so don't worry if you are outside the US, we welcome everybody, US and otherwise.
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Jane: Spider mites. If the mere mention of the name does not immediately set your teeth on edge, then you haven't experienced the true horror of full-onspider mite infestation. The first time spider mites comes to visit you, you probably won't realise until your plants are looking pretty miserable. The trouble with these tiny creatures is they take up residence without us noticing and hang around doing damage before we know they're there. And even after you've treated the plant they can come back for more when you least expect it. So, I thought I'd start this episode by going on a little tour of my own houseplants checking for any spider mite evidence.
Apologies in advance if it is a little bit noisy because we are having some building work on today but, I couldn't delay because I needed to go on a hunt for spider mites, or in other words, let's all go on a mite hunt, those of you without children may not get that reference, but anyway, we're going to look at my plant and see if there are any spider mites in evidence and my guess is that there will be because spider mites are everywhere. So I'm going to go over here and have a look at my little mini-windowsill ofMaranta group plants. I've got three up here, I got a Ctenanthe and a Calathea makoyana and a Calathea musaica. Now I know that the Ctenanthe has had spider mites in the past, so I think this is probably the number one suspect, and it just seems to be particularly vulnerable. And if you are looking at a Maranta group plant and you want to have an idea of how vulnerable it will be to spider mite, my, well, experiential learning has found that the more papery and thin the leaf is the more likely it is it will suffer from spider mite, so the Musaica calathea, Calathea musaica just seems to have thicker and more leathery leaves that don't seem so prone to infection. And looking at this plant it's a..., I love this, the leaves on this, they're just incredible, it's almost like a barcode. This looks fine, it's clear, it's healthy, it's green. The undersides of the leaves are showing no signs of any kind of residue or dust that may be the indication of spider mite, and similarly the peacock Calathea, which is one that's coming back from being abused, I was given it as a plant that's been suffering a bit, so, this one, even though it's only tiny and has had a difficult early life, if we canput it that way. It got lots of leaf damage but it hasn't got that kind of damage that tells me it's a spider mite. So the leaf damage here is kind of brown into the edges spider, which indicates lack of humidity but not the kind of mottled look that it tends to get with spider mite. And looking at the Ctenanthe I can see that it has had some spider mite damage because, particularly when you hold up the leaves to the light, on the underside you can see it is mottled and that's where the sap is sucked from the plant by the spider mites. So I can see there's been damage and now I'm going to have a really close look at the leaves to see if I can see any current spider mite infestation.
You might not notice the backs of the leaves starting to develop this grainy stuff that is the spider mites and their eggs because it's really quite hard to spot unless you were looking for it, and, but you may notice the overall plant, particularly with this Maranta group, just going to to sink and droop and little miserable, and if that happens, always check for spider mite. If I check for spider, mite, anyway because it's so prevalent, especially this time of the year, and looking at this plant I can see that it's... well... there's a tiny amount of evidence of spider mite right at the point where the leaf blade, the laminar meets the petiole or leaf stem, there is a tiny amount of white stuff, and if I get my magnifying glass out I can see that yes, that is a tiny amount of spider mite damage, so I need to move this plant away from the others and treat it again.
If you have a really severe infestation, you'll probably find that you get some webbing around the plants and the plants will start to collapse and look absolutely terrible and then you will be absolutely sure what you're dealing with. I'm glad to say I've never had one go that far, I always manage to stop it but I do find the plants that have been infected, and then actually weakened, then become very vulnerable to repeat infections, so you've just got to keep checking those leaves. And in the case of this plant, because it's got quite a lot of leaves, what I will probably do if I find any leaves that look seriously infested I'll just remove them wholesale, right down from the bottom of the stem, and other than that, I will treat the plant with some SP plant invigorator and generally give it lots of love and it should recover fairly quickly. Again, but it's just an act of constant vigilance.
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Jane: So that's my weekend of leaf wiping sorted out, but let's find out a little bit more about the weird world of mites. I manage to get, not one, but two entomologists to chat to me this subject. The first is Jules Howard, author of 'Death on Earth: Adventures in Evolutions and Mortality' and columnist for The Guardian, and Andrew Salisbury who Principal Entomologist at the RHS. And the first thing that I want to know is; where is this mite from exactly? Is it an interloper that arrived in the UK on a shipment of plants? Or has it always been here? Over to Andrew Salisbury.
Andrew: Almost certainly the red spider mite hopped over on a plant, but by the time we started looking at these things and people started growing plants more widely it become wide spread, so its actual origins probably aren't, actually that well known.
Jane: So here is the thing about red spider mites and indeed the whole mite family. Surprising though it may be, we really don't know much about this group of invertebrates.
Jules: They're kind of like a... one of nature's best kept secrets. You know, there's lots of species, every species is unique in its behaviour, in its habitat, in its niches, and to be honest, most people completely overlook them, so mites... well that's one of the many problems when we try to understand how to deal with mites pests. There is just no any scientist out there willing to look at tiny things, they are all out there studying lions and macaques and other animals like that, you know.
Jane: So as Jules Howard just explained there, mites, we don't know much about them. Fortunately, the glasshouse red spider mite, which is the pest that causes most, if not all of the damage to your houseplants, is one of the better-studied mites. Why? Because it's of interest to the horticulture community and indeed the industry of horticulture to get this pest under control, because after all, well, having your precious plants covered in spider mites and looking this poor may only cause you emotional stress. For professional growers of plants it really can impact on their livelihood.
So, what do we know about Tetranychus urticae, commonly known as the glasshouse red spider mite or the two-spotted spider mite. Each spider mite lives about four weeks and the females can lay about 10 or 20 eggs every single day. The larvae hatch anything from about 3 days to 2 weeks after they've been laid, and they generally hang out on the underside of leaves and suck sap out of plants cells, and that's why you get the mottled effect, that's where they just suck the life out of a cell and you're left with a very pale patch behind. Obviously this is all happening on a really small scale, hence the mottled effect as the spider mites take hold, the leaf will get more and more pale until eventually it will die. But the rate of expansion of a spider mite infestation can really depend on the conditions in which the plants are kept, as Andrew Salisbury explains.
Andrew: It is often said that plants growing in particularly high temperature in dry or clouded situations can be affected more badly than plants growing in sort of more humid, not quite so hot environments. It's sort of the case of a war of attrition trying to keep it sort of at more acceptable levels and their numbers down so they're not actually damaging plants too much.
Jane: Dry heat really is the perfect breeding ground for spider mites, which is why we really struggle at this time of the year when some of our plants can undergo heat stress as temperatures get very high behind the glass in our homes.
As we've already heard, spider mites can come in on plants that we've newly bought, which is why it's really important to not only examine every single plant that you bring into your home very very carefully, but ideally also quarantine them away from your other plants for a few weeks before you introduce them to the gang, because that way you can be sure there are no pests in evidence. I have to admit I don't do this every time, but I have paid the price in the past for not doing it, so if you can, if you've got the room to keep that quarantine area up and running, please do. Although the anything worth saying of spider mites is, bear in mind that although they don't have wings, they can actually float in through your window, and indeed, float from plant to plant if it happens to be a bit of a breeze from an open window or door. Yes, these things are little ninjas.
Jules: Mites are capable of, some species fly around in the wind, particularly the species like spider mites, spider mites that couldn't make silk, you know they can make little silk threads that catch the wind, and as you said, they fly around, and these animals are so prolific in terms of their breeding that if, exponentially increases what their capable of, I mean, only, you know, one or two mites can lead to a whole infestation, you know, after 20 days or so.
Jane: And one of the signs that you've got a really serious infestation is the sight of the webbing. Spider mites aren't actually arachnids as such, so, why do they produce this silky stuff? Back to Jules.
Jules: I'm writing about insects at the moment. You realise that a lot insect of these small insects do the same thing, it's basically a way for... if you are living for instance on a leaf or on the side of a tree, there is various things trying to attack and kill you, so you know, fungal, parasites trying to get you, bacteria, obviously other mites, so by producing a soft line of silk everywhere they move, if they are running around, they make a complex sort of blanket of silk, and it becomes a kind of protective barrier basicallyfrom the things that are trying to kill it. So a lot of colonies, because obviously as you said, they are interrelated very closely to one another, and your know, they're almost, it's like cloning, you know, so their whole colony can get covered up with these patches of silk, and it protects, it protects the babies.
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Jane: Gives the phrase, 'High and mighty' a whole other meaning. Well, we'll be back with more on 'How to fix the spider mite problem?' after the break, but first let's hear from our ad sponsor this week.
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In the battle against Tetranychus urticae, there're various weapons we can wield but which of them really works? That's what we going to find out next. Andrew Salisbury brings us the options.
Andrew: Yeah, on the pest side there isn't really a great deal available which will control the... control spider mites, and if you are down to, really things like the SB plant invigorators, the plant oil based products, fatty acids based products, neem oil might be recommended for use in placed like the States, but it's not registered for use in the UK so we can't recommend that it's used. It hasn't beentried, tested and registered for UK use. So really it is... using things like the plant oils, the fatty acids, the organic type of pesticide can help, the systemic neonicotinoid, acetamiprid is also available, but of course there are, you know, people who have concerns with the systemic pesticides these days. Other controls are... there are a range of biological controls, the most famous of which is the Phytoseiulus mite, but on a small collection of plants, houseplants, may not be as effective as in a large glasshouse.
Jane: Right, I hope you caught all that. I will be testing you later but let's have a recap.
I think that the most important element of red spider mite control is what we call in the business, "Cultural control", and that means just changing the conditions in which your plants live in order to ensure that they are always healthy and happy as possible, because that way they are less susceptible to pest infestations, because pests tend to target plants that are already weakened, and that may be the humidity level is too low for their needs, or they're getting stressed through the roots because the water supply is too uneven, or because the temperature is wrong, or because they are in too much light or too little light. So trying to keep your plants happy should help to keep pests away. That said, clearly, spider mites can still strike at any plant, and this is when pesticides come in. Systemic pesticides are the synthetic pesticides where the active ingredients are taken up into the plant at its actual system, and therefore they are quite long lasting and can be quite effective, however, these are the ones that contain neonics, neonicotinoids. The active ingredient in the one used for spider mites is called acetamiprid, and the reason why I don't use these is I tend to follow organic principles in my garden, indoor and out, and neonics have been found to damage pollinated populations, so I don't really want these in my garden, especially as I put my dead houseplants onto the compost heap. So I steer clear of those, but if you do use them just make sure you follow the instructions absolutely to the letter, and in fact that applies to any pesticide or product you are using on your plants. Please read the label, I cannot say that strongly enough. There are pesticides which have organic approval, however, and these usually contain things like plant oils or fatty acids, and these work on the bodies of the spider mites, and basically either kill them or damage them in such a way that they can't continue to reproduce. And depending on what country you're in, you'll find many of these products are available in sprays and liquids that can be used on plants. Again, read the label really carefully and check what the active ingredients are, that way you will know that your product is... does not contain any neonicotinoids.
There are other products as well, which are often called plant invigorators or plant defenders. These are quite new to the market and they are a mix of nutrients that the plant needs and surfactants, and these just literally reduce the water tension, the surface tension of water and make water wetter effectively, and allow the spray to stick to the leaves more easily, and they can help to loosen spider mites and help remove them, used in conjunction with a program of spraying and wiping. And in fact, any of these fatty acid or plant oil products you can use by spraying the plant and then wiping the debris off them. This is the way that I would proceed really, isolate your plant well away from any other plants, particularly things like other Calatheas that are particularly prone to spider mites, and start a program of washing down your plant, spraying something like SB plant invigorator or eco-effective plant defender, wiping the leaf, especially the underside, where the spider mites tend to hang out, and just keep doing it, just keep reapplying and reapplying day after day until all the signs of spider mite are gone. Then, a few days later check again, check, check, check and you will find there's probably some more eggs there, which need removing, and just keep going.
If you have any really seriously infected leaves, then sometimes you can remove those completely and give the plant a better chance of recovery that way. 'Perhaps the most fun way of dealing with spider mites might be a biological control; if I can put it this way is a bit like 'Game of Thrones' under a hand lens. So, effectively you introduce one mite to kill another mite. There is a predatory mite that Andrew mentioned called Phytoseiulus persimilis, and this is the one that is often used to kill off mites, the mites we don't want, the red spider mites. They're introduced into the houseplant area, usually a big glasshouse, and eat away at the red spider mites at every stage of their life cycle and can be very effective in your home though, where you might have quite a small plant population as Andrew Salisbury explained, possibly not so effective, but kind of fun though I do want to give it a try. Here's Andrew to explain a bit more about how it works.
Andrew: They do come in as living animals, so you throw one the biological control supplies up and they send it out and it will be on... They often come in little cardboard things where you peel back a strip or they come in a little sachet that you open and pour basically the nicolites, which taints the mites on the plants. They do come as living mites and they can get going straight away. The key with the biological control is always keep an eye on plants and spot those early signs, and when you do see the early signs of mite damage and mite populations, that is the time to introduce biological controls. Your mites have got out of hand and your plants are beginning to dry up and then the mite has really caused some serious damage, it's probably a bit late then to introduce biological control.
Jane: And when it comes to neem oil, I should mention this one too. This is become a very popular treatment for houseplants and is an extract from the neem tree. It is a naturally occurring ingredient, it's not a synthetic chemical, but is not approved for use in the UK as a pesticide, which is why I don't use it. If you do use it, if you are in a country where it's allowed to be used, please, please, please follow the instructions really, really carefully as you would do with any other pesticide, because just because it's "natural" doesn't mean that you can fling it around with gay abandon and not take any notice of the instructions. So make sure you dilute it to right levels and use it carefully. And while you are treating your plant, again, look at the levels of overall health and check if it needs repotting. All of these things can help to just give your plant an extra boost and get it back to its former glory more quickly.
One final mystery to solve before we put this subject to spider mites to bed, and that's, why are they called red spider mites? Because quite often a few, loot at them under a hand lens, they aren't actually red.
Jules: For those who actually see the spider mites under the hand lens, what you often see is a pale-coloured mite, often with two black dots on its dorsal surface, and you may then be wondering why they are called glasshouse red spider smite by most in the UK whereas North Americans tend to call them two-spotted spider mite obviously because of those two black spots. Well, a part of their life cycle, particularly when they badly damage plants or when they are going into over-wintering, the females do actually turn an orangey-red, and that's the only stage it's orangey-red, most of the rest of the time they are a pale green colour with two black dots.
Jane: And if I can leave you with a final mite-related thought it's this; although we do want to make sure that our plants are not inhabited a huge population of red spider mites, mites, as part of a huge group of invertebrates, are actually really important and it's really unrealistic if not completely wrong headed to ever imagine that we're going to get our houseplant soil to be free of these microorganisms.
Jules: Mites as a group, you know, a million species, a single handful of soil having absolutelythousands of individuals in it. You can't really control that, you know, to control that you would kill the soil. Springtails and mites are almost solely responsible for decomposing leaves into smaller and smaller chunks that make up the soil, you know, without them doing that, without that action, along with worms, we would basically starve, you know, within a year. So, yes, there's no such thing, I would say, like clean soil, you know, these animals are kind of... there, they're there to stay and I suppose the more we talk about them, the more we can view them in a positive light, dare I say.
Jane: Well, that's a great note to end our mite chat today. If you want to hear the full interview with Jules because he had so many fascinating things to say about non-plant dwelling mites that you might find interesting, you can do so if you happen to be a Patreon subscriber at $5 a month or more. An Extra Leaf number 30 is up now and you can hear my full chat with Jules in that episode.
And now, the question of the week comes from Brightondayson Instagram a.k.a. 'Charlotte', who wanted some trailing plant recommendations that were cat friendly for beginners, but not too common.
Well Charlotte, I'm not sure if this one is considered to be common or not, but I really love it, and that's what's called myers asparagus fern, also known as the foxtail fern. The normal asparagus fern plumosus has an upright habit, but the foxtail fern has a beautiful foxtail-like stem, which, once it gets long enough, will start to tangle over and trail, and it looks absolutely lovely, and it's not toxic to plants, and it's pretty easy to look after, so that one would be pretty high on my list for your requirements.
If you've got a really hot sunny spot, then Sedum morganianum, the burro's tail succulent, that one is fine for cats and it also would look beautiful in a head pot.I love it when they're displayed in that way, so that the stems look like Medusa's hair. That's another great one but only if you've got plenty of sun.
It is worth noting that as far as I know, and it's just worth double checking this before you buy, but all of the plants in the genera Pilea and Peperomia are cat-friendly, so that includes things like Pilea libanensis, often described as Pilea glauco or glaucophylla, a beautiful tiny-leaved Pilea that trails beautifully and is really really super easy to grow. I really like that one. And there's lots of Peperomia choices for trailing plants too, including prostrata, I find that a little bit more difficult to grow, but something like Peperomia quadrangularis, I think that is a super easy Peperomia that is definitely worth a try, and it may not be in vogue at the moment, but I think Cissus rhombifolia, the old-fashioned grape ivy is a great choice. [laughter] Sorry about that, it's a trailing plant with attractive looking leaves and it is easy to look after and won't kill your cat, and I don't think you can say fairer that that. I hope that's helped Charlotte, and if you've got a question for On The Ledge podcast drop me a line. My wonderful editorial helper, Kelly, is charting all of your queries in a wonderful spread sheet, so when I come to my Q&A special very soon I'll be able to answer as many as possible. Thanks to all of you who are getting in touch.\ Two things to mention before I go. Listen to Steve, he's hoping to organize a Listener Get Together in Glasgow, in Scotland, so if you happen to be a Scottish listener, do join the Facebook group 'Houseplants Fans On The Ledge' and you'll find Steve there, or if you just Google 'Glasgow' and 'Steve' you'll find him, or otherwise, just drop me a line and I'll put you in touch with Steve. Also, there's a London Plant Crawl happening on July 28th, unfortunately I can't attend but it's goanna be an East London tour of some of the best places to shop for houseplants, including Colombia Road Flower Market. So the details of that are in the show notes and I really hope that you can support that event.
That's all for this week show. I hope this has given you new insight into the invisible things going on all around our houseplants. I'll see you next week. Bye!
[music]\ The music in this week's episode was Roll, Jordan roll by The Joy Drops, Insectifyby Kid and Nasty and Lonely Spider by Colour. The ad music was by The Heftone Banjo Orchestra, the tracks were Dill Pickles and Whistling Rufus. All tracks on the show are licensed under Creative Commons. See janeperrone.com for details.
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Red spider mites may not be visible with the naked eye, but they damage they can do to our houseplants is considerable. I get an insight into the world of the spider mite with entomologists Jules Howard, who is the author of books including Death on Earth; and Andrew Salisbury, who is principal entomologist for the RHS, and look into the various options to keep mites under control. And I answer a question about easy trailing plants that are non-toxic to cats.
Check out the notes below as you listen to find out more about the fascinating world of mites, and how to control the red spider mite.
Spider mite specifics
The red spider mite, Tetranychus urticae, is also known as the two spotted spider mite.
its lifecycle lasts only up to a month: check out this video for a look at the spider mite all stages. The RHS has an excellent information page on red spider mite that is worth checking out too.
The red spider mite is the main mite that will attack houseplants, although the mite family is enormous. (Patreon subscribers of $5 a month or more can hear more mite insights from Jules Howard in An Extra Leaf 30).
It seems to favour papery-leaved plants of the Maranta group but can attack just about any houseplant.
Spider mite symptoms
The first signs of an infestation are the plant starting to droop and generally look miserable.
If you can use a hand lens, a magnifying glass or even your mobile phone’s camera is really useful to check the backs of the leaves - mites are not visible to the naked eye but can be seen with a bit og magnification. This is the one I have - a double botanist’s hand lens made by Ruper.
The leaves will develop pale mottling as the mites begin to suck sap from the plant cells.
The undersides of the leaves will display a grainy white substance, and more severe infestations may exhibit webbing.
The leaves will eventually turn yellow and die off, leading to total plant collapse.
Treating spider mite
Prevention is better than cure: making sure your plants are in good health will help prevent pests from taking hold. Pests tend to target specimens that are already undergoing stress: they love hot, dry conditions so are a particular problem in summer.
When it comes to pesticides to treat red spider mites, Acetamiprid is a systemic pesticide that is suitable for use on houseplants, although I don’t use it because I take an organic approach. Acetamiprid contains neonicotinoids which have been linked to bee decline.
Neem oil is a popular treatment for spider mites and other houseplant pests: however it is not licensed for use as a pesticide in the UK so I don’t use it. It is meant to be effective, but if you use it, please don’t assume that because it’s “natural” (it is extracted from the neem tree, Azadirachta indica) can be sprayed indiscriminately: you still need to follow the instructions closely.
There are a number of pesticides available that are based on plant oils or fatty acids: they work really well, but may need repeated treatments to work fully.
There are a number of ‘plant invigorator sprays’ on the market which contain a mix of nutrients and surfactants, such as SB Plant Invigorator, Neudorff Plant Invigorator and Ecofective Houseplant Defender.
When you first see spider mite symptoms, isolate your plant, and start treating it immediately. Washing or wiping down with a damp cloth daily will help remove mites and eggs: focus on the underside of the leaves.
Biological controls such as the predatory mite Phytoseiulus persimilis are best used early on in infestations, and for larger collections.
My Pilea libanensis, as mentioned in episode 99. Photograph: Jane Perrone
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This week's show featured the tracks Roll Jordan Roll by the Joy Drops, InsectifEYE by KidNNasty and Lonely Spider by Cullah. Ad music is by the Heftone Banjo Orchestra: Dill Pickles and Whistling Rufus. All tracks licensed under Creative Commons.
Logo design by Jacqueline Colley.