Episode 204: thrips
Blue sticky traps can help to monitor an infestation of thrips. Photograph: William Kirk
Transcript
Episode 204
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Jane: Hello and welcome to On The Ledge podcast, the podcast for people who love their plants. This week's show is dedicated to the topic of, brace yourselves, thrips! Yes, that pest which we've all come to fear comes under the beady eye of On The Ledge this week, so get your hand lenses at the ready - this one is not for the faint-hearted! Plus, I answer a question about whether a plant can recover from cat-related damage. Oh dear, pussy's got her claws out!
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William: Hello, I'm William Kirk, I'm a professor of applied entomology at Keele University in the UK and I've been studying the biology of thrips for exactly 40 years this year.
Jane: Now there is no such thing as a thrip?
William: That's right. You'll see lots of people mentioning it but it's a false assumption that the singular is thrip, whereas in fact it's from a Greek word which ends in the letter psi, which is a "ps" sound, so we're stuck unfortunately with the singular of thrips being thrips, so it does seem a bit strange when you talk about one thrips or a thrips, "I've seen a thrips..." but that is strictly correct.
Jane: Yes, it's a bit like data, isn't it? I think that's another word that scientists always have to struggle with. I think that this is going to probably trip me up, so I apologise in advance if I say thrip but I'm going to try to say thrips throughout. Now, this is a pest of houseplants that is present everywhere in the world, as far as I know, are we dealing with one species or lots of different species on our houseplants?
William: We're dealing with lots of species. In fact in the world there are 6,000 known species, although most of those just live in leaf litter on forest floors and don't really concern growers of plants. Then you get down to about 100 species that are pests of crops around the world. But on houseplants, you're probably talking only about 12-14 species that are significant pests that people are likely to encounter. Of those, there are perhaps two or three that are the main pests because they feed on so many things. Some thrips species are very specific, will just feed on just a few species, and then there are other ones that feed on lots of different species. You shouldn't take any notice of the name. When it has a name, something like Onion Thrips, well, okay, it's a pest on onions but it's also pest on lots of other things, so the names can be a bit misleading. About a dozen species are the ones that you'd encounter on houseplants.
Jane: Presumably this is another reason to be careful with importing plants from other countries because this is how they're spreading, coming in on plants and spreading to parts of the world that one species wasn't in before they've arrived.
William: That's right. We have quite a lot of invasive species of thrips. For example, the Western Flower Thrips, which is perhaps one of the major thrips pests at the moment, was originally limited to just the western part of the USA, that's where the 'Western' came from, but since about the 1970s it spread worldwide. Once the insect, the thrips species, becomes resistant to insecticides, then it's very difficult to stop it. Horticultural trade brings plants all around the world, so you have cut flowers coming in from Colombia to the UK, then propagation material coming into the Amsterdam flower markets and then the next day they could be in 400 garden centres all across the UK. Because the thrips are so small and easily overlooked, they get spread really frequently and easily by human trade.
Jane: Yes, this is a big problem. As you said, they are rather small and not the easiest thing to spot. I'm always going on about hand lenses on the show but as soon as you get a hand lens on one and get some magnification you start to see that there's probably more than you thought. What are they actually getting up to on our plants and what are they doing in terms of damage?
William: They feed by piercing and sucking. So they have a mouth part, a bit like a drinking straw, but they pierce a hole in the cells in the plant, or in pollen grains, or cells on the leaves or cells in the flowers, pierce that and then suck out the contents. So you'll sometimes hear people saying that they rasp or scrape. That's not true. They actually pierce and suck. So by piercing and sucking, they suck out the contents and that's how one of the common symptoms of thrips feeding is that you get this silvering pattern. So what's happened, is that the green chlorophyll, or the pigment of the petal, has been sucked out of cells and you're left instead with an empty cell full of air and that gives that reflective silvery look as a result of the feeding. So they move along, sucking out cells in turn and building up a silvery area of the damage. So the main thing they do on the houseplants, on plants in general, is feeding. So the larvae, the young stages, are continually feeding and then when they get to the adult stage, the adults are also feeding, so it's the feeding damage that's the real problem in houseplants. Outside, in crops, of course, thrips can also be vectors of plant viruses and that can be very damaging in greenhouses or sometimes in outdoor crops. I suppose in small collections of houseplants, you're probably not likely to have virus as a problem. It'll be the thrips but doing the feeding that's causing the problem.
Jane: We see one thrips on a leaf and think, "Oh, it's only one," but I imagine that they've got a life cycle that allows their populations to explode pretty fast?
William: Yes, that's the one of the main features is how fast they breed. So if we start with the adults, you get males and females in most species, the males and females mate and females lay eggs, the eggs are very small, about a third of a millimetre, and are inserted into the plant tissue. So you can't see the eggs or find them, except with very powerful microscopes. So they lay the eggs, those hatch quickly into larvae, you get two larval instars and they're feeding on the plant and then, typically, they drop to the ground for the pupil stage, which is usually on the surface of the soil or down the side of a flower pot. Sometimes they'll pupate on the plant in a little nook or cranny somewhere, not very obvious, but usually it's in the soil, or on the surface of the soil. Then they will emerge and the adults will fly up onto the plant again. That whole life cycle from egg to adult in a warm room, say 25C, can take place in less than two weeks. Females are laying a few eggs every day. They don't lay batches of eggs like a butterfly, just steadily laying eggs throughout their lifetime as an adult, which might be two, three, four weeks. All that time they're producing the eggs and every two weeks you've got more adults coming through. So from one female coming in you could have hundreds and hundreds within four to six weeks.
Jane: It's interesting about the eggs being laid in the leaf material. Would that mean that if I was, say, wiping the leaves to remove an infestation with a damp cloth, that I wouldn't be wiping out the eggs because they are lodged in the plant material?
William: That's right. They're nearly completely submerged, so wiping over the surface isn't going to do anything for the eggs. That's also a problem, of course, with any control measures. If you used any insecticidal approach, the eggs are protected within the tissue. So you might apply an insecticide of some kind and kill maybe adults and larvae but you wouldn't get the eggs that are about to hatch and you wouldn't get the pupae that are perhaps down in the soil. So that's why with an insecticidal approach you'd have to repeat it at intervals, so that you would then catch the ones that have been in the egg the first spray. By the time they've emerged, then you'd want the second spray because of all those protected stages. It's not just that the egg and the pupae rather can evade insecticide, the adults and larvae, a feature of them, they're not just small but they tend to retreat into very small spaces. So they might retreat into leaf buds or flower buds or small spaces and those small spaces are where insecticide often doesn't reach or where predatory biocontrol agents might not be able to get into, those small spaces. So they can evade all sorts of treatments and the stages can protect them as well.
Jane: This is a challenging pest by the sound of it. Do you find that when you're dealing with commercial nurseries, that this is a problem that is one of their worst glasshouse issues? It seems worse than red spider mites, in a way, because of those factors you've just mentioned. Is it considered more serious to have a thrips infestation than, say, red spider mite?
William: Well, it changes over time. When you get these invasive species, which we've got several of them at the moment increasing, when they appear, then the growers have an enormous problem because they haven't adapted or learned the best approaches for biocontrol say. So when Western Flower Thrips arrived, say in the UK, in about 1986, it was an enormous problem and the same in other countries when it arrived. One problem is that, say, in greenhouses, the growers in Europe, certainly, are now very much using biocontrol. Pesticides are very much not used. They've been very much reduced in the extent to which they're dependent on them; biocontrol has taken over. When you get a new pest that arrives and you don't have the biocontrol protocols, and things, worked out, the pest takes off. The thrips becomes a pest and then you end up having to spray because you've got no choice at that stage, there's no alternative. But, of course, when you spray then that wipes out the biocontrol for all the other pests and then the problem magnifies because then all the other pests, you lose control of them. So when a pest first arrives, it becomes a big problem, but I think I'd say that now the Western Flower Thrips, certainly in Europe, there are well-established biocontrol protocols that keep the thing under control. But we've got another thrips that's just arriving in the UK called the Japanese Flower Thrips - Thrips Setosus - and again, that's something where people haven't really got the methods worked out, which are the best biocontrol agents and so on. So that is likely to become a problem over a few years but then maybe that'll settle down as the biocontrol approaches are developed that are suitable for it. I'd also add that thrips are generally seen as a bit of a nightmare for growers both of protected crops under glass or plastic and outdoor crops. I've had pest advisors come up to me and say, "I'm responsible for aphids and thrips for northern territories of Australia. I'm okay with the aphids but I really don't know what to do about the thrips because they're really very difficult to control." As I said, they escape treatments by being very small and hiding, but they also develop resistance to insecticides very rapidly. So we have a problem of "How on earth do you control them?"
Jane: This is obviously something that's going to be an ongoing issue in our globalised world. We've talked a little bit about what signs we might be looking for in the plant, this silvering. I like the image of them sucking up the plant cell material through a straw-like device, so the plant might be silvered. Are you generally looking for a plant that's got that silvery effect but also probably just looking generally miserable as the life is literally drained out of it? Is there any other sign that they're going to leave... I'm just thinking of the equivalent in red spider mites where you see that white grainy stuff which is their shed skins?
William: If you hang up a yellow sticky trap, or a blue sticky trap, because the adults fly and are quite active, monitoring is one good way of picking up thrips infestations at an early stage. Of course, you might be using yellow traps for white flies, or fungus gnats, or shore flies, or something else. They could be multi-purpose. So if you can recognise a thrips, then you might detect it in early stage on a sticky trap, but on the plants themselves, it depends a bit on the plant. There are some thrips that particularly like flowers, so they will get into the flowers first because they like to feed on the pollen. If you look at the flowers, on the petals, if you've got coloured petals, what you'll see is some whitish areas, that's where they've sucked out the pigment and then you might also see some darker spots. So, let's say it's a purple petal, they feed on the purple pigment in the petal cells and then, after a while, they produce a kind of faecal droplet which has got a lot of the pigment in it and when that dries out, it dries a darker colour than the pigment of the flower. So on a purple flower you'd see these dark purple spots and you'd also see these little white patches where they've fed. So very few thrips in a flower can cause quite visually obvious damage to the flowers. So those are thrips that prefer to go into the flowers, but when they're on the leaves, they can retreat into leaf buds or into flower buds, so that would be perhaps a place to look but they will also feed on the undersides of leaves. So if you turn the leaf over and look for little patches of silvering or where the little insects, near the veins often, that's one way to spot them. As you say, with a hand lens, you can detect them at an earlier stage. So if you've got a houseplant that's got flowers, look in the flowers. You'll see the adults are much more obvious than the larvae, so it'll be the adults you'd probably see in the flowers, whereas on the underside of the leaves it's probably yellowish larvae, so you'd be looking for little yellowish thrips underneath the leaves. Within the flowers, the adults are usually light brown to dark brown to black depending on the species and they can look rather obvious within a flower. So if you can hold the flower close to your eye with the hand lens and look inside you would see them probably in amongst the stamens because they'd feed on the pollen.
Jane: It's a horrible shock when you find some thrips, but it is good to know what you're looking for. I found them on my neon green Thaumatophyllum Aroid and actually, the colour didn't really help me because the very neon green leaves hid the thrips quite well until they got to the adult stage when they turned darker and that's when I spotted them. So, yes, as ever with pests, eternal vigilance. Going back to the colour of the traps, the thrips traps seem to be blue but presumably they'll be attracted by yellow traps as well?
William: Yes, you'll still catch them pretty effectively on yellow traps. It's a strange thing that these blue traps seem to be particularly effective in the summer, in bright light I think more often, there's certain shades of blue that are particularly effective for trapping certain species of thrips. So they're often used by commercial growers because they're more selective. You don't get so many of the other species. You won't get your bees or your pollinators on a blue trap. Strange thing is that we don't really understand why that shade of blue should be particularly attractive to thrips. It's not just one species it's several species that seem to quite like blue traps and it's not the same as the blue of a sky. They're not mistaking it for the sky and flying towards it, because the traps are blue without the UV, so they would appear a different colour to an insect. The sky is blue when the clouds aren't there, it's blue with UV, so it'll look very different because insects can see the UV component, so it's a bit strange. Western Flower Thrips, which is probably the main thrips pest of indoor plants and glasshouses where they're growing houseplants, Western Flower Thrips does go for this blue but the Western Flower Thrips can feed on just about anything. It's very polyphagous, it feeds on many different plants and yet it seems to have this preference for a certain shade of blue. If you use yellow traps, you're still going to catch them, they're still highly attractive to thrips anyway, just the yellow ones.
Jane: That makes sense. Well, I've got both. I have got yellow traps and blue traps. My main issue is trying to avoid accidentally sticking them to the plant but getting the right angle and position to catch the insects without getting the plant involved. I guess when you're in a nursery setting you can have them hanging above just above plants, and things, rather than my Heath Robinson methods of putting them around the plants.
William: The best position is about a hand's width above the top of the plant, which means, of course, as the plant grows, if it's a fast growing in a glass house, you've got these pepper plants say, coming up, you'd have to keep moving them up to keep them out of the way of the top of the plant. More of a problem is something like a strawberry crop, the people coming along picking the strawberries and then they tend to get their hair stuck in them, which is not so nice.
Jane: Yes, I can imagine that.
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Jane: We'll be back to talk more about thrips with Professor Kirk shortly, but now some housekeeping. Thank you to SilverDevastation - you guys come up with the best names for your podcast reviews - SilverDevastation left a lovely review for On The Ledge on the UK Apple Podcasts. Pat and Jessica became Ledge-ends, that means they became Patreon subscribers to On The Ledge. Talking of Patreon, I've been sitting on my rug in my lounge writing out lots of cards to my Patreon subscribers at the Ledge-end and Superfan level and it's been an absolute delight! It's been a bit of a cottage industry in my family with my daughter and my husband helping to stick on labels and write out addresses because some of your addresses didn't go through the database particularly well. Some of those special characters got rather mangled! So I've been writing out these cards and it's reminded me of lots of lovely listeners who support the show, those who've been there since the very beginning and some of you who've joined in the last few months. You're all wonderful and it really got me in the back of the throat to think of all this wonderful support. So those cards will be going out in the next few weeks, in time for the festive season. So if you're a Patreon subscriber and you've added your address for mail-outs, then that'll be on its way and I really appreciate, as ever, all your support.
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Jane: On to Question of the Week, which comes from Sarah and concerns cat versus Hoya linearis. This is one of those stories that cats owners sometimes dread. Some cats just seem to leave plants alone, others just like to have a bit of a play and it seems that way with Sarah's cat. Sarah asks: "If a Hoya loses its leaves, can a new leaf regrow to replace it or will that spot be bald forever?" Sarah says that the vines grew and became in reach of the feline in question and, obviously, the cat decided that it looked like a good scratching post. Sarah asks: "Will those leaves grow back or should I try to propagate those leafless nodes?" What a great question. This is about Hoyas and I know I do answer an inordinate number of Hoya questions, but this applies to so many other plants that it's quite a useful question.
So when we're thinking about leaves, what have we got? Well, in the case of a Hoya vine, you've got that long stem and along it there are the leaves which dangle down, so it's hard to see what's going on, actually, in terms of where the leaves are attached to the stem. You've got those leaves attached to the stem all the way along and the plant is growing mainly from the end of that vine, the growing point, and that is called apical dominance. So the plant wants to grow out and along that vine and, in the case of Hoya linearis obviously, it can get quite substantially long. I've cut mine so many times but it just keeps on growing and it's probably getting on for six foot long by now I would think. So the good news is, Sarah, that your Hoya linearis will be absolutely fine provided the cat doesn't keep shredding it.
What happens when you cut the tip of that vine off, is that you remove apical dominance, to use the phrase from your botany classes, and that basically means that the message goes out to the side shoots, the axillary buds that are embryonic buds of leaves that sit in those nodes, the place where the leaf joins the stem, and the message is sent out to them: "Oh, we've lost our head, therefore you need to start growing!" and this involves the hormone auxin. I've just been reading some academic papers on auxin, actually. We don't really still understand how it fully works, which is true of so many things in life. These complicated hormonal messages are sent to these side shoots, these axillary buds, "Get growing!" and so these will grow out from that node and start so that the plant can put on new growth even after that growth tip has been removed.
So, I think in the case of your plant, it depends if the very end has been cut off or knocked off by the cat or if it's just done damage along the leaf. Hoyas are wonderful because what Hoyas do, is they have these very, very productive nodes, that when there is damage they will just naturally start putting out new growth and branching by producing axillary buds from those nodes and creating new stems. So I would imagine that your Hoya will start to bulk up in that area quite quickly. What it won't do is grow brand new leaves from exactly the same point where the old ones came out. Now, in the case of Hoya linearis, that doesn't really matter because the leaves are so numerous and they hang together so well that actually it won't take long for the plant to camouflage that damage quite quickly. If you had a Hoya which had really huge leaves and individual ones then you would have a different story in that those big leaves would be missing and you would get new growth side shoots coming from that node but you wouldn't necessarily have quite the same look, if you see what I mean.
So this is what happens to plants in the world all the time. Obviously, they're getting this damage and so they have ways of dealing with it. It's fascinating to know the different ways that plants can respond to damage. For example, lots of the epiphytic plants that put out aerial roots, they will put out aerial roots that go into a nearby tree, to anchor themselves, but they'll also put out these feeding roots and those feeding roots are really important. So, in the case of something like Monstera deliciosa the plant can actually survive the stem at the base, where it's growing in the soil, being totally destroyed, provided it's got a good number of feeding roots. The rest of the plant will just keep on growing because the plant has so well anchored itself and provided itself with a root system separate from the original growth point of the plant. So plants are really clever at this stuff. As I say, I think your Hoya linearis will be fine, difficult to know without actually seeing a picture of the plant, but what I would probably do is cut off that long stem and propagate it, because if it's a long piece of stem, you could cut it into several cuttings and then you could put them back in the top of the pot, you'll end up with a more full plant and the plant will naturally, from the point where it's cut, produce new growth that will be beautifully replete with leaves. So take that cutting off and there's a number of ways that you can propagate Linearis. I think, when I did it most recently, I put it in a clear plastic bag, normally I would say trim to just below a node, but with Linearis and with a lot of Hoyas actually, they tend to just produce a heck load of roots from all over the stem because that's the way they grow. Put them in the bag with a tiny bit of moisture in there, they'll grow roots and then you can chop off any roots that are too high up and any roots that are able to go into a well-drained potting mix can go into the well-drained potting mix once they reach 5 cm or so long. That way, you'll propagate more Hoya linearisand if you don't need to put it back into your existing plant to bush it out then you can always give some away to somebody else, which is a lovely thing to do!
I hope that is helpful, Sarah, and that your cat finds something else to play with! Perhaps he or she needs a new toy? I know it's hard to keep track of cats all day. My dog just sleeps on the sofa and he never touches my plants, so I tend to forget quite how much you need to pay attention to what cats are doing, but I'm sure your cat is adorable and the damage to your Hoya linearis is not going to be too bad. If you've got a question for On The Ledge podcast drop a line to ontheledgepodcast@gmail.com and, remember, I'm after your festive questions, so have a think about that. What Poinsettia horrors would you like to set loose on me? Have a think and do get in touch!
Now back to my interview with Professor William Kirk on thrips and we're going to talk about how to actually control the little blighters!
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Jane: What are the biological controls that work on thrips?
William: There's a range of options. If you're looking at biological control, you're usually talking about just managing the pest at a very low level, keeping it under control. So it's an ongoing process. It's not just a one-off. Unless you absolutely bombard the thing with biocontrol agents, you might possibly be able to eliminate the thrips but, usually when growers are doing this, they're just managing it at a low level. So one of the best options is to use predatory mites. These used to be known, for a long time, as amblyseius but the newer name is neoseiulus, so you might see them advertised as both and there are several different species. Neoseiulus cucumeris, it's obviously something to do with cucumber, but can be used to biocontrol thrips. That species is good at slightly lower temperatures, so if you had plants in a cooler conservatory or greenhouse, that might be appropriate. There's another one, called Neoseiulus swirskii, that's better at warmer temperatures, so for indoor, where you've got more than 20C, that's pretty good. Predatory mites, obviously being mites, they're pretty small, they can't actually kill and feed on the adults but they can kill and feed on the small larvae, so you don't get an instant effect. What you do with these predatory mites is have a little sachet so it's releasing them and that would keep it at a low level, but if you've got a great big infestation, in the first place, that's not going to work very well because they can keep something at a low level but they can't really bring it down from a high level. In fact, they're widely used now in glasshouses, say in the UK, for biological control without having to depend on insecticides. Bear in mind, of course, when you're using biocontrol, they're usually not at all compatible with any kind of insecticides, even things like insecticidal soaps can still be harmful to predatory mites, so you have to choose what strategy you're going to go down. Are you going to go for zero tolerance, where you use insecticides and you try to get rid of all pests completely, or are you going to manage them and keep them at a low level, in which case you could use the mites. There are also predatory bugs. The common name is Minute Pirate Bugs, which is a rather fancy name. They, again, need warmth. So, again, in a warm place where you've got houseplants, they might be alright. Their name is Orius. The downside of that is that they're a little bit bigger than the thrips, perhaps 3-4 mm, and if they land on you and they think you're a plant they will probe with their mouth parts and that can be a bit painful, a bit like a mosquito bite, so that might not be the best approach for houseplants if you're living in amongst them, but they're very effective as predators because they feed on both the larvae and the adults and that's something that commercial growers are using, particularly in warm glasshouses. Another possibility: nematodes. You make up a solution and you spray water over the foliage. There's a particular nematode called Steinernema feltiae and you find this under trade names such as Nemesis or Entonem, various possibilities, and this parasitises the thrips. So the little, tiny nematodes, which are too small to see, that when the thrips is in contact with them, the nematode will work its way into the body of the thrips and multiply within it and kill it. So they can be quite effective, nematodes. There are entomopathogenic fungi, that's fungi that breed and kill insects. So there's a well-known fungus called Beauveria, which is sold under various trade names, but that will kill thrips and probably also white flies at the same time. Again, with all these biologicals, you have to be careful how you look after them. They don't last forever in a fridge for example. You've got to have them fresh and active and use them fairly quickly.
Jane: Yes, that is definitely key, isn't it? I think sometimes people don't read the instructions properly and then realise that they've waited too long to apply them. Given that they're not super-cheap, it's definitely worth checking out the instructions. Is that the same nematode that's used for fungus gnat larvae?
William: I think it probably is, yes.
Jane: That name rings a bell.
William: Steinernema feltiae. Yes, I think it's not specific to thrips.
Jane: Oh, that's interesting. I guess that's a good one to choose if you've got both problems at the same time? You might be able to tackle both.
William: Yes.
Jane: As you say, it's not a one-off solution, is it? How many times a year would you say you need to blast your houseplants, if you've got an ongoing thrips problem, just to keep it at an acceptably low level. Is there any regime that you would suggest?
William: Well, if they're indoor houseplants and you're keeping a fairly stable temperature, it doesn't really matter very much. Orius bugs certainly need warm conditions, so if your conditions get cooler in the winter, both Orius needs warmed temperatures and Neoseiulus swirskii need warm temperatures. So, if you're warm all year round, then it doesn't really matter very much, but if you're using a cool conservatory then you'd be trying these things in the summer. But with biocontrol, generally what you're trying to do is you have to keep using them, you're not eliminating the thrips usually unless you really bombard them. I suppose that might be possible, but usually it's quite difficult because thrips can come back in again. They hide away, if you're bringing new plants in and out, even if you quarantine them for a couple of weeks, you may still find things appearing, so it's quite difficult.
Jane: Presumably, if you're talking about chemical control, you'd be wanting something systemic, a systemic insecticide? Does that help in terms of dealing with the overall problem if you want to go down that route, obviously bearing in mind that then biological controls are not an option?
William: Yes. Well, systemic is best because the thrips are actually feeding on the plant tissue inside the plant, so although they're in contact with the surface, if you want to get it to the insect very effectively - suppose the larvae or adults are deep within a flower bud - something that's just sprayed and has a residual contact, insecticide, isn't necessarily going to get to where the thrips are, but if it's systemic, of course, then it'll get through the system of the plant into the leaf buds or the flower buds and then the thrips will end up feeding on that material, so that's more effective. For private use, it's getting harder to find the appropriate insecticides. So many, certainly in Europe, are being withdrawn on safety or environmental grounds. There's also a problem, of course, of resistance growing. So Spinosad was quite widely used on various crops in the UK, but the resistance has been spreading quite fast and sometimes, for example growers of strawberries that were reliant on Spinosad, find they're spraying it and nothing much is happening, the thrips are taking off. So you've got a resistance problem, as well, and a limited number of choices.
Jane: I guess that's really where the biological controls do come in, in that they're going to just go on working and resistance is not a problem, I guess, unless the thrips figure out a way of not being eaten! I don't know how quickly they can evolve? Usually I'd be recommending lots of cultural controls for dealing with pests. Is there much you can do other than just keeping your plant healthy and quarantining new arrivals to try to prevent your plant from getting a bad infestation in the first place?
William: Quarantine is important because the usual way they'll come onto houseplants is on a plant you've bought in from a nursery or garden centre. Sometimes we buy orchids in from garden centres and sometimes you can find there - I haven't seen many thrips on them - but certainly they can be pretty infested with pests. So they're not immune, even if they look a bit clean, look closely and quarantine them for a few weeks. We get pot chrysanthemums, we actually buy in a lot of these because we actually keep thrips as part of our research. So we're keeping Sainsbury's going by buying three or four pot plants of pot 'mums every week and then we feed them to our thrips. Sometimes we notice the odd thrips is coming in on the plants, so they come in and they're almost impossible to spot, so the quarantine is important. Of course, they like healthy plants because juicy leaves, healthy with flowering well, lots of pollen, those are all good for the thrips. So, culturally, keep your plants sensibly watered, if you stress them that's going to weaken their resistance to thrips. Another approach, which I haven't seen used in houseplants but might be something people could try out and it's something that is done with commercial crops sometimes, is because the thrips drop to the ground or drop onto the soil surface, some people put some kind of special mulch down on the surface which might be a reflective powder or something that has some kind of insecticidal effect, coating the soil surface.
Jane: Yes, that's an interesting one. I have heard of people who say that when they do get a thrips infestation that the first thing they do is, literally, wash off all the soil off the plant and just get it back to zero in terms of substrate, wash everything off and repot in a fresh substrate just to ensure that any pupae that were in the soil have been removed. I suppose that's a bit of a, in a way, waste of time because there could be lots of eggs sitting in the leaves waiting to start the cycle again, but I can understand why people do that as a first measure. I have heard people who will take all the soil off and cut all the leaves off on plants that can regenerate from their rhizomes or whatever as a kind of scorched earth approach, which I guess can work.
William: It's kind of dramatic, but I'm not sure you need to go that far. When you've got an infestation, the larvae will drop to the soil but we also find that if the soil dries out a little bit then there's quite a big crack between the pot and the soil down the edge. It doesn't even have to be big, just a very small crack, that's where the larvae seem to walk to the edge of the soil and get down there. So if you pull the whole plant out with its root ball and the soil you can often find the pupae down there, so it's not just the surface of the soil but to change the pot or wash the pot. In fact, actually rinsing the whole plant with water can wash off quite a few thrips, so it's not going to get rid of all of them but it's certainly going to set it back. So, in crops that are grown outdoors, when there's been rain, that knocks the thrips back and sometimes when people are doing experiments they have a control, compared with some insecticide or some treatment. The control might be spraying with water and it's surprising how often the spraying with water actually knocks the thrips back quite a bit.
Jane: That's a cheap solution, isn't it? It's just your time, I suppose, if you've got a lot of plants to wash. I think washing down your plants is good for any number of reasons and I guess among them is control of thrips. Well, as you say, it seems like you need a multi-pronged approach to this particular pest, is there anything else you can do that we haven't mentioned in terms of controls of any kind, or have we covered the main bases?
William: I think we've covered the main bases. I think if you've got sticky traps, then you're going to, possibly, with luck, you might mop up early arrivals. So, certainly, if you're quarantining your pests, have some sticky traps all around them because a plant could come in with just one female thrips on it and that would be enough to start an infestation. So, if you just happen to catch that one, that would stop the infestation. In fact, most species of thrips have got males and females but, in fact, it's a strange feature of the thrips biology, but a female thrips that's not fertilized, when it lays eggs, they become males and if it fertilises those eggs they become females. So we've got the situation where a larva could arrive on a plant, it emerges as an adult female, it doesn't have to mate, it can then start laying eggs which become males and then that female can mate with its own sons and be fertilised and then produce females. So one larva is enough to start an infestation. So it shows how rapidly...
Jane: Wow, no wonder they're so successful! Wow, that's quite some reproduction regime! That's amazing. You've spent 40 years studying these thrips. Is there anything else that we need to know? Any more fascinating thrips facts to add in?
William: Well, yes, I suppose what I've learned over 40 years, I haven't cracked the problem yet - I need another 40 years! - but one thing is that even though they're very small, their biology is no less fascinating than larger insects, or even larger animals. So you've got various curious things about their biology; I mentioned this thing about unfertilised eggs becoming males. Even the common species have some fascinating biology. So, for example, the Western Flower Thrips, which is probably the main pest of houseplantsk, that has several different pheromones it produces for communication. It has alarm pheromones, but the larvae communicate to each other to warn them of danger. Males produce an aggregation pheromone that attracts the females, the males also apply to the females an anti-aphrodisiac pheromone, so when they've mated it deters other males from mating with them. So you've got all these fascinating areas of their biology. In fact, some species in Australia, not ones that are pests on houseplants, are social, like bees and wasps. So you have a large colony of these thrips and then you even have soldier thrips that fight to defend the colony. So you've got some fascinating biology going on.
Jane: It's amazing to think all this is happening on our plant leaves and in the wider world! I have increased respect for thrips, but also you've increased my fear level because I just think the idea of one tiny larva coming in and an outbreak... I have to say, the plant that I had the worst outbreak on has been exiled to the garden and I need to make a decision in the next couple of days as to whether I'm going to bring it in for winter, or whether it's going to go on the compost heap, so I'm not sure whether it's worth it or not, but I'm sure there are some more thrips kicking around. They do seem to be attracted to particular plants in my experience and I've had other plants standing right next to the one that had thrips that didn't get any at all, so I imagine there's a lot going on in terms of what their preferred food sources are and lots more to learn. It's been really fascinating to talk to you Professor Kirk. Thank you so much for joining me and I think we'll all be checking our plants for thrips now!
William: Thank you. My pleasure!
[music]
Jane: Thanks so much to my guest, Professor William Kirk, and, as I always say, please go and check out the show notes because I beaver away and put lots of information there which will back up what you've listened to in this episode and it's a great resource for dealing with thrips, so do go and have a look at that. In the meantime, wishing you a fantastically planty and thrips-free week - that's hard to say: "thrips-free!" I will speak to you in exactly seven days. Bye!
[music]
Jane: The music you heard in this week's show was Roll Jordan Roll, by The Joy Drops, The Road We Used To Travel When We Were Kids, by Komiku and InsectifEYE, by KidNNasty. All tracks are licensed under Creative Commons, visit the show notes for details.
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Got thrips? You need to listen to this episode! I am joined by entomologist and thrips specialist Professor William Kirk to talk about these houseplant pests. Plus I answer a question about a plant damaged by a cat.
Note for anyone who has been asking me about houseplant courses: former guest and friend of the show Dr Scott Zona is running another of his botany of houseplants course in January 2022. I have no doubt this course will be excellent for anyone wanting a systematic look at this topic! Details from scott.zona@yahoo.com and further details in this tweet.
This week’s guest
Professor William Kirk is Professor of Applied Entomology at Keele University in the UK, and he has been studying thrips for 40 years. Professor Kirk’s book on thrips is available on Amazon and you can see his talk “The Secret Life of Thrips” here.
Western flower thrips eggs embedded in a pepper leaf. Photograph: Professor William Kirk.
Typical thrips damage on a flower. Photograph: William Kirk.
Check out the notes below as you listen…
Thrips facts
There is no such thing as a thrip! The singular and the plural is thrips.
Not all thrips species affect houseplants. There are 6,000 species of thrips around the world, although most of them live in leaf litter.
Around 100 thrips species are pests of food crops, and a further 12-14 species are significant pests you are likely to encounter on houseplants.
Here’s a useful Thrips identification poster. This was produced for UK growers, but includes the main houseplant pest species.
This free-to-access research paper by Professor Kirk explains the mating behaviour of western flower thrips: the “supporting Information” section includes a video of thrips mating.
Thrips that feed on houseplants are often spread around the world by the houseplant trade.
Western flower thrips (Frankliniella occidentalis) arrived in the UK in 1986, and can feed on many different plants. Japanese flower thrips (Thrips setosus) are just starting to arrive in the UK.
Some species are specific and some are more generalist. Onion thrips don’t just eat onions, for instance.
They feed by piercing and sucking parts of the plant - such as cells or pollen grains - with a mouth part that’s a bit like a drinking straw.
Thrips can be vectors of plant viruses which can cause more damage too.
The thrips life cycle can complete in less than two weeks in some species at higher temperatures, but 2-4 weeks is probably more typical on houseplants at room temperature. The eggs are a third of a millimetre and are inserted into plant tissue rather than sitting on the surface. They hatch into larvae; there are two larval instars (developmental stages), then they drop to the ground for the pupal stage - either on the surface of the soil or down the side of the pot, or sometimes in nooks and crannies on the plant. The adults emerge and fly back onto the plant. A female can lay eggs and mate with its male offspring and produce females - so one larva is enough to create an infestation.
Thrips symptoms
One of the symptoms of thrips is silvery markings. This is where the contents of a cell - including its pigment - has been sucked out and you are left with an empty cell full of air.
Thrips they feed on the pollen, so look near the stamens - plus whitish areas to the leaves, and dark droplets which is faeces.
Thrips can retreat into leaf buds and will feed on the undersides of leaves and look for yellowish larvae near the veins and signs of silvering. A hand lens means you can detect them at an earlier stage.
Controlling thrips
Monitoring is a good way of picking up an infestation at an early stage. You can use blue or yellow sticky traps to catch the thrips. Thrips are attracted to both colour traps, but the blue ones are more selective, ie other things like fungus gnats are not attracted to these traps.
Biological controls allow you to control thrips and keep infestations at a low level, but are not a way to completely eradicate thrips. They are not compatible with any kind of insecticide, including horticultural soap sprays. There are a few options:
Predatory mites Neoseiulus cucumeris is good for lower temperatures such as greenhouses and conservatories, swirskii is better at warmer temperatures of 20C (68F) and above. These mites cannot kill and feed on the adults, only the larvae, so they do not have an instant impact, and are better for minor infestations. Hypoaspis mites - Stratiolaelaps scimitus - will also eat thrips pupae as well as fungus gnat larvae.
Predatory bugs The minute pirate bug (genus Orius) are bigger than thrips at 3-4mm and if they land on you they will probe with their mouth parts and give you a bit of a bite. They are very effective as predators, as they feed on larvae and adults. Make sure you use them quickly and follow the instructions for any biocontrol.
Nematodes of the species Steinernema feltiae work on thrips as well as fungus gnats: make up solution and spray onto the leaves and these microscopic worms parasitise the thrips.
The entomopathogenic fungus Beauveria bassiana parasitises on arthropods including thrips, and is sold under various trade names to kill thrips and whiteflies.
Cultural controls
A reflective mulch on the soil can help to stop the larvae from pupating in the soil.
If the soil dries out, the larvae will often end up in the crack between the soil and the side of the pot, so rinsing the whole plant with water and the container it’s in can help. Rinsing plants can knock back populations of thrips.
Quarantine new arrivals in a different room to make sure you don’t bring any pests in on new plants. Put sticky traps around them to pick up any thrips.
Wiping the leaves is helpful for removing adults and larvae but will not remove the eggs because they are lodged in the plant material. (The same applies to insecticides applied to the leaves.) Therefore you need to repeat wiping at intervals so you catch the next generation of larvae. The adults and instars (developmental stages) can retreat into small spaces where insecticides or predatory biocontrol agents can’t get into.
Chemical controls
Horticultural soap sprays can help, but the note above about the eggs lodging in the plant tissue still apply.
Systemic insecticides can be used, but thrips are developing resistance to various insecticides.
QUESTION OF THE WEEK
Sarah’s cat has had a tangle with her Hoya linearis … so is wondering whether the bare stems will reclothe themselves with leaves? The short answer is yes: new leaves will not grow out of the identical spot, but the plant will grow axillary buds from the nodes along the stem and fill out the vine, especially if the end is snipped off (either by the cat or Sarah!) - this is known as removing apical dominance. The apical bud is the growth point of the vine, and when this is removed a plant reacts by moving the hormone auxin to the axillary buds. Sarah can cut off the bare stems and propagate, or leave them alone to work it out for themselves.
Want to ask me a question? Email ontheledgepodcast@gmail.com. The more information you can include, the better - pictures of your plant, details of your location and how long you have had the plant are always useful to help solve your issue!
LEGENDS OF THE LEAF
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CREDITS
This week's show featured the tracks Roll Jordan Roll by the Joy Drops, The Road We Use To Travel When We Were Kids by Komiku and InsectifEYE by KidNNasty.