Feeding damage caused by thrips to a pothos leaf before it has fully developed, resulting in distortion and necrotic patches. Regrettably out of focus image © Plantscape Designs, Inc.; image retrieved from their blog.
Feeding damage caused by thrips to a pothos leaf before it has fully developed, resulting in distortion and necrotic patches. Regrettably out of focus image © Plantscape Designs, Inc.; image retrieved from their blog.
Amblyseius cucumeris, a mite predator of thrips, in a moment of glory. Image © Rosemarije Buitenhuis; image retrieved from an article on thrips on the Greenhouse Canada website.
Amblyseius cucumeris, a mite predator of thrips, in a moment of glory. Image © Rosemarije Buitenhuis; image retrieved from an article on thrips on the Greenhouse Canada website.
Feeding damage and feces left behind by thrips, the rude little buggers. Image © the respective photographer (there are several credited on the page); retrieved from the University of California, Riverside

When one reads about pests in the interior landscape, spider mites, scale and mealybug are the most often mentioned, and with good reason: these creatures can, if conditions are right, completely overwhelm a plant, literally sucking the life from it. Even if they don’t, they can cause plants to be unsightly, and, in the case of the latter two insects, can cover the plant and the immediate area with sticky honeydew which is a bitch to clean. The signs of their presence are usually fairly obvious: spider mites create webs which can cover leaf surfaces and their feeding damage gives foliage a speckled appearance; scale will populate the foliage and stems with their dark, bumpy selves, and mealybug will crowd leaf axils and stems with their cottony badness.

And then there are thrips; they are small enough that most of the time you won’t know they’re there, and the damage they inflict on plants is often confusing and can lead one to believe that something else is responsible. And often when I’m assessing a problematic plant and mention them, people will frown and ask, ‘What are thrips?’ Hence this post.

Thrips is a large family of insects, many species of which are serious agricultural pests. (And yes, the singular and plural pronunciation and spelling is the same.) Because they have the ability to reproduce so rapidly, they can quickly develop resistance to chemical insecticides, making them a serious nuisance.

Because they feed primarily on new growth (including leaves, flowers and fruit), the damage they do is often caused before this new growth develops fully: flowers and leaves then open with distorted shapes, necrotic patches and streaks that can resemble disease, mechanical damage or nutrient deficiency. They can also feed on matured foliage, leaving behind silverish patches which in some species can become bronze-coloured or corky with time. They are, as I mentioned above, quite tiny, and when hidden deep in a flower bud or the like can be practically invisible. Knocking the plant part while holding it above a white sheet of paper will often dislodge a few and reveal their presence, and they do leave dark little globs of feces near feeding damage as well.

t is not merely enough for them to be sneaky, tiny, and voracious; they are also incredibly mobile. I have visited sites where every single plant on an entire office floor was afflicted (granted, it was stocked mostly with thrips’ favourite food, but still). They are capable of flight and also of being carried by the breeze due to their tiny size (not that this is as much a problem indoors). They also breed at a pace that would make a rabbit blush; they can complete the cycle from egg to breeding adult in as little as two weeks if temperatures are high enough to permit it.

This latter fact is one of the main challenges with their management by chemical means; as they reproduce so quickly, they are able to just as quickly develop resistance to insecticides, and thus these only offer a measure of control for a short time before they are useless. Irresponsible use of these chemicals by producers (failing to rotate chemicals, calendar spraying, and incorrect technique) have not helped the situation, and in fact, probably the most widespread of these beasts in Canadian ornamental horticulture, the western flower thrips, is now resistant to most chemical insecticides.

But before your despair causes you to throw your scarred old pothos into the compost, wait: there is hope in the form of biological control. There are many predators of thrips in nature, and several are able to be commercially raised for use in controlling them. Natural Insect Control, Inc. of Stevensville, Ontario is our most local option here in Toronto, and offers several of these in varying increments, the most common and economical being Amblyseius cucumeris, a predatory mite with a healthy appetite for thrips. This is a great solution if you’re not anti-bug in general (you’ll be intentionally releasing thousands of mites into your plants, after all), though to their further credit they are also very small and innocuous. I won’t get into the importance of knowing a little bit about the life cycle and natural history of both thrips and these mites in order to have optimal success with this technique; that’s a whole other blog post, at least. Suffice to say that it’s definitely a good option, particularly if you have many afflicted plants or what you’re trying to save is of high value (A. cucumeris are reasonably priced, by biological control standards, but certainly cost more than the average plant that one might buy at a garden centre or the like). There are several other thrips predators available to consumers such as Orius insidiosus and several other mite species, but as mentioned above, A. cucumeris is the most economical.

Another option is using products containing spinosad; these have only recently become available to consumers in Canada, so far as I know. They contain spinosyns, which are compounds which were discovered by fermenting a bacteria found at a rum distillery in the Virgin Islands(!); you can read the dry version of that here. Spinosad is wonderfully effective at controlling thrips, as it happens; I first read about it perhaps six years ago as a good tool against thrips in people’s orchid collections, and it has certainly knocked down populations that I’ve treated. There have been reports of thrips developing resistance to spinosyns, so applying sparingly and rotating with another product and/or in conjunction with a thrips predator so that the spray is used less often would help to minimize this effect.

Some products containing spinosad are approved for organic agriculture, which is enticing, though it should be noted that it is very noxious to bees, and so care should be taken with the product in that regard. The LD50 for this stuff is absurdly high compared to pyrethrin, a derivative of chrysanthemum flowers which is used in over-the-counter insecticides, which effectively means that it’s safer (the LD50 actually means that it takes whatever the number is worth of milligrams of compound per kilogram of an animal’s body weight to kill 50% of a population of said animal; a bit strange, but that’s the standard): according to the LD50, presuming you’re as sensitive to this compound as mice or rats and near median human weight, you would need to ingest 310 grams of it in order for it to be fatal; a feat, to be sure.

There are other non-chemical options available to commercial producers which are effective, and which seem to be trickling down and becoming available to the rest of us: the entomopathogenic fungus Beauvaria bassiana has been used as an ingredient in commercially available products for at least several years, and consumer-packaged products are available for order online from out of the country. There is not yet such a product being sold on shelves in Canada, so far as I know.

If you have these pests, you’ll likely be wanting to do something, as they can overwhelm a plant if they like it enough. The pothos pictured above is a common example of a member of the family Araceae, which all the way across the board has been a magnet for thrips, in my experience: Monstera, Philodendron, Anthurium, Spathiphyllum; all of these have fallen prey to the tiny rasping mouthparts of these miniature beasts, while other plants have grown in close contact without sustaining so much as a nibble. There are other susceptible species amongst common and rare indoor plants, and they love many species grown for cut flowers such as chryanthemums and gerbera daisies, to which they cause huge economic damage every year in production greenhouses; tap your bouquets over a sheet of white paper before you set a vase full near your houseplants!

Because their life cycle is so rapid, treatment will need to be fairly frequent in order to ensure they do get knocked down. Biological controls should be released at regular intervals, as many predators such as A. cucumeris feed only on younger thrips and can’t manage the adults (O. insidiosus certainly can, and it’s a joy to watch under magnification, but these guys don’t come cheap).

So that’s the story about thrips; controlling them can be a challenge, so have a close look at any plant you’re thinking of taking home, because you know what they say about an ounce of prevention…


If you’re unfamiliar with the chemical reaction above, then you may also be unfamiliar with the fact that life as we typically tend to think of it would not be possible without plants.

Photosynthesis is responsible for the capture of solar energy that in turn powers nearly all life on earth: everything we eat is either plants, or other animals that formerly ate plants (or that ate other animals that ate plants). Plants absorb sunlight and convert it into chemical energy which is stored within the plant for its own uses; we eagerly exploit this by consuming them and thus the sum of the solar energy they’ve stored. Good deal for us, bad deal for the plants (not that they seem to care).

There are not many other ways to capture and metabolize energy in this way, save chemosynthesis (which is why I need to keep referring to ‘almost all life’ above, which, while definitely less dramatic, is more accurate, as there are organisms which are able to capture energy from chemical reactions, most notably in deep-sea communities colonizing hydrothermal vents, and so have no need of sunlight). So plants really are the foundation of nearly all life on the earth.

And not just regarding energy, either. Though a bit more oft-toted, the fact that plants maintain the planet’s oxygen levels is equally prevalent. This does bring up the subject of conservation, but I can save that for another time. I will add in a shameless plug, though, that plants indoors will raise local oxygen levels and just generally improve the air quality indoors. You can read this post for more information if you like: Plants at Work: The Science Behind how Plants Improve Life Indoors.

For anyone interested in the equation who doesn’t understand the chemistry, basically the plant takes 6 molecules of carbon dioxide and 6 of water, and splits these to create free oxygen (which is released by the plant), and a few other goodies which combine with the solar energy captured by the chlorophyll in the plant to create carbohydrates (the C6H1206 in the equation above) which contain that solar energy. Pretty simple, but critical to life on earth.

It’s a little humorous to me that our industry (speaking very broadly here, of course) provides, in a manner of speaking, a product that no one can live without. Maybe that’s why everyone tends to like plants so much: I’ve met many people indifferent to them (and have changed a few minds there), and many more people who love them but can’t seem to stop killing them (and I can only hope I’ve helped a bit there), but have never really met anyone who’s said that they actively dislike plants (except maybe recent victims of poison ivy or the like). Maybe it’s a stretch to assume that we as a species are that aware of the inexorable connection we have with the rest of life, but for whatever reason the biophilic instinct is certainly alive and well.

We, as an industry, nearly always tote the benefits of interior plants, and I’m here to tell you that it’s not just bullshit: there are measurable effects in the way people think, behave and feel when they are in an environment that contains plants versus one that does not, and plants actually are able to clean the air we breathe.

Instead of doing what everyone else does, which is usually just to concisely (we all know that’s my strong suit, ha ha) list the same key points, I’ve done the legwork and actually rustled up a few of the papers from which said points were drawn from, and will point you to them so that you can read for yourself the results of some of the various studies that have been conducted over time.

By all means be skeptical, and don’t take our word for it: we’re very few of us scientists in this industry, but there has been real scientific work done which really confirms what we’ve been saying all along: that plants indoors have a direct effect on things like employee productivity, reduction of airborne pollutants, and combating stress and fatigue.

Cleaning the Air

So for starters, do plants actually clean the air? It would be a boring blog post indeed if I said no, and here are a few papers which highlight some of the work that different plants (and their associated colonies of soil-dwelling microorganisms) do to remove harmful chemicals from the interior atmosphere. What I’m not going to do is tote the old NASA study that gets thrown around so often: you can look that up for yourself, but Dr. Wolverton (and others) have continued to do good research into this phenomenon since the first study was published in the 80’s.

Plants and Soil Microorganisms: Removal of Formaldehyde, Xylene, and Ammonia from the Indoor Environment

This study found that quite a few plant species, notably Boston ferns, chrysanthemums, and dwarf date palms, were able to remove appreciable amounts of these chemicals from the air inside sealed chambers. Based on prior research into indoor air pollutants by the EPA, the authors calculated that an average-sized office constructed of typical building materials would contain 3916 µg (micrograms) of formaldehyde (to use the most sinister example in the paper). A single Boston ferns was shown to remove 1863 µg of this formaldehyde- per hour! The mums and palms were not far behind, and there was a decent list of other plants which were also quite effective at removing formaldehyde from the air.

Figure 1 from the study linked above, showing formaldehyde 
concentrations being removed by a Boston fern.
Figure 1 from the study linked above, showing formaldehyde concentrations being removed by a Boston fern.

The other part of the study looked at the microorganisms which colonize the rhizosphere (the area immediately surrounding plants’ roots), and the role they play in the removal of these chemicals. They found that unsterilized soil was able to remove formaldehyde from the air while sterilized soil was not, and that soil containing a plant was more effective still. They found that different types of bacteria had an effect on how much formaldehyde was removed, and the data indicated that different plants harbour different types of soil bacteria. Check out the paper for yourself: I’ve linked to it above.

Purification Ability of Interior Plant for Removing of Indoor-Air Polluting Chemicals Using a Tin Oxide Gas Sensor (sic)

This study performed similar experiments to the one above (you can read it yourself for the full details), with a slightly different method. Their results were similar: plants and their associated bacterial communities removed airborne pollutants quite effectively from the atmosphere. One point of note is that temperature and light had a large effect on the experimental results, suggesting that plants are more effectively cleaning the air when they are actively growing (see the portion in the discussion on uptake of gases through stomata if you like), which really bolsters the case for optimizing plant health in the interior landscaping in order to maximize this beneficial effect.

Improving Employee Productivity

This is a claim that is often used because it seems to infer a real economic benefit to the client. I’m inclined to agree with the science, and I can see that this certainly makes interior landscaping more marketable, but it almost feels like a bribe: surely plants can be desirable of their own merit, and surely the effect they have on people should not be measured in terms of productivity but of general mental and physical health? Do clients actually purchase plants to get more out of their staff? At any rate, the effect has been measured in the following papers (and I’m sure there are more); let’s call it here just an added bonus to the addition of plants to the workplace.

Interior Plants May Improve Worker Productivity and Reduce Stress in a Windowless Environment

This is one of the commonly cited ones, in which the authors noted a 12% increase in productivity (measured as reaction time to a computer task). I’m not sure that this is really a rock-solid study, and I wish I could have found another paper which replicated the experiment, but it’s here, for what it’s worth. One more interesting point in the study is the result on the blood pressure of the participants, which measured significantly lower during and after completing a computer-based productivity task in participants in a room with plants versus that of those in a room without.

Effect of Ornamental Foliage Plants on Visual Fatigue Caused by Visual Display Terminal Operation

I’m hoping that someone’s Japanese is better than mine and they might comment on this paper, but based on the English abstract and the figures in the results, these researchers found that viewing plants while performing tasks on a visual display terminal (presumably a computer screen of some sort) resulted in reduced visual fatigue when measured as critical flicker fusion frequency (a somewhat complicated phenomenon that you can look up on your own). I can’t comment much on this one, as I can’t even read it, but the numbers are there.

Plants Enhance Productivity in Case of Creative Work

This is more of a press release than an actual paper, I think, but it highlights the results of an experiment carried out by researchers in The Netherlands, which found that, while no improvements to productivity tasks were noted, there was a marked improvement in performance of creative tasks. These improvements were even more dramatic with test subjects with self-reported stress or exhaustion (the study used students as their guinea pigs)

In Situ Plants interior landscapes, vertical gardens, and other plant installations in Toronto
Figure 2 from the productivity paper above, showing the number of correct associations by students who reported a high level of physical exhaustion.

General Health and Wellbeing

This is probably the most important one for me, because it has much to do with the concept of biophilia, which I will be addressing soon (likely at great length), and which is tied very closely to In Situ’s raison d’être. We believe that humans have an innate subconscious need for proximity to natural elements, and keeping plants indoors proves to be a noteworthy way of satisfying this in our modern urban settings.

General mental health seems a difficult thing to quantify, but the works below are able to convey a few measured benefits to having plants around us while spending, as we typically do, the majority of our time indoors.

Stress Recovery during Exposure to Natural and Urban Environments

This almost creepy lab study measured several parameters (heart rate, muscle tension, etc.) during and after showing the poor participants videos of people getting into violent industrial accidents, followed by a video of either a fast-moving stream, a wooded area, or varying degrees of busy vehicle or foot traffic. The results clearly showed that the wooded scene was very effective in recovery from the stress indicated in the physical tests.

The self-report from the participants also indicates that the nature scene was the most positively affective by far, and best able to reduce anger, aggression and sadness.

In Situ Plants interior landscapes, vertical gardens, and other plant installations in Toronto
Figure 2 from the study above, showing changes in pulse transit time while watching first a gory video and then one of several other types of scene.

Greening the Great Indoors for Human Health and Wellbeing

This extensive study looked at various aspects of how keeping plants indoors relates to human well-being, from mental and physical standpoints. In section 5, the authors had their subjects complete Profile of Mood States questionnaires (apparently a widely accepted method for measuring different psychological states) before and after the placement of varying numbers of plants in their workspaces for a period of three months. The questionnaires covered such feelings as tension/anxiety, fatigue, and confusion.

The data shows that plants did in fact affect these parameters, and that the control group with no plants scored even worse on the questionnaire than it initially had done, while the subjects with plants saw their scores improve markedly.

The Role of Nature in the Context of the Workplace

This is also mostly a literature review, and includes quite a few statistics from other authors’ papers (which is why I’ve included it here), but the author points to two of her own studies, and I’d like to summarize here the gist of the second one: in a survey rating employee satisfaction, the availability of a view out of doors was considered far more valuable and restorative if it contained natural elements, and became even more so the more natural elements could be seen. Further to this, respondents with clear outside views to natural elements reported feeling more positive about their work in general. From the above:

“These results point to the range of impacts that a view of nature can affect. Those with a view of nature felt less frustrated and more patient, found their job more challenging, expressed greater enthusiasm for it, and reported higher life satisfaction as well as overall health.”

Pretty interesting stuff, I think. It will be interesting to stay on top of the science and see what further studies come from this quarter. If anyone has any further information on this they want to share (for or against, of course, though I bet you’d be hard pressed to find a study against plants in buildings), be sure to include it in the comments.