Thrips: A Tiny but Noteworthy Pest of the Interior Garden

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…