Thrips are tiny, winged insects that are classified as a pest for crops, houseplants and gardens. They come in a wide range of species, but are overall very resilient and polyphagous (which means they can and will feed on a variety of plants).

These little guys are the devil for many plant owners, as they can be persistent and highly damaging for your beautiful houseplants. The adults and larvae cause damage on the leaves as they scrape the chlorophyl out of the cell, or suck the juicy plant sap out of the wounds.

Sounds gruesome but it gets worse, some species of thrips will migrate to the soil of the plant when they are in their larval form to transform into their adult phase, making them even more incognito and almost impossible to get rid of. As they can populate almost all parts of the plant, it is important to have an all-round strategy when trying to combat them.  Both the looks and health of infected plants will slowly deteriorate as the plague grows bigger, that’s why it is crucial to nip it in the bud as soon as the first signs appear.

Thrips parvispinus (Pepperthrips) – Left: Adult // Right: Larva

Let’s run you through the signs of a Thrips infestation and give you some battle strategies to be victorious over them on the battlefield of pest-control.

Damage Recognition

Different species of thrips can have different modes of feeding or might target other organs, but in general the signs of a thrips-infestation can be rounded up as such: Silvering of leaves and discoloration (yellowing/browning) in combination with slower or distorted growth with a possibility of seeing small, elongated insects on the plant. Damage will show as soon as the first thrip insect starts feeding, but seeing the animal itself should be rare. If you’re seeing these bugs casually strolling on your plant, it might already be to late to save. Let’s show you some pictures of what the damage looks like, so you can jump to action before it’s overdue.

Left: Monstera variegata (underside of leaf) – browning/yellowing of the leaf // Middle: Philodendron scandens – bite/scrape marks on young leaves // Right: Schefflera actinophylla amate – distorted growth.

Left: Philodendron pedatum (underside of leaf) – bite/scrape marks // Right: Philodendron Elegans – idem.

Left: Ficus lyrata bambino – bite/scrape marks on new leaves // Right: Schefflera arboricola – bite marks and distorted growth.

These pictures are made in our greenhouse where we have camped with some Pepperthrips, they tend to go for young leaves and growing nodes. However for other species the damage might look similar, the most important thing to look out for is the silvering in leaves, and brown/ yellow spots! More on these different species later…

Battle Strategies

Now that we have positively diagnosed our plant with thrips, it is time to take action. It is often advised to go straight to a chemical treatment with ‘Spinosad’ or other crop-protective agents, however these chemicals are increasingly harder to find, and some are already outlawed. Personally I am not in favor, as there are better, more sustainable ways of dealing with these critters. Let’s run ‘em down:

• Social distancing: With the first signs of damage on one or more of your houseplants, isolate these from the plants that still seem healthy. This way we can prevent the infection from spreading throughout your whole indoor jungle before taking aggressive action on all of them, reducing your cost- and workload.
• Oils and Soaps: As an alternative to harsh chemical treatment, oils and natural soaps can help a great deal. Neem oil specifically acts as a natural insecticide and can help control the spread of the thrips population. When you buy neem oil, read the instructions carefully and follow them to the letter. It is important to dilute the substance before applying it with a plant-spray all over your plants. Focus on the underbelly of the leaves, as this is where the little devils like to hide. Repeat the process every 7-10 days until the infestation is under control. The same applies to green soaps, the ones you’d use to remove green stains from your floor tiles. Both of these substances can dramatically reduce the thrips number, but be sure to test them on one plant before spraying them on everything, to account for adverse effects.
• Natural predators: Now this is my personal favorite, natural predators are the way these pests are controlled in their natural habitats. There are several effective species of predatory mites available on the market (Amblyseius swirskii, Transeius montdorensis, Neoseiulus cucumeris, to name a few we use ourselves). These are generalist mite species that will prey on the living thrips in the foliage. Apart from mites there are also predatory bugs like Orius insidiosus or lacewing larvae that are ferocious hunters on many pest-species. However these might be too hardcore for treating a few plants in your home, they could be useful for owners of greenhouses however, or when the infestation has grown out of control. Now I mentioned before that some thrips species have part of their life cycle in the soil, for these it can be beneficial to inoculate the soil with nematodes or soil-mites, but a good soil change with washing of the roots can already help a lot. Very important to note, if you are using any insecticides, this will kill the beneficial insects too. So time your treatments correctly and do not combine biological and chemical solutions.
• Prevention: Of course the best cure is not getting sick at all. Healthy plants are generally less susceptible to thrips infestation, so keep your plants safe, watered and lighted to reduce the risk of infection. Improving air circulation around your plants is one of the important cornerstones of thrips control, as high humidity and still air is the ideal breeding ground for them. You can also use sticky traps (preferably blue or yellow) to monitor if there are thrips on or around your plants.

To Know Your Enemy is…

… hard to figure out sometimes. With more than 7000 species in the Thysanoptera order, it is normal to get a bit overwhelmed with the possibilities. To make it a bit easier on you, I’ll identify the 3 most prominent species that are found in European homes and greenhouses. A full list of important pest species commonly occurring  in Europe is attached to the bottom of this text, this list and identification sheet is created by Wageningen University to help growers and plantlovers battle the ever-growing thrips problems.

But now, the 3 thrips species we ourselves have had to deal with, and the most common and hence most likely to end up in your home:

• Western Flower Thrips (Flankiniella occidentalis): One of the most common thrips species to occur. Their feeding causes silvering of the leaves in combination with deformations. They live on the leaves or in the soil and hence are a pretty tough cookie to crack.
• Orchid Thrips (Chaetanaphothrips orchidii): Contrary to what you might think, this species is not orchid-specific but can also appear on decorative plants like monstera and anthurium. When colonized, the plant will be unable to open its flowering bodies while the leaves become deformed and brown colored. In our greenhouse we found these critters on the leaves closest to the soil, possibly living there after they pupate in the pot.
• Pepper- or Tabacco Thrips (Thrips parvispinus): The new kid on the block, shipped over from India. They are resistant and persistent and proper regulation strategies are still being researched. Pepperthrips are most commonly found in the growing tips of plants, where they feed on the young leaves still incapsulated by the bract. New leaves will emerge already damaged, deformed and/or spotted.

For more common thrips species, follow the next link and download the identification sheet: https://www.wur.nl/nl/onderzoek-resultaten/onderzoeksinstituten/plant-research/business-units/glastuinbouw/show-glas/nieuwe-versie-tripsherkenningskaart-beschikbaar.htm

Victory, at last!

We hope this overview of the damage, treatment and recognition of thrips helps you to take care of your indoor jungle. Some of the bio-controlling mites are available in our webshop as ‘pest-control bundles’ or separately available. We wish you good luck with the battle on these pesky insects, and if you gain any knowledge on how to beat them, we are all ears!!

By Senne Note, a Plantloving biologist.