How to Control Thrips

Thrips are an amazing pest! Some struggle with them, while for others, thrips merely come and go throughout the season. If you’re here, reading this then I assume you fall into one of these categories:

  1. “What the heck is a thrips?”
  2. “Do I have thrips?”
  3. “I have thrips – help me get rid of them!”

Hopefully, I can address all your needs here by combining my knowledge of thrips, the bio-control of thrips and commercial and amateur growing.

1. What the Heck is a Thrips?

A single one of them is still called a “thrips,” so you can have many thrips or one thrips. Actually, I lie, no one ever has only one thrips! These insects are small and can be numerous. In small numbers they are rarely noticed, but populations can explode causing major damage to foliage. Typically the greatest concern over thrips is in vegetable production and ornamentals, where thrips damage in flowers can render the plant or vegetable unsellable.

There are many species of thrips, but some of them are of major concern: the onion thrips is numerous and can do damage but is usually controlled with chemicals. The Western Flower Thrips is devastating as it is more chemically resistant. While I could list a bunch more, let’s focus on the common ones, as we can group them together. However, it’s worth IDing your thrips, because some are beneficial predators! Update: GreenHouse Canada published a new article by thrips expert Dr. Sarah Jandricic in the February 2022 edition that identified Onion Thrips as a growing concern: It is most likely to grow in populations indoors throughout the winter, and is showing signs of more chemical resistance.

Here’s their life-cycle:

Adults are winged but generally don’t fly. Instead they tend to “tumble” towards your plants in gusts of wind. They feed on plant tissue and pollen, and their toxic bite will discolour plant tissue and warp flower formation (sometimes leaf formation as well.) They are relatively big as adults, and they are fast and aggressive so they can often fight off some predators.

Adult females will lay eggs INSIDE plant tissue. This protects them from predation – a major problem for natural predators. As the egg hatches, the larva will feed on plant tissue before popping out of the leaf.

Larval thrips look similar to the adults but without the wings. They are usually smaller and sometimes have less colour. They are still quick at this stage, but are usually able to be attacked by predators.

At the end of the larval stage, thrips will pupate in the soil. This is a key time for predation – if they can be found.

After pupation, adults emerge and either return to the plant or move-on.

About movement:

As mentioned, despite being winged, they don’t really fly. Instead, their feathery wings are extended in order to be suspended by differentials in electro-magnetic fields……no joke. They essentially levitate. While that sounds bizarre, it is relatively common in the insect world. During levitation, and particularly during times of atmospheric convection (thunderstorms) they will be carried up and away by the wind. (Their nickname in Holland is Thunderbugs or Thunder-flies, for this reason.) This is also the reason they suddenly come in droves. Usually thousands of them are blown into a greenhouse or yard all at once. Fortunately they are as likely to take off and leave again as they are to stay. But it’s when they stay, and lay eggs that the problems start.

Tired of reading? Here’s my Youtube video on managing thrips.

2. Do I have Thrips?

Thrips damage can be mistaken for spider mite damage by the tiny patches of discolouration on plant tissue which is common of both pests. However, thrips damage is usually looks like longer scrapes of the leave tissue. And the big give-aways are the texture of the damage (usually a depression on a leaf) or the curling, or gnarling of plant tissues due to their toxicity.

With a simple hand-lens or some great vision, you will often see the adults and larva where there is damage. While this is a major generalization, I typically tell thrips apart simply by their long body and their type of movement. But, beware, predators like lacewing larva are long and skinny too.

Unless you can tell they are thrips from the above, it’s best to get a closer look. Here’s what you can do.

  1. Gently bend a flower head over your hand and tap it. Usually, if thrips are present they will fall onto your hand before scurrying away.
  2. Use sticky cards. Thrips love flowers, and so are attracted to both colour and scent. While blue sticky cards were designed for thrips, a recent study found that yellow still attracts more thrips. Try placing a card to see what sticks….but be prepared to stick and kill lots of things….so this is best used in greenhouses or indoors.
  3. Use a bucket of soapy water with a floral extract in it. Try something like peppermint extract, vanilla extract or a floral-based perfume. They’ll hop in for the smell and drown in the water.

3. I Have Thrips! – Help me get rid of them!

Ok, here is how we control them in a commercial setting, which is also applicable to anyone:

  1. Use the bucket of soapy water method as above. But every week, empty it and rotate through extracts. Every adult you catch is 50-100 eggs that won’t be laid. Also, seeing one or two a week, and then suddenly seeing 30 will let you know that they have arrived, which is important for some of the next steps.
  2. If you use sticky cards, date them. The same applies as above. Use this to get rid of adults but also to identify when their numbers are increasing.
  3. Apply the predatory mite Neoseiulus cucumeris (common name: Cucumeris). These predatory mites are super cheap so you get lots of them. (1L contains 50,000 mites). Apply these to your plants when adults are present. They will not eat the adults, but will eat the larva as they hatch from the leaves and they’ll eat the second stage larva as well. These should be applied every week when thrips are present or every 2 to 3 weeks if you want to prevent them.
  4. Never spray anything – no chemicals, no soaps, no oils…..nothing. Natural predators rely on these boom and bust populations to drive their own population growth. So if you are trapping adults and applying cucumeris, there is no need to do anything. If you don’t have the patience to wait, it is simply a matter of adding more Cucumeris.
  5. You must have a predatory mite in your soil. This goes for all house plants, all garden beds, pots and lawns. Stratiolaelaps scimitus is the most common option, but Gaeolaelaps gillespiei (available in Canada) is best. When adult thrips pupated, they have to do so in the soil. Predatory mites are so in-tune with this that both of the above mites are proven to reduce thrips populations by 80% just in the soil alone. These can be bought by their Latin name through distributors here, or on Amazon under the name “Grub Grenade.”

4. Bonus question: “What the heck is Cucumeris?”

Cucumeris is a commercially produced predatory mite. It’s tiny, and used primarily for thrips control. However, they also control eriophyid mites like russet mites and broad mites.

They are sold as mites of all life stages in loose carrier material for easy distribution or in slow release sachets.

500 mites per square foot, per week is a good knock-down amount. That means 1L will cover 1,000 square feet. However, in practice, truly getting rid of them usually requires 750-1,000 per square foot.

In Canada, Anystis the Crazee mite is being produced at Applied Bio-Nomics in order to help with Thrips predation. In current trials the combination of Cucumeris and the Crazee mite out performed other bio-controls or either alone. It is currently in development stage.

You can purchase Cucumeris through a local retailer or distributor here.

If you’re in a garden and worried about thrips, try not to be. They are seldom a problem unless in a mono-culture or with some susceptible flowers. Generally, thrips are a problem in protected greenhouses and indoors where they get in, but predators don’t. If you are worried about thrips on a house plant, sometimes, the bucket of water is all that is needed. Other times, washing the plant leaves with water in the tub is enough to get rid of the thrips after one or two times.

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