What the Thrips?! A Deep Dive to Understanding Them & Treatment Solutions

Thrips are perhaps one of the most hated pests for all growers. They are notoriously difficult to get rid of and even having a couple on a bloom can ruin a stem’s salability.

In 2023, we had an abnormally mild winter followed by a very dry May/June with relatively low humidity so they descended in a fury! Previously, I had only ever seen them on my snapdragons. As the first of my feverfew started blooming, I noticed a serious thrips infestation and that is how I ended up in this thrips rabbit hole.

In order to determine the best way to manage this pest, we must dissect the following:

1) Biology & reproductive cycle

2) Weather variables that impact thrips reproductive rates and mortality

3) How they damage crops

4) How they “arrive” on our crops

5) Treatment options

But first… what are thrips?

Thrips are slender insects with fringed wings. There are over 6,000 species but about 100 are considered to be pests. In the flower farming world, we primarily deal with Western Flower Thrips (WFP, Frankliniella occidentalis) and Gladiolus Thrips (Thrips simplex). WFP particularly love roses but will really feast on anything - on my farm, they love snapdragons, ranunculus and feverfew but I’ve heard of them feasting on sweet william, campanula, and anemones just to name a few. Next to not getting enough water (which exacerbates the thrips issue), Gladiolus Thrips are usually the culprit for ruining gladiolus crops. They will also overwinter on gladiolus bulbs, so make sure you check carefully before planting!

What makes thrips the bane of many farmers’ existence?

• Small & stealthy: Thrips are just visible to the naked eye but sometimes it’s hard to tell if it’s dust or a thrips. If you blow on it and they scurry, it’s thrips. But when you get a really bad infestation, you can see them crawling all over. Because they’re so small, it is difficult to eradicate every single one of them. This is a challenge with sprays that work only if the spray comes into direct contact with the insect (ie Neem oil - more on this later!)

• Just “chilling out” until it’s 50F (10C): Thrips overwinter in the soil and have been known to survive freezing temperatures. I’ve been told that historically, thrips didn’t overwinter in my region (6b) but climate change is changing this. The 50F piece is important because often times, it’ll feel like a thrips infestation appears out of no where. In reality, the thrips are just hanging out, waiting for their moment! Note that high intense heat (ie over 100F) can eradicate thrips but freezing temps don’t seem to do much based on this study.

• Relatively fast reproductive cycle that lasts awhile: In a single growing season, there can be up to 8 generations of thrips. Thrips thrive on heat and low humidity - their sweet spot is 73F (23C) to 86F (30C) with 40-70% humidity. I don’t know about you, but that sounds like summer in alot of places even though our humidity can be quite high. So at the height of the growing season, we’re looking at a full reproductive cycle every 2-3 weeks! Research shows that it takes 14 days for a WFT to hatch and mature into an adult at 77F (25C). This is why a seemingly small thrip problem can exponentially turn into a big one.

Before we can determine the best course of action to take, I think it’s really important for us to dive deeper into the 4 topics I laid out above, starting with their biology and reproductive cycle. I see alot of well meaning flower farmers take a “ready, shoot, aim” approach by ordering beneficial nematodes or spraying neem oil before they fully understand the root of the problem they’re trying to solve.

1) Biology & reproductive cycle

As you can see in the graphic above, an adult thrips can live for a bit over a month where after, they spend their life laying 150-300 eggs. More importantly, they can reproduce both sexually and asexually (ie presence of a male is not needed)! And more fascinatingly, if a female WFT does not mate, it would produce male offspring asexually. If thrips mate, they produce mostly female offspring. There is also generally a higher percentage of females vs males. Bad news from an egg producing perspective!

What does this mean? It means that one must either be:

1) Proactive about managing their thrips situation. If you’ve seen thrips and had an infestation, it is very likely it will happen again eventually.

or

2) Diligent about monitoring for thrips and act immediately. My research has shown that “natural” options are more limited once there is an infestation.

Weather variables that impact thrips reproductive rates and mortality

There are three weather related variables to discuss here — temperature, humidity, and precipitation.

We already know that thrips prefer temperatures between 73F (23C) to 86F (30C) with 40-70% humidity. However, moderate to heavy rainfall negatively impacts thrips survivorship in the following ways:

• Heavy precipitation can kill larvae, which means that the thrips population will recover more slowly due to the high larval mortality

• Direct rain or heavy dew can prevent thrips take-off as their wing setae become saturated

More information can be found via this study.

My personal experience validates the above. After discovering my thrips infestation on my feverfew, we had a few hours the next day of relatively heavy rain totaling to 1 inch. There were so many thrips on my Sunnyball feverfew that it was impossible to miss. I also saw dozens of thrips on my Madame Butterfly snaps. The next morning, it was still possible to spot thrips on both of these crops, but quite easy to miss if one were not paying attention. More importantly, I did not observe thrips on my Magic Single feverfew and very few on my Virgo feverfew. All hope may not be lost!

The key here is the time it took for that 1 inch of rain to disperse. It happened over the span of 2 hours versus over an entire day. I am sure that the heavier volume of rain in this short period of time helped my cause and the same amount of rain over the duration of a full day would not have had such a significant impact. The good news is, we have rain forecasted multiple times over the next 10 days. If that rain does come to fruition, I feel some of my feverfew could be salvageable.

Post-rain Sunnyball feverfew. If you look very closely, you can spot some thrips. Prior to the rain, the exterior blooms were teeming with thrips.

How do thrips damage crops?

Pests generally damage crops one of two ways — by either sucking moisture out of the plant or chewing on the plant. Thrips belong in the sucking category. My observation with thrips is that they have rarely done enough damage by sucking to compromise the ability for my crops to produce good, viable stems. Rather, their damage is cosmetic. Who wants these tiny, disgusting crawlers all over their flowers? I think it’s also a bit irresponsible to knowingly sell stems that have thrips because if the thrips land on houseplants, they can do damage there too. They especially love a good monstera and what plant loving person doesn’t have at least one monstera in their house?

The other way that thrips damage crops is by carrying viruses — Impatiens Necrotic Spot Virus (INSV) and Tomato Spotted Wilt Virus (TSWV). TSWV has an extremely wide host range, most of which are vegetative crops and a concern for me personally since we grow alot of our personal consumption of tomatoes, peppers, and eggplant in the same growing space as our flowers. While INSV has a fewer hosts, it does affect many of the ornamentals flower farmers grow such as asters, snapdragons, marigolds, host, delphium, ageratum, chrysanthemum and unfortunately, the beloved dahlia.

How do thrips “arrive” on our crops?

We know that thrips can overwinter in the soil and begin laying larvae once the weather hits 50F, but why do they continue to persist even after a full generation is eradicated? In other words, why do treatment plans recommend continued use throughout the growing season? The answer is because thrips can both fly and are able to migrate long distances via the wind. They can only fly short distances and it’s more like “hopping” but the wind is quite a viable mode of transportation. This is why greenhouses continue to battle thrips even after eradicating them, though sometimes greenhouses bring in new crops that are harboring thrips.

So even if you can successfully eradicate thrips from your bed of snapdragons, if you have thrips persisting somewhere else, say in that bed of clover weed a couple hundred feet away or at your neighbor’s property, a new infestation is possible.

I specifically call out clover because WFT has been known to overwinter in clover in New Jersey where I live. Many weeds and even cover crops (rye, vetch, and wheat) are also hosts that I suspect are helping WFT overwinter. My lawn stays green during drought because we have a spectacular amount of white clover. In early June, it can look like snow on the ground from far away.

I did not do a great job of maintaining this clover, especially in my flower field. The clover has crept into a few of my growing beds and I suspect that’s where the source of my thrips infestation began. And perhaps it probably wouldn’t hurt to do a rain dance every so often. We sure need it as our summers appear to be more dry!

Treatment options:

Herein lies the million dollar question — how do we get rid of these pesky pests? And the answer I’m about to give you is not going to be a good one. To fully eradicate them, you really need the big guns aka chemical insecticides and even then, there is no silver bullet. Rose growers in Kenya are struggling to keep thrips at bay and one grower estimated that they spend 30% of their insecticide budget on thrips!

With chemical insecticides, you need to be very careful about not creating a situation where you help the thrips develop resistance. You would likely need to apply 2 different agents among and within different generations. While I personally would never take this route, I realize that some people’s livelihoods depend on the salability of these crops so I highly recommend watching this webinar hosted by Syngenta if this is the route you want to take (minute 56 begins the section on Resistance).

Since I employ a “no spray” mindset, I want to spend some time talking about natural alternatives. But first, let’s break down why I DON’T recommend using something like neem oil.

There are many “organic” growers who spray neem oil on any pest infestation. I put organic in quotations here because even though neem is a natural occurring pesticide, it is still toxic and doesn’t discriminate between pests and beneficials. But in our context, neem is also extremely ineffective. Neem oil works by covering the insect’s body with the oil which blocks their breathing openings, causing it to suffocate. It only works if the oil comes into direct contact with the body. Remember how small a thrips is? It is impossible to spray every single thrips in an infestation. Thrips also like to hang out on the underside of the leaves which is where the stomata are located (where a plant “breathes”). So spraying on the underside of leaves would also clog the stomata and therefore weaken your plant. To add insult to injury, you’re likely also killing off some beneficials that would have at least helped the thrips situation. Let’s add up this equation:

Some thrips left unsprayed (can still exponentially reproduce) + compromising plant’s ability to breathe + killing off some beneficial predators = you’re better off not spray with neem oil!

What does that leave us with? Integrated Pest Management (IPM).

A word of caution before you read any further. If you already have a thrips infestation, IPM is unlikely to solve your problem, although it can help keep the infestation to a manageable level. The one main advantage of IPM is that you never hear of thrips developing resistance to predators!

In my research, IPM is better employed as a proactive measure to prevent a thrips infestation. The goal is to eradicate the larvae from ever becoming adults.

• 1) Applying beneficial nematodes (best done as a proactive measure in the early spring or as a part of a recurring regimen): Nematodes are microscopic predacious worms. You apply them in a water drench into the soil where they then hunt down and kill thrips pupae. Their limitation is that they only work their magic in the soil and won’t be of much help on the thrips that are already on your leaves and flowering buds.

• 2) Releasing predatory beneficials such as predatory mites, pirate bugs, ladybugs, and green lacewings. A local farm, Moonshot Farms, has shared that they found the mites to be superior to pirate bugs as the mites (Cucumeris or Amblyseius Swirskii) will eat all stages of thrips and are alot less mobile. Obviously the pirate bugs, ladybugs, and lacewings can all fly away. Moonshot Farms applies beneficial nematodes weekly during the high pressure times or biweekly as a preventative measure plus the predatory mites. They use Evergreen Growers Supply for their beneficials and while I have not personally bought from them or employed this regimen (yet), I highly respect Moonshot Farms and would trust their recommendations.

In my current situation, the goal isn’t to save my infested crops but to prevent it from spreading. I liken this to when my dad had a central retinal artery occlusion (CRAO) aka an eye stroke. While he was spending his energy figuring out how to save the vision of his right eye, his eye doctors were more concerned about preventing it from happening to his left eye and I was personally more concerned about getting him checked out for a stroke evaluation as CRAO is a harbinger of heart disease. In my case, the snapdragons and at least 1 bed of feverfew are gone in my book. The only things that can potentially save some of my remaining feverfew is the additional forecasted rain as well as all of the ladybug larvae that are feasting. I was wondering why the larvae were still on my feverfew even after they took care of my aphids!

My primarily goal is to prevent thrips damage from my more valuable crops such as my lisianthus (which I paid alot of $$ in seedlings for) and my dahlias which are still very much “babies” at this point. I will be purchasing beneficial nemotodes and it remains to be seen whether I will purchase predatory mites. And of course, I will be making sure that I diligently weed, especially in the areas close to these crops. Although I didn’t experience thrips infestations in my dahlias last year, I am hearing from other growers that they are seeing thrips on crops they never had issues with before. Better safe than sorry then!

Other questions you may have…

• Can I rinse the blooms in soapy water and sell them? Hypothetically yes. Technically, nobody is preventing you from selling thrips infested blooms except perhaps your conscious. But I personally wouldn’t do it. I think it would be a bit irresponsible especially knowing now that thrips can jump onto houseplants and cause an infestation there. I am also very vigilant about ensuring my customers have a good experience with locally grown flowers because they would never find thrips from grocery store bouquets (those have been heavily sprayed to prevent this)! I also sell to florists who do large, expensive floral events and it would be a huge risk for me to sell them something that could compromise the perception of the florals by their clients.

• Would you use an infested crop as a “trap crop”? I am leaning towards no, given how quickly thrips can reproduce. Knowing what I know now, I am going to cut down any stem that has thrips and into the compost they go! I’ll also reassess my feverfew beds in a week after the rain has passed to see how bad the infestation is. If it hasn’t improved, the weed whacker is coming out! Luckily, both feverfew and snapdragons can overwinter and I’ll apply beneficial nematodes both in the fall and spring as a proactive measure.

• What are ways to detect a thrips infestation earlier? There are sticky tapes that emit pheromones (this is how thrips communicate) and it could be worth having some of this tape next to crops prone to thrips as a “canary in the coal mine” measure to bring in beneficial insects or start a spraying regime.

Concluding thoughts:

For many of us, there is a clear pattern emerging. Our springs are ephemeral, our summers are getting hotter, and rain is less abundant. All of this is creating prime conditions for thrips, so I expect the problem to worsen in the future versus getting better. From the research I’ve seen, most studies have been conducted focusing on chemical insecticides. There is very little concrete data on specific IPM strategies. The best resource is other farmers. Scour the ASCFG Facebook group for posts or reach out to fellow flower farmers who deal with thrips via beneficials.

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