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The Case of the Onion Thrips: An IPM Mystery

If your thrips biocontrol program isn’t working, it may be because your thrips have changed.

February 28, 2022  By Ashley Summerfield, Dr. Sarah Jandricic, Dr. Rose Buitenhuis & Dr. Cynthia Scott-Dupree

This budding chrysanthemum plant is damaged by onion thrips. Photo credit: A. Summerfield

Thrips were already the most formidable pest tormenting greenhouse ornamental growers in Canada to begin with, but if you’ve noticed that they are even harder to control than they used to be, you’re not alone.

Over the past few years, growers started seeing more thrips outbreaks, and their biocontrol programs weren’t always keeping up. That’s when Dr. Sarah Jandricic of OMAFRA discovered we might have a mystery thrips on our hands.

It turns out there was a new villain on the scene: onion thrips (Thrips tabaci). They make up on average around one quarter of the thrips found in floriculture greenhouses in the Niagara region of Ontario, but may account for more than half depending on the year, crop, or location.


With the culprit identified, an investigation began (in the form of a Master’s thesis) to learn more about our new foe and, most importantly, how to stop it.

Criminal Record: a brief history of thrips in Canadian greenhouses

Western flower thrips (WFT; Frankliniella occidentalis) have been the primary pest thrips in greenhouse ornamentals since they found their way from the southwest coast of the US to the rest of North America and the world in the 1980s-1990s.

Onion thrips (OT) on the other hand, have been here for much longer. They were first described in North America 140 years ago. Before widespread synthetic pesticide use, OT were a common pest in greenhouses, but are now mostly associated with outdoor crops, and onions in particular.

In the 1990s, changes to pesticide spray practices in onion crops for controlling onion maggot  in Ontario inadvertently led to increased OT pressure in this crop.1 Around the same time, greenhouse growers were moving away from calendar sprays of pesticides and adopting more IPM tactics (e.g. monitoring-based spraying, forays into biocontrol) in response to increasingly pesticide-resistant WFT populations.

This combination of increased OT populations outside and the almost complete adoption of biological control for greenhouse pests by 20182 likely led to OT resuming its criminal activity in greenhouses. With fewer pesticides incidentally controlling this less pesticide-resistant thrips species, we now have a classic case of pest resurgence on our hands.

Breaking & Entering

Thrips enter your greenhouse in one of two ways: you either bring them in on imported plant material, or they come in from the outside on their own.

Past research at Vineland Research and Innovation Centre demonstrated just how many thrips you could be bringing in on cuttings like chrysanthemums or spring annuals (up to an average of 12 thrips per 25 cuttings in certain crops3). We repeated this research to find out which thrips were coming in on propagation material. Interestingly, we only found WFT.

This led us to suspect OT were breaking in from the outside. Sticky cards set up outside three greenhouse operations from spring to fall 2019 confirmed a consistent presence of OT directly outside Niagara greenhouses. OT represented about 15 per cent of the thrips caught on the cards, on average.

Why is this important? It turns out that when it comes to thrips species composition in your greenhouse, what happens outside can have a direct impact on what happens inside.

At our research sites, the greenhouse with the highest proportion of OT outside had consistent OT presence in their crop all year. The species composition in their crop was perfectly matched to what was happening outside, as well (Figure 1). Similarly, the greenhouse with the lowest proportion of OT outside had very few OT inside.

Figure 1.
Generally, the percentage of thrips that were OT caught outside on yellow sticky cards (yellow bars) seemed to reflect the percentage collected from the greenhouse crop inside (green bars).

However, outside pressure isn’t the only thing that can influence species composition. At our third research site, the proportion of OT in the crop was actually a lot higher than what we expected based on outside catches. This suggests that this greenhouse’s IPM program may be less effective for OT compared to other sites.

At all sites, though, we saw thrips populations for both species peak in July and August. So, if you want to prevent OT from becoming a problem in your greenhouse (and messing up your biocontrol program for WFT), this is a good time to go bananas with mass trapping! This is especially important for side-venting greenhouses, where thrips can more easily break in.

Catching Onion Thrips in the Act

Mass trapping, if used properly, can be an effective way of reducing the number of thrips that make it through your vents and into your crop.

You may have heard of study results from Europe that blue is the best card colour for trapping thrips. However, in 2016/17 trials looking at the efficacy of yellow versus blue sticky cards and tape in Ontario greenhouses, Dr. Jandricic found that yellow was better than blue for catching thrips, regardless of the manufacturer.4 But this research was done in greenhouses with predominantly WFT populations – what about OT?

To answer this, we put up yellow and blue sticky cards inside three potted chrysanthemum greenhouses throughout 2019. As it turns out, in addition to their love of chrysanthemums, OT and WFT also share a preference for yellow sticky cards (Figure 2). However, the strength of this preference can vary between greenhouses.

Figure 2.
When tested in chrysanthemum greenhouses, yellow sticky cards caught more thrips than blue cards. On average, 60% of thrips were caught on yellow cards, although this preference varied by site. At some sites, over 75% of thrips were caught on yellow, while at other sites it was closer to 50%.

Surveillance operation

Since sticky cards are used for monitoring as well as mass trapping, we also wanted to know how accurately sticky cards reflected the thrips species composition in the crop.

We collected thrips directly from the crop using plant taps and compared the ratio of OT to WFT with what was caught on the cards. Overall, when we looked at the averages throughout the growing season, the cards gave us a pretty accurate picture of thrips species composition in the crop. But, week to week there is a LOT of variation.

Unfortunately, this means that weekly monitoring cards aren’t a reliable tool when it comes to pest management decisions based on how many OT versus WFT you’ve got in your crop (i.e. to spray or not to spray). If you want to get a clear picture of what’s happening in your crop, you’ll need to collect thrips directly from the plants themselves.

Motive, Means & Opportunity

Although outside thrips prevalence plays a role in initial infestation, the differences in what OT and WFT do inside the greenhouse is where the trouble really starts.

You may notice that in late fall when it’s too cold for thrips to fly outside, WFT numbers start to steadily decline in the crop. We found that OT numbers, on the other hand, remain steady. They can even increase in number throughout the winter and early spring. Since there is no source of incoming OT (on cuttings or from the outside), this confirms that OT may be a more cunning adversary than WFT.

There are two possible explanations for why OT populations can increase over the winter/spring. Either 1) OT reproduce much faster than WFT on greenhouse crops, or 2) biocontrol-based IPM programs don’t work as well for OT as they do for WFT in cooler months.

Both thrips species have been well-studied by researchers all over the world, and there is no indication that OT has a faster reproductive rate than WFT. As for the efficacy of biocontrol products, a lot of literature, plus all of our laboratory tests to date (looking at predatory mites, microbial pesticides, and nematodes), indicate that the biocontrol products we currently use for WFT should work as well for OT – or maybe even better (see Figure 3)!

Figure 3.
Number of thrips larvae eaten by predatory mites and thrips adults eaten by Orius in lab trials. This data suggests that current biocontrol products should work for both OT and WFT.

The Mystery Continues…

We now understand who the new thrips villain is and where they come from, but so far we haven’t identified why they are able to evade our tiny police force of predatory mites and other biocontrol agents.

We’ve gathered plenty of evidence in the lab, so the next step will be to examine the scene of the crime. Which is to say we’re going to conduct greenhouse trials to see how OT, WFT and thrips biocontrol agents interact on plants. Maybe this will give us the clues we need to control OT without resorting to pesticides.

Stay tuned for installments of this gripping biocontrol mystery!

Which thrips are which?

Western Flower Thrips
Key features: Larger than the other yellow-coloured thrips you’ll encounter, WFT have bright red ocelli (three spots between their eyes). They also have plenty of long coarse hairs on the top and bottom of their “shoulders” (called the pronotum). WFT body colour ranges from common pale yellow to a very dark brown (called a “dark morph”).

Damage pattern: Widespread, dispersed damage in crop; frequently causes damage to flowers.


Onion Thrips
Key features: Pale grey ocelli (eye spots); coarse hairs occur only on the bottom of the pronotum (none on top); their bodies range in colour from pale yellow to brown. Smaller than WFT.

Damage pattern: Crops damaged in small clusters of plants; heavy damage to the foliage that makes the plant unsellable; less damage to flowers. 

Note: With the right gear (a microscope) anyone can learn how to identify the usual suspects that show up in their greenhouse. For an identification guide designed specifically for growers, go to 



MacIntyre-Allen, J., C.D. Scott-Dupree, J.H. Tolman, C.R. Harris. 2005. Resistance of Thrips tabaci to pyrethroid and organophosphorous insecticides in Ontario, Canada. Pest Management Science 61: 809-815.

Summerfield, Ashley. 2019. Biocontrol thriving in Canadian floriculture greenhouses. Greenhouse Canada, March 26, 2019.

Buitenhuis, R., Lee, W., Summerfield, A., & Smitley, D. (2019). Thrips IPM in floriculture: cutting dips to start clean. IOBC-WPRS Bulletin, 147, 130–135.

Jandricic, S. 2019. Making mass trapping work for you. GrowerTalks, June 1, 2019.

Ashley Summerfield is a senior research technician in biological control at Vineland Research and Innovation Centre and an MSc. candidate at the University of Guelph. Sarah Jandricic, PhD is the greenhouse floriculture IPM specialist at the Ontario Ministry of Agriculture, Food and Rural Affairs. Ashley’s co-advisors on the onion thrips project are Rose Buitenhuis, PhD, (Biocontrol Lab; Vineland Research and Innovation Centre) and Cynthia Scott-Dupree, PhD, (Professor and Bayer Crop Science Chair in Sustainable Pest Management). Questions? Email Ashley at


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