Mass Trapping For Summer Thrips, Part 1: Why Trap and When to Start (Answer: This Week!)

Sometimes your thrips biocontrol program is working great. Other times, it seems to get completely overwhelmed.  Thrips can get out of control in Summer, when fall crop production is ramping up. This is where mass trapping can really help.

This post will cover where thrips come from, which species we find on cards, and when the best time is to add mass trapping to your thrips IPM program.

And stay tuned for Part 2, which will cover ways to optimize your mass trapping!

1. Where Do Thrips Come From?
2. Outside Pressure = Inside Problems?
3. Preventing Onion Thrips Infestations
4. What About Western Flower Thrips?
5. When to Start Trapping
6. Take Home Messages

Where Do Thrips Come From?

Figure 1. Thrips numbers and species were monitored directly outside the greenhouse of both side- and top-venting facilities.

Although we know that thrips come in on plant material, including cuttings and whole plants, this is only half of the story. (For more info on thrips on cuttings, and why we do cutting dips in Ontario, check out this post). The OTHER major infestation route we sometime forget about, is thrips flying in (or being sucked in) from outside.

This applies to species that are either endemic to Ontario or are now naturalized (i.e. they’ve adapted to our harsh winters). Species that can invade from outside include onion thrips, chrysanthemum thrips, and western flower thrips, as it’s had over 30 years to adapt to our outdoor conditions since it first arrived from the west coast of the U.S.

Outside Pressure = Inside Problems?

To see how well thrips pressure outside the greenhouse correlated to inside pressure, Ashley Summerfield (Vineland Senior Research Technician) conducted trials as part of her M.Sc. in 2019. She put sticky cards outside and inside 3 commercial ornamental greenhouses in Niagara, and tracked them over the summer, from the end of May to the beginning of October.

She did indeed find a link between increased thrips pressure outside leading to more thrips coming into the study greenhouses. But, to our surprise, the main thrips we found at all 3 greenhouses was eastern flower thrips (Frankliniella tritici – EFT). These comprised 70% of the thrips caught, and we saw huge peaks of this species outside the greenhouse in mid-July (see Figure 2). At the peak, we were catching an average of 40 EFT per card per week inside the greenhouse!

Figure 2. Average number of Eastern flower thrips (Frankliniella tritici) caught on yellow sticky cards outside and inside 3 commercial greenhouses in Niagara.

Although eastern flower thrips (EFT) is a close cousin of western flower thrips (WFT), it is not known to damage ornamental crops (although it is a big pest of strawberries). We confirmed this by sampling for thrips in the crop itself, but only found a few EFT in any of our samples.

But does that mean mass trapping cards aren’t catching important thrips species then? Absolutely not.

It only means that a large volume of EFT can mask influxes of other thrips. It’s just something to be aware of, especially if you are relying strictly on monitoring cards in the summer for your pest management decisions.

Knowing all the thrips on your cards may not be pest thrips can save you from unnecessary panic. This also tells us the BEST methods of MONITORING thrips pressure in your greenhouse are plant taps and walking the crop for damage, not sticky card monitoring, per se. This is mainly because differentiating EFT from WFT with just a hand lens, or even a dissecting microscope, is very difficult, and would be too time consuming to be useful.

Preventing Onion Thrips Infestations

Along with EFT, the data also showed that onion thrips (Thrips tabaci – OT) populations did increased on cards inside the greenhouse somewhat when populations were high outside. We know from previous thrips surveys in Ontario greenhouses that OT is an important pest of chrysanthemums, gerbera, osteospermum, herbs, cannabis, and other greenhouse crops in Ontario.

Given that we never found any OT on cuttings in our survey of incoming chrysanthemum material, this study confirmed that fly-ins from outside are the primary route of entry for OT in Ontario greenhouses.

Figure 3. Average number of Onion thrips (Thrips tabaci) caught on yellow sticky cards outside and inside 3 commercial greenhouses from May to October.

We also know from previous research that OT can become established in greenhouses over winter months. As this thrips is harder to control with biological control than WFT, and pesticides can disrupt your natural enemy programs for WFT, it’s important to head OT infestations off at the pass with mass trapping.

Although the numbers on this graph might seem low (max. 5 OT caught per week), remember that is PER CARD (10 x 20 cm). If we multiply this out by the standard number of cards (120 large sized cards (40x20cm) in a 15,000 sq ft greenhouse), this would mean we removed up to 1200 OT per week in high pressure weeks, or 12,000 OT removed over the life of a 10 week crop. As each female OT can have up to 80 offspring in her lifetime, this ounce of prevention is certainly worth its pound of cure!

What About Western Flower Thrips?

As always, WFT like to keep things interesting for us in floriculture. Ashley DID catch WFT on cards outside and inside the study greenhouses. And, she DID catch WFT at higher rates than OT. But, there wasn’t really a relationship between pressure outside and pressure inside the greenhouses (Figure 4).

Why did this happen? Well, for a few reasons.

All of these greenhouses were chrysanthemum greenhouses, and we know that for this crop, WFT arrive on cuttings, at rates between 0.03-0.7 thrips per cutting (depending on time of year and variety). So, pressure is coming from outside (as evidenced by the outdoor cards), but pressure is ALSO coming from the crop itself, making the relationship between outside and inside less clear.

Figure 4. Average number of western flower thrips (Frankliniella occidentalis) caught on yellow sticky cards outside and inside 3 commercial greenhouses.

Add to this the fact that all our current biocontrol programs for thrips were developed specifically for WFT. This means that even if more WFT did enter the greenhouse, biocontrol programs already in place inside the greenhouses may have offset this somewhat.

The important thing to note is that WFT can come from BOTH inside and outside the greenhouse, meaning that mass trapping for this pest should be considered year-round.

When to Start Trapping

This study above indicated that the biggest influxes of all three thrips species happened in July, but as we know, thrips management is best when it’s PREVENTATIVE. So:

• Put up your cards in the last week of May or early June. This will allow you to catch the start of fly-in season. It also will help trap the WFT proliferating in your crop due to warmer temperatures. An average rule of thumb is to use 8 large cards / 1000 sq ft. to start.
• Monitor closely starting in June: Although card counts can be misleading if there’s a lot of EFT, you can pair card counts with damage assessments and plant taps. This can allow you to pivot your pest control strategy, as illustrated in this grower case study, if a large fly-in of WFT or OT happens. (And don’t forget about chrysanthemum thrips, another fun local thrips pest that can fly in and cause destruction!)
• Maximize mass trapping in July & August: In Ashley’s research, all 3 thrips species peaked outside the greenhouse in late July. So you may want to increase your number of cards at the beginning of July (or at least make sure you have some fresh ones up).
• If you have a side-venting greenhouse, concentrate mass trapping cards or tape 2-13 feet away from vent openings, as previous research by OMAFRA has shown that you don’t get much bang-for-your-buck after this distance.

Take Home Messages

To me, any thrips caught on a card is 1 less to worry about in your greenhouse, so mass trapping is a no-brainer. Although there haven’t been any published economic analyses done in commercial floriculture greenhouses, a trial in greenhouse strawberries in the UK showed that mass trapping reduced thrips numbers in the crop by 76% and feeding damage by 63%. This reduced shrink and saved the grower over $2K per ha.

Mass trapping also reduces the incidence of other greenhouse pests, such as fungus gnats. In fact, a grower I was working with that decided to stop mass trapping ended up with a sudden resurgence of gnats. In the end, they decided mass trapping was a better strategy for them then weekly nematode applications, and put the cards back up!

Lastly, mass trapping can take some of the pressure off your biocontrol program. A sudden fly-in from outside can overwhelm mite sachets and lead to needing to put up tape or cards anyways, as in this grower case study. So having the cards up BEFORE there’s an issue acts as a form of insurance. I’ve also know growers that were able to reduce their pest management costs by using just mass trapping and weekly BotaniGard sprays for thrips in certain crops, instead of a mite-based program. So it’s worth experimenting with on your own farm. So get out there and get sticky!