Where Do Most of Your Thrips Come From? The Answer May Surprise You.

Are your thrips numbers on the rise?

Sometimes your thrips biocontrol program is working great. Other times, it seems to get completely overwhelmed.  Thrips often get out of control in Summer, when fall crop production is ramping up. But why is this?

Several factors can contribute to rising thrips numbers, but the major reason involves HOW and WHEN thrips like to invade your greenhouse. Read on to learn about this, and what you can do to stay on top of thrips this summer!

Where Do Thrips Come From?

From Incoming Plant Material:

OMAFRA summer student Elizabeth Huber-Kidby washing mum cuttings to find thrips.

When western flower thrips (WFT) first arrived on the scene in Canada in the 80s, they were NOT adapted to our Canadian winters. This meant they didn’t survive outside when it was cold, and that the ONLY route of infestation was via plant material.

This is still a common occurrence. A study done by the Buitenhuis Lab at the Vineland Research and Innovation Centre showed that up to 75 thrips can come in per 100 cuttings of chrysanthemum, and can also arrive on spring cuttings.  

A joint survey by OMAFRA and VRIC of 4000 mum cuttings in summer 2019 confirmed that the thrips species coming in on mum cuttings are almost entirely WFT.

Tropical thrips species such as Echinothrips, chili thrips, and Thrips parvispinus also arrive via tropical plant cuttings and rooted plant material brought to Canada for finishing.

From Outside, Via Vents, Doors and Other Openings:

After many years of Canadian residency, WFT are now adapted to our climate, and can overwinter outside. So, as soon as days are warm enough (usually mid-May), WFT populations build up on outdoor crops and weeds, and can start to fly in. This is especially true in hot, dry summers, when many of their normal weed hosts may be dying. They can also simply be blown in on wind currents, as thrips are notoriously terrible flyers.

Other endemic and naturalize thrips species, such as onion thrips, can also enter this way.

Sudden, drastic thrips damage or high numbers of adults on cards or plant taps can indicate a large number of thrips have blown or flown into your greenhouse from outside.

As a lot of focus has been given to thrips hitchhiking on cuttings over the past few years (and the importance of controlling them early in your production cycle), it was also important that we looked at the roll of thrips fly-ins in thrips management.

How Exactly Does Outdoor Thrips Pressure Affect My Crops?

In 2019, Ashley Summerfield (M.Sc. student at the University of Guelph) wanted to see how thrips pressure outside greenhouses affected what was happening inside. So, she set up monitoring cards around the perimeter of 3 commercial facilities, as well as at various points inside, and monitored them biweekly from May to November.

You can see in the graph below that the number of WFT caught on sticky cards inside closely mirrored the WFT pressure outside the greenhouse in any week. How can we be sure that the thrips in the greenhouse didn’t just come from the crop, though? Well, a mathematical model indicated that the number of thrips outside the greenhouse explained roughly 83% of the thrips inside the greenhouse over the summer.

Average number of western flower thrips caught on cards placed outside (green line) and inside (blue solid line) commercial greenhouses. Thrips pressure inside the greenhouse generally mirrored what was happening outside. Although the above graph only shows WFT, the same trend was seen for onion thrips (OT) as well.

This indicates that between late June and early September, your MAIN source of WFT infestation in your crop is actually from outside, rather than from infested cuttings.

Entering through greenhouse openings is also the primary route of entry for onion thrips (Thrips tabaci), which have become a serious problem in several ornamentals in recent years.

Once temperatures are below the flight temperature threshold for thrips (around 15°C or 60°F), this pressure is reduced. However, any thrips that successfully make it in before that can become established in greenhouses over the winter months (a common source of ongoing onion thrips infestations in ornamentals). So, it’s important to head them off at the pass, if you can.

What You Can Do To Reduce Thrips Populations in Summer:

It’s recommended to use at least 4 large mass trapping cards per bench, or 120 cards in a 15,000 sq ft zone.
  1. Monitor closely starting in June: pay close attention to card counts in starting in early June and watch for sudden increase in thrips numbers through to late September.  Also look for higher than normal damage. This can allow you to pivot your pest control strategy, as illustrated in this grower case study, if a large fly-in event happens.
  2. Maximize mass trapping in July & August: In Ashley’s research, both WFT and onion thrips populations peaked outside AND inside the greenhouse in late July. Putting up mass traps starting in mid-June will prevent a large number of thrips from actually reaching your crop. Many growers go with at least 8 large cards per 1000 sq ft, but, honestly, you can never have TOO many mass trapping cards 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.
  3. Continue Dipping Your Cuttings: Dipping chrysanthemum cuttings and other weekly crops in reduced-risk pesticides in the summer is still a good idea, as this helps reduce the number of pesticide-resistant WFT that come from propagation facilities. This way, if you do need to take action with pesticides on certain crops, they might just have a chance of working.
  4. Consider screening your vents: although this is a higher-investment solution, it’s one that hasn’t been considered seriously enough by the ornamental industry. Although screens can reduce airflow, which is always a concern, there are currently no scientific articles showing this increases the incidence of plant diseases such as powdery mildew. However, there are many articles showing how screening reduces pests from outside (more on that in a future blog post!).

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