Which Thrips are in Your Flower Crops (Part II): How to ID and Control Onion Thrips

Thrips tabaci, or Onion thrips. Photo courtesy of Thrips-ID.com.

If you were at the Canadian Greenhouse Conference (or are regularly reading this blog!) you’d know we’ve recently identified Onion thrips as a pest of floriculture crops in Ontario (see this post).

Outside of Ontario?  Well, this still may apply to you, as a recent study in France also indicated that up to 47% of pest thrips in floriculture greenhouses were Onion thrips.  So, this issue could be wide-spread.

My last post covered the extent of the problem in Ontario’s industry.  This post will help you identify if YOU are dealing with Onion thrips (OT) along with Western flower thrips (WFT), and what to do about it.


What to Look for:

One thing we’ve realized in Ontario is that the presence of Onion thrips should be suspected if the following cluster of symptoms are seen (at least on potted plants):

  • Intensive foliar damage
  • Damage often clustered on a small group of plants, with nearby plants seemingly unaffected
  • Appears despite good biocontrol coverage
  • Can appear even if thrips levels on monitoring cards are low

Have all or most of the things on this checklist?  Then move on to identifying your thrips species to be sure what pests you are dealing with.

Onion thrips (OT) damage often appears as intense foliar damage on a small cluster of plants (right), with seemingly unaffected plants nearby (left).  This is possibly due to OT “colonies” forming instead of migrating throughout the greenhouse.


How to BE SURE:

Confirming your suspected Onion thrips problem is a must if you want to avoid unneccesary sprays.  Here’s what you need to ID your thrips:

  • Pan or shallow Tupperware container for doing plant taps, AND/OR yellow trap cards
  • Aspirator for collecting thrips (optional; see below)
  • A microscope with at least 40X magnification
  • A small dish to place thrips in
  • A fine paintbrush or probe to manipulate thrips under the microscope
  • This simplified thrips key made just for Ontario growers: Key-to-important-thrips-pests-of-Ontario-greenhouses-2018

For problem plants, I currently recommend doing plant taps of 3-5 different plants in the same white tray, then collecting at least 30 ADULT thrips with an aspirator (randomly selected).  Try to get ones that seem both big and small, so you don’t bias your results (OT are slightly smaller than WFT). You can then put the aspirator directly in the freezer to kill the thrips (about 45 min should do it) before identification.  If you don’t have an aspirator, then plants can be tapped directly above a wide, shallow container filled with about a centimeter of soapy water to kill the thrips.  Give them a few minutes to die, and then fish out the thrips with a fine paint brush or eyedropper for identification.

Diagram of a mouth aspirator, a handy tool for collecting insects easily.   Image courtesy of Handsontheland.com

To get an idea of the entire thrips population in your greenhouse (e.g. if you’ve been struggling with a thrips biocontrol program), you should collect at around 50 adult thrips per compartment from random areas/varieties.  This way, you can see if the ratios are different between areas of your greenhouse.  Alternatively, it is possible to ID thrips from sticky cards placed under the microscope (place in a clear ziplock bag or cover in Saran wrap so your scope doesn’t get sticky!), but it requires a bit more practice, in my opinion.

Control Options:

We are still figuring this out.  But right now, we know that Success (spinosad), sprayed for WFT,  should effectively kill concurrent populations of OT (though effects on WFT are likely to be minimal due to resistance). However, chemicals should ONLY be considered when absolutely necessary (e.g. when damage appears, or if a large number of OT are seen on plants or cards), since even “softer” chemicals like Success can affect the efficacy of mite-based biocontrol programs for WFT and spider mites.  The efficacy of other chemicals for OT are currently unclear.

onion vs. WFT control
A mnemonic by S. Jandricic to help growers remember which control options are effective for commonly-found thrips species in Ontario greenhouses.  Western flower thrips are generally not controlled by chemicals, while Onion thrips can be susceptible.

Another option that works is mass trapping.  As in WFT, yellow cards are preferred over blue by OT, according to a quick-and-dirty study I recently completed in an Ontario greenhouse with a high population (>60%) of OT.   The suggested rate is “as many as you can physically put up” (not joking here), with a minimum rate of 8 cards/1000 sq. ft (or roughly 8 cards/100 sq. m).  For more information on mass trapping, see this post

yellow sticky cards
Yellow sticky cards have been shown to attract Onion thrips, as well as Western flower thrips, in trials by S. Jandricic (OMAFRA).  Cards are just as effect when placed down post rows as they are within the crop itself.

What We Are Doing About It:

Given the lack of information we have at the moment, OMAFRA, the Vineland Research and Innovation Centre, and the University of Guelph are joining forces to hopefully support a M.Sc. student to confirm which bios and chemicals work (and don’t work) against OT.  In the meantime, I’m looking at whether OT reside in greenhouses over the winter, or are just a summer fly-in pest.  Flowers Canada is currently pursuing label expansions to have other thrips species besides WFT put on Canadian pesticide labels.

More Resources:

If you missed my presentation at CGC on the state of Ontario’s floriculture industry with regards to thrips, here’s a PDF copy: CGC _Thrips Workshop_State of Industry_FINAL 2

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