What you NEED TO KNOW about Thrips and Whitefly Control: September Update

thrips and whiteflyIt’s that time of year again where two of our biggest crops cross over: fall pot mums and poinsettia. This means growers have to simultaneously keep an eye on the two biggest pests in the industry: thrips (usually western flower thrips) and Bemisia whitefly.

Here’s how things are shaping up with these pests and where they might be going.

Thrips, Where Art Thou?

Due to OMAFRA/UofG’s ongoing project looking at onion thrips (Thrips tabaci) as a pest of flower crops, we’ve been monitoring thrips species levels both inside and outside several local mum operations in Southwestern Ontario for the last few months.

card set-up Schenck (3)
Blue and yellow monitoring cards set up outside a local greenhouse operation to monitor for various thrips species.  Photo courtesy of A. Summerfield, M.Sc. Candidate, UofG.

The good news is that all thrips levels (both onion and WFT) seem unusually low right now and don’t show any sign of going up again as temperatures cool.  This is likely a result of a combination of things.

Low outside thrips pressure is likely due to a relatively wet spring/summer: thrips have lots of weeds to eat this year (ask any allergy sufferer) and don’t need to seek out your greenhouse crops.  Low inside thrips pressure points to better grower awareness and management of thrips on cuttings.  This includes the wide-spread use of cutting dips, thanks to efforts by VRIC, and a focus on biocontrol in propagation/early sticking (check out this Webinar if you need a refresher).

Taking Advantage of a Respite from Thrips:

Thrips levels have been so low over the last month or so that some growers have been able to decrease the amount they are spending on biocontrol.  For example, one large local grower that produces both indoor garden and pot mums is managing thrips  with just two weekly sprays of Beauveria-containing pesticides (e.g. BotaniGard or Bioceres) and good coverage with large sticky cards.

trap cards hung from heating pipes_move up and down
Sticky cards should be placed at a MINIMUM density of 8 cards/1000 sqft (or 120 cards per 15,000 sqft growing area).  OMAFRA studies have shown that cards are equally effective when placed directly above the crop (pictured) or along post rows. Sticky tape along post rows (pictured) can also be used, as long as it is changed regularly (every 8-12 weeks).

This doesn’t necessarily mean you should abandon any and all plans for adding mite sachets to your usual thrips IPM program; they generally form the backbone of any solid bio program and provide insurance for sudden pest fluctuations.  But it DOES mean that you may be able to find some way to trim program costs this Fall.  Ultimately, the greenhouse is a dynamic system, and pest pressure from a variety of sources fluctuates across time.   Although following a “tried and true” recipe can be reassuring, it CAN mean missing out on opportunities to lower production costs.  Constant re-evaluation is the key to staying competitive.

So, this Fall may be the perfect time to experiment with what your mu program looks like under low thrips-pressure conditions.  This could include changes such as removing nematodes from your program, using Cucumeris instead of Swirski, using regular instead of “long-duration” mite sachets, or focusing bios on more sensitive varieties (e.g. dark coloured moms that show petal damage more readily).  Talk with your bio company rep, and make sure to keep up with your weekly monitoring to ensure any changes you make to your program are still working as time goes on.

What’s Up with Whitefly?

You’ll notice that this week is the dreaded “whitefly tipping point” I’ve referred to  in several blog posts and articles (see the latest issue of Greenhouse Canada Magazine),  when growers have to decide whether to continue their biocontrol programs in poinsettia or chance chemicals.  Switching to pesticides later than this may mean that you aren’t able to fully get on top of the whitefly population, and can lead to credits.  But abandoning biocontrol and using chemicals also runs the risk of running up against pesticide resistance.  It’s always a gamble…

So which way does it look like it’s tipping this Fall?

WF pupae on poinsettia.jpg
An 8-inch poinsettia plant with new growth infested with whitefly pupae/nymphs (white spots on underside of leaf between the leaf veins).  Picture taken Aug. 28, 2019.

Unfortunately, it’s hard to say.  Although the season started strong, with good quality cuttings and few rooting/disease problems, the whitefly numbers are creeping up.  As usual, colours (whites, pinks) and earlier cuttings (8-inch and up) seem to be affected more.  But the good news is that evidence of parasitization and host feeding is being seen in the crop, even in greenhouses experimenting with Encarisa-only releases (as opposed to the traditional Eretmocerus-Encarsia mix).

Ultimately, your decision will have to be based on several factors particular to your farm, including:

  • Is your main red variety heavily affected?  In all areas or just one?
  • Are you seeing evidence of your biocontrol program working? (Again, in all areas?)
  • Are you seeing whitefly colonization on old growth (original cutting leaves) only, or has it spread to new growth?
  • What is the pressure like (densely colonized leaves? Or 1 or 2 adults or pupae here and there)?

Ideally, to stay the course for biocontrol, we’d like to see evidence that a) the parsitoids are working, b) generally low pressure/density on new growth, c) less pressure in your main crop than in the colours, and d) less than 20% of the total crop infested (see here for more details on how to determine this).

If 3 out of 4 of these are NOT true for your crop, then it may be time to seriously consider pesticide application.  Check out this post for which chemicals are most likely to be (or not be) effective.

That’s all the news we have so far for mums/poinsettia!  If you are seeing something different out there, or have tried some of the new chemistries registered for whitefly control, please feel free to add a comment!


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