Leafminer control in Ontario’s greenhouse crops -what’s working?

Fig. 1. Leaf mines on gerbera leaves.

As much as we all hate thrips, there are, frankly, worse problems to have.  And it’s name is Leafminer.  These flies cause just about the ugliest damage we see in floriculture (Fig. 1), and they have incredible pesticide-resistance capabilities. Outbreaks seem to go in cycles, and I’ve had quite a few gerbera and mum crops come across my desk with leafminer this past 2 weeks.

This post covers chemical options (BawHawHawHa!!! Oh… Sorry… I’ll get myself under control now) and non-chemical options for leafminer, as well as how their control fits into the big picture in greenhouse IPM programs.  

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Ramping up thrips biocontrol BEFORE they get out of control!

WFT on hibiscus_Caitlin McDonald_1
Western flower thrips adult on an open Mandevilla flower.  Photo credit: Caitlin MacDonald, USEL student.

Now that the warm weather is finally upon us, it’s time to start worrying about thrips control.

What we’ve learned over the years is that pesticides just don’t cut it – the only reliable chemical for western flower thrips in Ontario is DDVP, which requires constant application.  This means biological control is your best bet.  Here’s a summary of the most effective tools, tricks, and timing, to ensure your biocontrol dollars are well spent.

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Can you safely mix nematodes and pesticides?

Entomopathogenic nematodes – used to control fungus gnats, shoreflies and thrips – are often a “gateway bio” into biocontrol use in greenhouses.  This is because not only are they effective and easy to use, but they’re generally compatible with insecticide use.   Readily applied with regular spray equipment or through drip lines, nematodes can even be tanked mixed with pesticides to save on labour costs.

In this post, I’ll share some of my research at NC State, looking at which commonly used pesticides in Canadian and U.S. greenhouses are safe to use with nematodes.

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Spray technique affects nematode rates

Spraying nematodes in your greenhouse for fungus gnats, shoreflies, or thrips?  Then check out this article published in the January 2015 issue of Hort Matters (OMAFRA’s Horticultural newsletter):


Researchers realized that nematodes can “stick” to the inside of spray tanks, reducing the number of nematodes coming out of the nozzle as you spray.  This is unlikely to be a problem using backpack sprayers (since the volume is so small), but could be an issue if you use a towed sprayer on wheels.

What’s the solution? Agitation, agitation, agitation.

To keep your nematode numbers consistent throughout the application, do your best to keep nematodes in suspension.  And, if you have any concerns about your application technique, it’s easy enough to assess if you have a microscope (or have a friendly neighbourhood OMAFRA agent with one).

Nematodes (S. feltiae) preparing to enter an adult western flower thrips.
Nematodes (S. feltiae) preparing to enter an adult western flower thrips. Photo by R. Buitenhuis (Vineland).

Nematodes are around 0.5 mm long, and can be easily seen using the low magnification setting on a scope.  A black background makes them easier to see. Live nematodes are usually serpentine or “J” shaped, and often wiggle slightly.  Dead nematodes are stick straight.

By counting the number of live nematodes in a small volume of your original spray solution (1 mL should do it), you can compare this to the number of nematodes in the same volume (1mL) from the nozzle dribble at the end of your application.  Seeing a huge reduction here?  Then you may have a problem.

To ensure nematode health, also follow these other tips:

  • Air temperature should be less than 30 C at application time
  • Apply during low light levels since nematodes are UV sensitive
  • Nematodes can be stored in a refrigerator (4C) but should be used within 4 weeks of receipt
  • Do not apply nematodes though sprayers that exceed 300 psi
  • Use a nozzle aperture of more than 0.5 mm