Poinsettias: To Spray or Not to Spray, that is the Question

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T’is poinsettia sticking season once again, and the question always comes down to the same thing: do I use chemicals to control Bemisia whitefly and hope it works this year? Or do I switch to biological control?  Here we show some head to head comparisons that can help you decide.

Pesticides Vs. Bios: Efficacy

Because Bemisia whiteflies originate on cuttings, and all commercial producers currently rely on pesticide programs for management, your whitefly will likely have been exposed to multiple applications of several groups of pesticides before they enter your greenhouse.

WF control strategies

Fig. 1. in 2017, chemicals were as effective as biological control using parastioids (with or without a predator) in a commercial greenhouse test (1 bay per treatment). Whitefly present on less than 15% of the crop is acceptable to most commercial buyers and is considered a successful program.  Predatory mites by themselves were not effective.

Depending on the suit of chemicals chosen at the producer end, this means that in SOME years (e.g. 2017, Fig. 1), we will still have effective chemicals to choose from in Canada.  But, in other years (e.g. 2018, Fig. 2) we won’t have ANY.

This is because overexposure to chemicals can drive Bemisia populations to be primarily composed of  the “Q” biotype.

Currently,  the Q biotype looks to be resistant to all whitefly chemicals registered in Canada. This is based on grower trials (below). This inlcudes Altus (flupyradifurone), Beleaf (flonicamid), Distance (pyriproxifen), Dynomite (pyridaben) and Kontos (spirotetremat). Read more about whitefly biotypes in this article.

bios vs pesticides for wf

Figure 2.  Multiple chemicals were applied against whitefly in 2018, and all failed. All 3 biocontrol programs tests (see below) were effective.  Tests were conducted in the same commercial greenhouse as 2017.

Overall, several years of grower testing demonstrates that biocontrol is the only CONSISTENT approach.

Pesticides Vs. Bios: Economics

One thing growers are rightly concerned about when it comes to switching to biocontrol from pesticides is cost.  Tweaked for the individual farm and pest pressure, biocontrol programs, depending on company and products, can range from a nominal 3 cents (CDN$) to around 12 cents per pot.  It takes a bit of playing  to determine what works for your particular farm (and poinsettia varieties grown) for the lowest costs, which is why it can be good to work with an independent consultant.

BC of Bemisia

Figure 2. Multiple biocontrol agents are often needed to control whitefly, which can drive up costs.

While it’s true that chemical control programs for whitefly in poinsettia are cheaper – somewhere around 2 cents/pot in commercial trials in Canada – this assumes the pesticides will work.

In years where whitefly populations coming in on cuttings have developed significant resistance, pesticide costs can skyrocket as multiple (ineffective) sprays are made (see Figure 2). The resulting pesticide costs and crop losses ultimately end up being much higher than a biocontrol program would have been.

And, as one grower put it, “costs are irrelevant if you can’t control the pest”.

Which Bios to Use:

A successful biocontrol program relies on knowing which bios to use, and when to use them. Canadian growers are finding success using one of the three following programs. One thing all three have in common is the use of multiple natural enemies, to target multiple life stages of sweet potato whitefly (Figure 3).

EncarsiaVsEretParasBemisia2

Fig. 4. Bemisia whitefly parasitized by Encarsia (left) and Eretmocerus (right).

  • Option 1: Using high rates (ca. 9/square meter) of a mix of Encarsia formosa and Eretmocerus eremicus (see Fig. 4). Contrary to some reports, BOTH wasps parasitize AND host feed on Bemisia whitefly nymphs (see refs here , here and here for Encarsia feeding on
  • / parastizing Bemisia).  As an added bonus, Encarsia will also host feed on pupae (see this reference).  Encarsia and Eretmocerus come in handy “combo” emergence cards from many suppliers, like Koppert, Biobest and BioLine. Cards can go in as early as on the misting bench – just cover the cards with a Styrofoam cup to prevent them from getting soggy and moldy.
  • Option 2: Lower rates of Encarsia plus the predatory beetle Delphastus. Sometimes referred to as the “West Coast Program”, this method was pioneered by British Columbia’s Applied Bionomics, and relies on “fresh” Encarsia that are purported to have better searching abilityEncarsia (ca. 0.25-2/ square meter) are released weekly for 9 weeks. Delphastus are added to help take care of any “hot spots” that might develop.  Given its sensitivity to pesticides, Delphastus shouldn’t be applied until 4 weeks after cuttings are received (when pesticide residues from the propagator end have dissipated).
  • Option 3: The “kitchen sink” approach. This includes any of the parasitoids above, plus Delphastus AND mites like Swirskii or Limonicus to attack whitefly eggs. Limonicus or Swirskii can be applied on the misting bench (broadcast) and throughout the crop cycle, and can be given supplemental food such as pollen or Ephestia eggs to help their populations build up.  On their own, mites aren’t enough to control whitefly (see Fig. 1).

Check back soon for posts about how to monitor for whitefly and parasitism in your poinsettia crop, and knowing what to do if things reach critical levels.

 

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