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Controlling Whitefly on Poinsettia

Unfortunately, poinsettia and sweet potato whitefly (Bemisia tabaci) go together like cats and fleas; if you have one, you’ll have the other. By this time (June) you should already be considering your whitefly management strategy. Here, we make a case for biocontrol as both a viable and economical way to control whitefly based on leading research supported by the American Floral Endowment (AFE) and many years of grower experience in Canada.

The Problem with Pesticides

Management of Bemisia whiteflies with pesticides can be difficult, if not impossible, in some years. This is because whiteflies that originate on cuttings (and those that fly in from field crops, as in the Southern US) have likely been exposed to multiple applications of several groups of pesticides before they even enter the greenhouse.

Overexposure to pesticides is a sure path to the development of resistance in whiteflies, especially in the whitefly species referred to as the “Q” type, which is common in poinsettia. For some good reading on B and Q type Bemisia (now officially called MEAM1 and MED, respectively) and the importance of rotating pesticides if you choose to manage whitefly chemically, see the article by J.C. Chong in the June 2018 edition of this newsletter.

However, pesticides aren’t the only option. An alternate management strategy we know works well in Canada for whitefly control is the use of preventative methods (i.e., cutting dips, outlined below) coupled with biocontrol. Compared head-to-head, this strategy has been shown to be just as effective as pesticides in years where there is little pesticide resistance in whitefly populations. 

In years where pesticide resistance is high, though, biocontrol is the only reliable method. Relying primarily on biocontrol to manage Bemisia in poinsettia has helped Canadian growers avoid anxiety and crop losses when chemicals start to fail.

Cutting dips

One of the main entry routes of whiteflies is on propagative material (cuttings). As we know, some years are worse than others, depending on the success of the whitefly control program at the propagator. The first step to a successful whitefly biocontrol program is reducing the number of whiteflies at the beginning of the crop. Otherwise, high whitefly numbers at the start of a production cycle may overwhelm the control capacity of biocontrol agents or significantly increase the cost of your biocontrol program as you add new natural enemies to control increasing numbers of whitefly “hot spots.”

Originally pioneered in Ontario, Canada, research has shown you can effectively reduce the numbers of whitefly at the start of your poinsettia program by dipping your unrooted cuttings in “soft” but effective insecticidal products upon receipt. These include insecticidal soaps, oils, and/or biopesticides. Not only does dipping provide full coverage, ensuring a high number of whitefly nymphs and pupae are killed, but the products used leave minimal residues and are compatible with biocontrol agents applied after the cuttings are stuck.

Research at the Vineland Research and Innovation Centre has shown that dipping unrooted poinsettia cuttings lowered Bemisia whitefly numbers on the cuttings by an impressive 70 percent. Best results were obtained against whiteflies using either of two dip recipes: 0.1% (v/v) SuffOil-X or a combination of 0.5% (v/v) Kopa insecticidal soap mixed with 1.25 g/L BotaniGard 22WP. Further greenhouse trials (Figure 1) demonstrated that dipping cuttings delayed whitefly population development on the plants, effectively setting back the clock by several weeks. This significantly improved the effectiveness of whitefly biocontrol programs thereafter, as it gave the biocontrol agents more time to start working.

The strategy of dipping cuttings to reduce incoming pest pressure in floriculture crops has now been adopted by 73 percent of commercial Canadian floriculture growers that grow from cuttings and is considered a critical tool when fighting difficult to control pets.

A few “Tips for Dips”:

Figure 1. Average number of sweet potato whiteflies per poinsettia plant. In the dip treatment (solid yellow line), cuttings were dipped in insecticidal soap + BotaniGard 22WP. In the parasitoid treatment (blue line), parasitoids (Eretmocerus eremicus) were released weekly. The final and best treatment combined dips and parasitoids (dashed yellow line).

Choosing Biocontrol Options in Production:

After dipping, the next step in a successful biocontrol program is knowing which bios to use and when to use them. Unfortunately, there’s still no “one-size-fits-all” recipe for biocontrol of whitefly on poinsettia. However, many growers find success using one of the programs outlined below throughout production. One thing all three of these programs have in common is the use of multiple natural enemies to target multiple life stages of whitefly.

Figure 2. A successful biocontrol program targets multiple life stages of the pest. For example, Delphastus or mite predators mainly target whitefly eggs. The addition of parasitoids greatly improves the control of whitefly nymphs and pupae through parasitism and/or host feeding.

Effective Bemisa Whitefly Biocontrol Programs:

Option 1:

The “standard” program uses high rates of both Encarsia formosa and Eretmocerus eremicus. While many growers think Encarsia only attack greenhouse whitefly, in fact, both wasps parasitize Bemisia whitefly nymphs. In Ontario, we’ve also seen evidence of heavy host feeding on Bemisia pupae by Encarsia – this, in fact, may be the main way they contribute to Bemisia biocontrol. These wasps 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 molding. Other growers choose to put out parasitoid cards at the time of potting up.

Option 2:

Sometimes referred to as the “West Coast Program,” this method was pioneered by British Columbia’s natural enemy supplier Applied Bionomics. The backbone of this program relies on “fresh” Encarsia (i.e., not chilled for transport). Scientific evidence shows that un-chilled wasps have better searching ability, flight capacity, and other “fitness” metrics compared to wasps that have been chilled for storage for any length of time. Because of this, this program can get away with lower rates of Encarsia than Option 1, significantly reducing costs. However, it also recommends adding in the predatory beetle Delphastus, which can bring the cost back up. Delphastus are predominantly added near the beginning of the program to help eat whitefly eggs early in the season, but additional numbers can be added later to help take care of any “hot spots” that might develop. Given its sensitivity to pesticides, Delphastus shouldn’t be applied until four weeks after poinsettia cuttings are received (when pesticide residues from the propagator end have dissipated).

Option 3:

What we like to call the “kitchen sink” approach. This program includes any of the parasitoids above, Delphastus, plus the use of predatory mites like Swirskii or LimonicusThe mites can be applied on the misting bench (broadcast) and used throughout the crop cycle by giving them supplemental food such as pollen or Ephestia moth eggs to help their populations build up. Although this option tends to be more expensive than the other programs (simply because of the number of natural enemies used), several growers in Ontario swear by this approach.

Economics

One thing growers are rightly concerned about when it comes to switching to biocontrol from chemical control is cost. Biocontrol programs are tweaked for the individual farm, and yearly pest pressure can range from a nominal 3 cents (CDN$) per pot to around 12+ cents (which can start to cut into profits).

While it’s true that chemical control programs for whitefly in poinsettia can be cheap – somewhere around 2 cents/pot in commercial trials in Canada – this assumes that they work. In years where whitefly populations coming in on cuttings have already developed significant pesticide resistance, insecticide costs can skyrocket as multiple (ineffective) sprays are made.

You can see an example of where this happened on an actual farm in Ontario in Figure 3. This data was collected as part of a joint research project between the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), a consulting company, and a large commercial greenhouse. In 2018, during a high whitefly-pressure year, the resulting costs from excess sprays and crop losses ultimately ended up being much higher in the “pesticide only” compartments (containing around 10,000 poinsettia plants each) than in the biocontrol compartments testing three different types of biocontrol programs. Had this grower chosen to use an entirely chemical-based program for whitefly control throughout their farm in 2018, the results could have been disastrous.

And, as this grower put it, “cost per pot of a particular pesticide is irrelevant if you can’t control the pest .”Since we have seen over and over in Ontario that bios are effective even in years when pesticide resistance is a major issue, biocontrol is simply your safer bet when it comes to your yearly poinsettia program.

Figure 3. In a year where heavy pesticide resistance was encountered, only the biocontrol control programs prevented crop losses from whitefly in a commercial greenhouse test. Low numbers of whitefly present on less than 15% of the crop is acceptable to most commercial buyers (both Canadian and U.S. customers) and is considered a successful program. Here, all biocontrol programs kept the number of plants infested with Bemisia under this threshold, with some closer to 5-10% infestation rates near the time of sale. Each program was tested on the same commercial farm in separate compartments containing ca. 10,000 poinsettia plants each.

Finishing

The final step in a successful biocontrol program for whitefly (besides dipping cuttings and choosing your bios) is careful monitoring of whitefly levels. This is needed from early August (when Bemisia numbers can start to build) up to about a month before the sale begins (mid-October).   This is to ensure your biocontrol program stays on track. Generally, simple presence/absence numbers every week is a quick way to give you the information you need. This involves sampling around 5% of the crop, or a minimum of 10-20 plants per bench, and simply recording if they do or don’t have whitefly. This gives you a percentage of plants sampled that are infested, as in Figure 3.

However, it’s not a bad idea to also keep track of color and variety separately, as some are hit harder than others (with colors – especially whites – tending to attract higher whitefly numbers than red varieties).

In Canada, a general rule of thumb developed by OMAFRA is that no more than 15% of your crop should be infested with whitefly in mid-September. More than this, you risk whitefly populations being out of control come late October (which is too close to sale time for comfort). Less than this, and you can be fairly confident your biocontrol program will see you through to sale without the need for pesticides.

If you start to see your whitefly numbers creep up, especially in certain colors or hot spots, then Delphastus can be added to help reduce the pest population. However, if infestation levels are near 20% of the plant in mid-September, and it’s not just restricted to a few areas or varieties, then it might be time to come in with pesticide applications. The good news is, by this point, much of the pesticide resistance present at the beginning of the crop should have been “bred out” of the whitefly by now, meaning your pesticide applications will have a higher chance of working.

In short…

The Canadian experience has shown that starting clean (through dips) and using multiple biocontrol agents early in the crop gives growers the best chance of success against Bemisia in poinsettia. In some years, biocontrol has been sufficient to manage whiteflies right until sale. Other years, one to two pesticide applications may be needed later in the crop cycle to clean up residual whitefly populations. Overall, though, years of research have demonstrated that this strategy can reduce pesticide applications by 50 percent or more compared to a pesticides-only program. Finally, it beats any pesticide rotation strategy in terms of preventing resistance development in whitefly populations.

Overall, you may want to consider using a combination of cutting dip treatments and biocontrol for Bemisia whitefly in poinsettia IF:

Acknowledgments: The whitefly cutting dip project was funded in part by Dümmen Orange, BioWorks, Koppert Canada, and Flowers Canada (Ontario) Inc. This work was continued with funding from AFE to develop dips against thrips. Thank you to the growers and consultants (Graeme Murphy, BioLogical Consulting) that shared their data from commercial trials.

By Dr. Rose Buitenhuis, Senior Research Scientist Biological Control, Vineland Research and Innovation Centre, Vineland, ON, Canada, Rose.Buitenhuis@vinelandresearch.com and Dr. Sarah Jandricic, Floriculture IPM Specialist, Ontario Ministry of Agriculture, Food and Rural Affairs, Vineland, ON, Sarah.Jandricic@ontario.ca , ONFloriculture.com


Be sure to check out more of AFE’s research, watch Dr. Rose Buitenhuis’ recent webinar on Biological Control of Whitefly on Poinsettia, Dr. Sarah Jandricic’s session on Developing an Integrated Pest Control Program for Whitefly on Poinsettia and register for upcoming sessions