Poinsettias cuttings are here! Unfortunately, they will likely arrive with unwanted “presents” in tow. This includes Bemisia whitefly, a pest that can be hard to control with natural enemies and can ALSO be resistant to pesticides. So what’s a grower to do?
By implementing some preventative measures RIGHT NOW, and understanding WHEN it’s appropriate to spray, you can help save yourself a lot of headaches later.
Warding Off Whitefly:
Research has shown that being preventative in your whitefly strategy lowers the population you’ll struggle with later.
As an example, work by Rose Buitenhuis’ lab at the Vineland Research and Innovation Centre demonstrates that dipping unrooted cuttings in insecticidal soap and biopesticides like BotaniGard as soon as they come in the door can reduce final whitefly populations by 70%. And, they’ve recently confirmed this is without risk of transferring poinsettia pathogens like Erwinia (check out Rose’s poster on this!). Here’s more information on VRIC’s research methods and results for dips as well as a super handy video demonstrating proper dipping techniques (also applicable to products registered for thrips on mum cuttings).
Although we’re still working on registrations for this, in the future this technique could also likely be applied after rooting, if necessary (say, if you forget to/can’t dip right away), while the cuttings are still in trays. Trays would have to be turned upside-down in the dip tank with care taken to avoid contacting the dip solution with the root area as much as possible.
The same basic principle holds true for applying natural enemies: by applying biocontrol agents EARLY in the crop (i.e. right after they are potted up), many growers are able to get a jump on low, initial Bemisia populations that come in on the cuttings. This way, they can avoid, or at least reduce, the need for pesticides (more on this later).
Unfortunately, there’s still no “tried and true” recipe for Bemisia biocontrol on poinsettia. However, some growers have good luck using both Encarsia and Eretmocerus (both wasps will attack juvenile Bemisia whitefly), along with a predator like Delphastus or Limonicus.
More information on each of these whitefly biocontrols and how to implement them can be found here. Finding the right rates and combination of natural enemies will take some experimenting to see what works for YOUR farm.
To Spray or Not To Spray?
Not everyone chooses to manage their whitefly populations in their greenhouse with biocontrol. I’ve seen some greenhouses attempt biocontrol year after year without a high enough success rate to justify the costs. In these cases, success has been seen the last few years using Distance (pyriproxifen; IRAC Group 7) and/or Kontos (spirotetremat; IRAC Group 23) sprays to manage whitefly nymphs. Sometimes this is able to control the entire whitefly population; other times, Dyno-Mite (pyridaben; IRAC Group 21A) is needed as a crop clean up for adult whiteflies (with more than 1 application likely necessary).
However, for a pesticide-based strategy to have even a remote chance of being successful, you absolutely NEED TO AVOID PESTICIDE APPLICATIONS AT THE BEGINNING OF THE CROP. (And yes, this is an important enough point to warrant the use of all-caps; a.k.a. digital “yelling”).
Why? Because the Q-Biotype of Bemisia (you know – the one that’s resistant to EVERY chemical we throw at it?!?!?) thrives when pesticides are applied. Spray too early and too often and the Q-Biotype will out-compete the easier-to-manage B-Biotype, and you’ll be left with a greenhouse full of unkillable pests and unsellable plants.
To prevent this, delay your first pesticide spray until the END OF AUGUST at the EARLIEST (see the figure, below). I’d even prefer you wait a little longer than that. This way, the Bemisia population will have enough time to switch predominately to the pesticide-susceptible B-Biotype (its reproduction is favoured under no-spray conditions).
The Elephant in the Greenhouse
The big caveat with a spray-based program is simply that we don’t know how long it will work for. The B-biotype of Bemisia still has the ability to adapt and become resistant to chemicals — it just doesn’t do it as fast as the Q-type. Additionally, we can’t control what pesticides are used (or abused) at the producer end, potentially promoting resistance.
To be honest, I don’t even know if a totally chemical-based control program would be successful if we lost even ONE of the above pesticides.
However, we have gained 2 new pesticides for whitefly, recently: Fuji-Mite (fenpyroximate; IRAC Group 21) and Talus (Buprofezin; IRAC Group 16). Although their efficacy for whitefly in poinsettia remains to be proven in commercial operations in Ontario, they may additional options for nymphal control.
Overall, an IPM strategy combining biocontrol, then delaying pesticides till towards the END of the crop might be your best bet. By tackling whitefly nymphs as soon as they come in on cuttings, and applying biocontrol agents right at potting, you’ll likely be able to avoid the need for pesticides for as long as possible. This way, if you DO need to use them, they’ll have more of a chance of working for you.
For more information on the pesticides mentioned, and their side effects on whitefly natural enemies, refer to my Downloadable Pesticide Table: registered-insecticides-2016_omafra_2-0
Note a new, updated copy is coming to the CGC in the Fall, including Fungicides and Plant Growth Regulators!