The floriculture industry in Ontario seems to have mostly avoided heavy aphid AND thrips pressure this spring/summer. But from everything I’ve been hearing, we are NOT going to be that lucky with whitefly on poinsettia this year.
This post goes over preventative measures that should be taken as soon as cuttings come in the door, biocontrol programs for whitefly on poinsettia, and WHEN and WHAT to spray for this so you can avoid resistance issues.
Warding Off Whitefly:
Research has shown that being preventative in your whitefly strategy lowers the population you’ll struggle with later. This is likely going to be especially important this year, as it looks like whitefly pressure on cuttings could be high. The Farmer’s Almanac is also predicting a hotter than usual August for Southern Ontario, which will likely cause whitefly populations in your crop to flare in Aug/Sept. So wouldn’t you want to start off with a lower initial pest population if you could?
Work by Rose Buitenhuis’ lab at the Vineland Research and Innovation Centre demonstrates that dipping unrooted cuttings in a reduce-rate insecticidal soap (i.e. 0.5%) and biopesticides like BotaniGard WP (at label rates) as soon as they come in the door can reduce final whitefly populations by 70%.
Dips are 100% compatible with whitefly biocontrol programs and pose little risk in spreading poinsettia pathogens like Erwinia. They may also benefit whitefly control by helping to rinse off pesticide residues, allowing for better survival of natural enemies. This is especially important in years where there is likely heavy pesticide use at the propagator end.
Here’s more information on VRIC’s research methods and results for dips as well as a video demonstrating proper dipping techniques. Dips are fairly simple, but taking care to make sure ALL the cuttings are immersed in the solution is key. Although FCO is still working on registrations for this, in the future this preventative technique will likely form the backbone of ALL whitefly control programs, whether they be pesticide OR biocontrol based.
The same basic “prevention is worth a pound of cure” principle also holds true for applying natural enemies: by applying biocontrol agents EARLY in the crop, many growers are able to get a jump on low, initial whitefly populations that come in on the cuttings. This way, they can avoid, or at least delay, the need for pesticides, which can be VERY important (more on this later).
Unfortunately, there’s still no “tried and true” recipe for biocontrol of Bemisia whitefly on poinsettia. However, many growers find the following program works best:
- Using BOTH Encarsia and Eretmocerus
- Both wasps attack whitefly nymphs; Encarsia will also host feed on pupae.
- Can be applied right on the misting bench (put cards under styrofoam cups to prevent them getting wet and molding), OR start right at potting.
- PLUS a predator like Delphastus or Limonicus.
- Limonicus or Swirskii can be applied on the misting bench
- Delphastus should be applied 4 weeks after cuttings are received as it is very sensitive to pesticide residues. Although it’s relatively expensive, it is an excellent egg predator and can help clean up outbreaks.
To Spray or Not To Spray?
Ultimately, choosing whether to use biocontrol or chemical control for whitefly comes down to a lot of personal factors (e.g. your success with each in the past, your farming philosophy, etc.).
Both strategies can provide good control (at least in low whitefly-pressure years). The proof? Here’s how different biocontrol programs stacked up against chemical applications in 2017.
However, the big caveat with a spray-based program is simply that we don’t know how long it will work for. Unfortunately, Bemisia whitefly develops chemical resistance easily. In high whitefly pressure years, where likely everything but the kitchen sink has been thrown at them in propagation in the U.S., whitefly are more likely to come in already resistant to chemicals, meaning none of our registered Canadian products will work.
If you do chose to go down the chemical pathway, success has been seen the last few years using the following chemicals to manage whitefly nymphs:
- Distance (pyriproxifen; IRAC Group 7)
- Kontos (spirotetremat; IRAC Group 23)
- Beleaf (flonicamid; IRAC group 9C): Note that this chemical may cause phytotoxicity in poinsettia if applied more than 1x per crop as a drench
- Dyno-Mite (pyridaben; IRAC Group 21A) can be used as a crop clean up for adult whiteflies (with more than 1 application likely necessary).
Increasing your Chances of Success:
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). 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).
Unfortunately, there’s no good way to tell if you have predominately B’s from Q’s going on in your crop without genetic testing. This can be done through a USDA lab in Florida (contact me at OMAFRA for more information).
When to Switch Horses:
If you start with a biocontrol program for whitefly in poinsettia, monitoring will be your best friend to help you determine whether to keep going or pull the plug. Simple presence/absence numbers (from 10-20 plants per bench) is quick and will give you the information you need.
The tipping point seems to come in Mid-September. Are more than 20% of your pots infested with whitefly at this point? If numbers per plant are generally high, and this is affecting the major variety you grow, it’s likely time to abandon ship (see below).
Overall, an IPM strategy relying mainly on dips + biocontrol, with pesticides LATER in the crop, may ultimately prove to be the best general strategy for whitefly in poinsettia crops. 2018 will certainly put all these strategies to the test.
(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)