Poinsettias hit hard by whiteflies this past season
Early biocontrol practices can help prevent the spread of whitefly to different areas of the greenhouse. They also help skew the Bemisia population towards the B-type, rather than the Q-type which is more resisistant to insecticides.
September 1, 2022 By Greta Chiu
Ontario poinsettia growers saw a surge in whitefly pressure last season.
During a discussion at the Sawaya Gardens’ poinsettia trial open house in Waterford, Ont. last November, a number of growers attested to high whitefly (Bemisia tabaci) pressure at various points during the past season. Some found them in earlier weeks, others in later shipments.
For Jeffery’s Greenhouses in the Niagara region of Ont., they observed at least five times as much whitefly pressure as they did in previous years while still on the rooting bench, early on in propagation. Shipments to their greenhouse were hit-and-miss. Some turned out to be highly infested while others were completely clean. Head Grower Albert Grimm says they were forced to throw away a few hundred cuttings after thoroughly inspecting each cutting tray. Based on their initial observations from weeks 30 to 32, they ramped up their biocontrol program accordingly and doubled their spending to curb a potentially devastating whitefly problem later on in the season.
“Very early scouting using methods that are suitable to detect whitefly nymphs, long before they show up on yellow sticky cards, were crucial,” Grimm said.
Of the growers who shared their methods for whitefly control, some used dips in BotaniGard and/or Kopa insecticidal soap, depending on their past degree of success with these products. However, many underscored the importance of scouting and starting a biocontrol program early on.
“If you didn’t use biologicals, chances are you would have to start spraying by September then spray every week after,” said Mike Short, IPM consultant and owner of Eco-Habitat AgriServices. “If you’ve been using biologicals all the way along, what you’re doing is you’re mitigating the amount of whitefly migration and intensity in the crop.”
From experience, Short has found that biological control methods help keep whitefly from spreading to different varieties in the greenhouse. Furthermore, these practices can help skew the proportion of the whitefly population towards the B-type rather than the Q-type species of Bemisia, where the latter is resistant to most insecticidal sprays. Starting with a biological control program early on would help improve the efficacy of sprays in the fall for a final clean-up of poinsettias, if needed.
A number of poinsettia cuttings suppliers were also present, giving growers an overview of pest management strategies used in their offshore facilities. They continuously monitor for whitefly on plants, with many relying on chemical-based inputs to keep them clean.
At Dümmen Orange however, technical support specialist Rick Rabb explained that the supplier changed up their approach to pest management by implementing biological control on their poinsettia. In 2015, the company introduced their GreenGuard program, which builds on foundational IPM principles, monitoring pests regularly and using non-chemical methods when possible. They use spot sprays along with bio-friendly pesticides throughout the growing season up until harvest.
“We’re making great progress on our biological control,” he said, pointing not only to the poinsettias, but to other crops as well, including their spring annuals.
Over the past six years, Dümmen Orange had reduced their chemical inputs by 80 per cent. Last year marked the first full season where their poinsettia in Ethiopia were grown entirely under the GreenGuard program, and their farm in El Salvador will soon follow. They use predatory mites Amblyseius swirskii or Transeius montdorensis to target whitefly eggs, parasitic wasps Encarsia formosa and Eretmocerus eremicus to manage whitefly larvae, as well as entomopathogenic fungi Beauveria bassiana and Lecanicillium muscarium to manage both whitefly adults and larvae.
For diseases, they use the biofungicide Bacillus subtilis to manage Botrytis and Alternaria, as well as Trichoderma harzianum to manage root diseases.
For growers who discover infested shipments, one key suggestion was to immediately advise the supplier. “If we can identify the bench, we can either treat that bench or take it out of production,” said Gary Vollmer, Selecta One’s technical support manager. Using the crop ID tags that arrive with each shipment, his team would be able to trace the contents back to the original house and bench.
As for the 150 different varieties trialled at Sawaya Gardens, plants were propagated by Linwell Gardens then sent to trial organizer Melhem Sawaya. The greenhouse consultant and owner of Focus Greenhouse Management decided not to use chemical pesticides and growth regulators. The result? Very clean plants – apart from one or two varieties in white, a colour that has historically attracted more whitefly than others.
“We started our biological program as soon as the plants were planted at week 35,” said Tara Celetti, biological program specialist at BioWorks Inc. Tasked with managing pests for Sawaya’s trial poinsettias, the team put out Dalotia coriara (previously known as Atheta) and Stratiolaelaps scimitus (previously Hypoaspis miles) to begin with. “These two work together to control fungus gnat larvae, shoreflies, as well as thrips pupae that are in the soil.”
They also released Encarsia and Eretmocerus, two small parasitic wasps that feed and parasitize both the Bemisia and greenhouse whitefly.
“By week 44 we found a hotspot [for whitefly],” Celetti recollected. “We released Delphastus which is a little beetle that feeds on whitefly eggs. We only use them for hotspots. You wouldn’t use it as a preventative treatment.”
At Jeffery’s, Grimm said they also released Delphastus in addition to Amblydromalus limonicus and Encarsia. “Delphastus feeds on eggs before it eats anything else,” he said. “This year, we put four Delphastus per square metre on the crop…. You won’t see reproduction, but they clean up the eggs.” For them, the results weren’t noticeable until three to four weeks after release, and they continued this treatment all the way to the end of the program.
The absence of reproduction in Delphastus is normal, even though the system is doing its job. As Ron Valentin, then director of technical business at BioWorks explained, “Delphastus needs to eat 160 whitefly eggs per day to start reproducing.”
As for Encarsia and Eretmocerus, he said, “if you look between the two wasps, Encarsia does approximately four, maximum five, L1s – the first larval stage of whitefly – per day. Eretmocerus can do twenty to thirty L1s per day.” There’s a significant difference in feeding capacity between the two biocontrol agents.
While both wasps will feed on both the greenhouse whitefly and Bemisia, Valentin says that parasitism on the latter really requires Eretmocerus. “[With] Encarsia, maximal parasitism on Bemisia is less than 17 per cent,” he says in a follow-up email. This is not considered “control.”
Celetti emphasized the importance of prevention and early detection. “Scouting is really important… If you notice that there is whitefly, mark that plant as having whitefly, whether it was 1 or 100.” She suggested calculating percent infestation using a minimum of 100 plants. “If you can do it by variety, that’s ideal. Then calculate what percentage of your plants have whitefly.”
While some growers shared less-than-ideal experiences with certain dipping methods, Michael Brownbridge, PhD, biological program manager for disease control at BioWorks, underscored the importance of ensuring the correct BotaniGard and insecticidal soap rates for dipping. Dipping at the recommended spray rate for insecticidal soap could potentially lead to plant damage.
Using BotaniGard at the label rate in a 0.5 per cent (v/v) solution of insecticidal soap, he said, “probably eliminates around 70 to 75 per cent of whiteflies through one dip.” Based on studies in other ornamental crops, he speculated that insecticidal soap may also help reduce any chemical residues lingering on the cuttings. “Knocking your whitefly population back early helps your biocontrol program succeed later on.” Horticultural oils like Suffoil-X– at a 0.1 to 0.25 per cent rate, significantly lower than the recommended spray rate – are another suitable option.
For growers concerned about Erwinia, Brownbridge said the benefits of dipping in insecticidal soap likely outweigh the risks. “You have to have such a high level of Erwinia bacteria to even infect the poinsettia cutting”.
Sarah Jandricic, PhD, greenhouse floriculture IPM specialist at the Ontario Ministry of Agriculture, Food and Rural Affairs agrees. “The chances are very, very low.” As she tells Greenhouse Canada, it’s important to use common sense around sanitation, changing the dip solution between days, at a minimum. Previous research by the team at Vineland Research and Innovation Centre has shown the risk of Erwinia to be very low, and recommends sanitizing and cleaning the dip bath before and after use.
As for phytotoxicity, Jandricic hasn’t seen any direct correlations with soap use. However, she notes that it’s important to rinse off the soap properly after dipping, simply by putting the cuttings under mist as soon as possible, to reduce any lingering residues.
For a combination treatment of Kopa and BotaniGard against whitefly on unrooted cuttings, Vineland recommends 0.5 per cent Kopa + 1.25g/L BotaniGard WP. Rates may need to be adjusted based on crop sensitivity.
To see dipping rates developed by Vineland, a video of Vineland researchers performing the procedure, as well as Jandricic’s updated article on how, when and why to dip, visit greenhousecanada.com and ONfloriculture.com.
So far, Ference insecticide (cyantraniliprole) seems to be the only pesticide that continues to be effective against the Bemisia whitefly. The product was registered for greenhouse ornamental use in Canada back in 2020.
However, since the Q-type whitefly has been known to develop resistance rather quickly, Jandricic has concerns about the longevity of this insecticide, particularly as new chemistries for whitefly are hard to come by.
Cary Gates, pest management director of Flowers Canada Growers (FCG) says there are other potential insecticides coming to Canada for these uses in the future.
One already available in the U.S. is Rycar (pyrifluquinazon), which was submitted for registration with the PMRA in 2019 for whitefly and aphid management in greenhouse ornamentals, as well as a number of greenhouse vegetable crops.
“Flowers Canada has been working with Nichino to get this into Canada since 2010 but there were a number of barriers,” says Gates. He hopes to see it registered for 2022. “With that said and despite the long timelines, we’re really grateful to both Nichino and Belchim for supporting the greenhouse industries. This will be a very welcome product for growers.”
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