By Janet Kanters
For over a decade, researchers in Ontario, Canada have tracked the stealthy spread of pests hitchhiking on unrooted plant cuttings. Among the most persistent offenders: whiteflies on poinsettias, and western flower thrips and two-spotted spider mites on chrysanthemums. And most recently, a new invasive player has entered the scene: Thrips parvispinus, a tropical pest now surfacing on mandevilla cuttings.
The problem lies not just in the pests themselves, but in their ability to hide. Many arrive as eggs or immature nymphs, virtually undetectable to the human eye.
“Insects are really smart. Inevitably there’s some thrips or aphids or spider mites or whiteflies that can
make it on the cuttings to the growers,” says Rose Buitenhuis, PhD, director, Biological Crop Protection with Vineland Research and Innovation Centre, in Ontario, Canada. “So then when the growers already start with a few pests right at the beginning of the production cycle, you can imagine it can only get worse.”
A 2018 study underscored the scope of the issue, revealing that most chrysanthemum cuttings sampled that year had one to two thrips per 20 cuttings – and nearly half also carried spider mites, sometimes in numbers exceeding 100 per sample. Multiply that across thousands of weekly shipments, and the potential for rapid infestation is staggering.
These pest species are not regulated, notes Buitenhuis. “The Canadian Food Inspection Agency has not said ‘if you have this pest species, you have to shut down or destroy it.’”
Buitenhuis adds that it’s almost impossible to find them [the pests] without having to destroy the cuttings. “To find them we take a few cuttings per batch, put them into a soap or ethanol solution and shake them, and pour the solution through a filter paper,” she says. “We find maybe one or two pests. It’s really low numbers, but it’s enough to cause problems later on – one or two pests can become 100 or 1000.
“And with thrips, including the new species Thrips parvispinus, their eggs are inserted into the leaf tissue, so you’d never find them.”
Making matters worse, many pests arrive already resistant to chemical pesticides. “The problem is that at the propagator, the mother plants that these cuttings are taken from are being treated with a lot of different insecticides,” says Buitenhuis. “So, resistance is very likely.”
That’s where cutting dips come in.
What’s a cutting dip? It’s exactly what it sounds like. Before sticking unrooted cuttings into soil, growers immerse them in a solution of reduced-risk pesticides, biopesticides or insecticidal soaps. These dips coat the entire plant surface, including the hard-to-reach undersides of leaves where pests like to lurk.
“The cuttings come in a bag of 50 or 100, just these little bits of plants. You basically spread out the plants in a tray and you immerse them in a biopesticide solution. It’s pretty simple,” says Buitenhuis.
Indeed, trials in both Ontario and the U.S. show promising results – cutting dips can reduce thrips, whiteflies and spider mites by 70 to 80 percent. Unlike foliar sprays, which struggle to reach all surfaces under mist or inside plastic tunnels, dips offer full coverage in one go.
“In poinsettias and chrysanthemums, the growers have really seen big benefits,” notes Buitenhuis. “And other crops, such as spring bedding plants – a lot of growers dip those as well. And now with Thrips parvispinus that comes in on tropical plant cuttings, cutting dips is again a really good method to stop them at the beginning.”
What works – and what doesn’t?Not all dips are created equal. Researchers caution against one-size-fits-all solutions and emphasize the importance of formulation, concentration and plant sensitivity.
Mineral oils (SuffOil-X, Landscape Oil) are among the few products confirmed to kill thrips eggs, which are notoriously hard to reach due to being embedded in plant tissue.
Oil-based fungal products, such as those containing Beauveria bassiana, may damage sensitive cuttings. In these cases, wettable powder formulations are recommended.
Entomopathogenic nematodes, while excellent when applied to soil, showed poor performance as foliar dips for unrooted cuttings.
Double-dipping – applying the treatment again after cuttings have rooted – may offer added control but should be approached cautiously to avoid phytotoxicity.
As with any pest control method, dipping isn’t a silver bullet. The residual effect is short-lived, meaning biological control agents must still be released early to keep pests in check. However, research shows that dips paired with biocontrol strategies outperform biocontrol alone.
Are dips safe?One lingering concern has been the potential for pathogen transmission,
particularly in shared dipping tanks. But recent studies with Pectobacterium carotovorum – a bacteria known to cause soft rot in poinsettias – suggest the risk is minimal.
Even though bacteria were found in dipping solutions at all sampled greenhouses, no increased disease incidence was found in dipped versus non-dipped plants. Only when unrealistically high bacterial concentrations were introduced did disease levels spike.
“To have a strategy and products that work well at the beginning of the crop cycle that have no resistance issues, it’s really important,” says Buitenhuis. “Also, a lot of growers want to use biological control because they’re dealing with pests like thrips or spider mites and whiteflies that typically are resistant
to a lot of chemistries. They needto use biological control.”
Growing momentumOntario growers are already embracing the method. A recent survey found 73 percent of poinsettia and chrysanthemum producers are now dipping their cuttings. Encouraged by these results, several product manufacturers have updated their labels to include dipping instructions.
And for those still not ready totake the plunge, alternative disinfection methods are being explored – from steam treatments to UV-C irradiation. ●
Pro tips for successful dipping
Always test new dip products on a small batch to avoid phytotoxicity.
With soaps and oils, use lower concentrations than spray rates.
Stick with wettable powders over oil-based fungal products for sensitive crops.
Maintain good hygiene—clean tanks and refresh solutions frequently.
Combine dips with early biocontrol for best results.
Rose Buitenhuis, PhD, director, Biological Crop Protection
Example of a dipping setup. Photo: Vineland Researchand Innovation Centre
