Every year at this time, growers start to think about strategies to minimizing western flower thrips on their chrysanthemum crops. This year, Thrips parvispinus is causing extra concern, given that chrysanthemum is one of its reported hosts.
Although there have been no reports yet of Parvispinus damaging chrysanthemums in North America, it’s still good to exercise a bit of caution. However, jumping the gun and spraying chemicals will likely just disrupt your biocontrol program for western flower thrips, leaving you with a bigger mess on your hands.
This post contains information on things you can do to put your mind at ease about all sorts of thrips this chrysanthemum season.
- Dips for Thrips
- Best Practices for Cutting Dips
- Continued Monitoring and Pest Management for Thrips
- The Bottom Line
Dips for Thrips
Many of you reading this post might be worried about Thrips parvispinus this year, as we get everything from tropicals and foliage plants, to chrysanthemum and gerbera cuttings, from Florida.
The good news is that cutting dips are a proven tactic against thrips pests in Canada and several products were recently registered in the U.S. for this use. This technique uses reduced-risk pesticides such as oils, soaps and microbial insecticides to effectively smother thrips when they are briefly submerged in these products, reducing incoming populations.
Figure 1. Dipping cuttings in reduced-risk products significantly decreases incoming thrips populations on cuttings. This technique is widely adopted in the Canadian floriculture industry and has helped reduced the need for pesticides for thrips in many crops. Photo courtesy of A. Summerfield.
We know this effectively reduces western flower thrips arriving on potted mum cuttings when used properly and gives natural enemies time to build up on crops and work more effectively. This work on dips came out of the Biological Crop Protection Lab at the Vineland Research and Innovation Centre.
More recently, Rose and her Senior Lab Technician, Ashley Summerfield, provided information on the efficacy of cutting dips on Thrips parvispinus. Similar to results with other thrips species, their work showed that dips in BotaniGard WP or mineral oil reduced Thrips parvispinus on plant cuttings by around 70%.
Figure 2. Results from dipping mandevilla cuttings infested with T. parvispinus in reduced-risk products. Figure courtesy of R. Buitenhuis, Vineland.
Best Practices for Cutting Dips
We recommend that ALL growers begin implementing dips for both potted and garden mum cuttings coming in, especially those coming from Florida. (Note that oils are NOT recommended for garden mums due to phytotoxicity).
Vineland and OMAFRA recently released a series of “How-To” videos when it comes to greenhouse IPM. Proper dipping technique for cuttings is demonstrated here:
Figure 3. Video demonstrating proper dipping technique. Produced by the Vineland Research and Innovation Centre.
In terms of which products to use, label registrations in Canada include:
- BotaniGard WP: registered as a dip on all greenhouse ornamentals for whitefly. This is also effective against thrips.
- Kopa soap: registered on all greenhouse ornamentals for whiteflies
- Landscape oil: registered on all greenhouse ornamental crops for thrips and whitefly
- Suffoil-X: registered on all greenhouse ornamentals for thrips and whitefly
Dip registrations in the U.S. include the following (from Michigan State Extension’s Greenhouse Insect Pest Management Recommendations 2024):
| Insect management products labelled for cutting dip application | |||
| Trade name | Active ingredients | MoA Group | REI (hours) |
| BotaniGard 22WP | Beauveria bassiana | Insect pathogen | 4 |
| EpiShield* | Botanical oils | Unclassified | 0 |
| Hexygon IQ | Hexythiazox | 10A | 12 |
| LalGuard M52 OD | Metarhizium brunneum | Insect pathogen | 4 |
| M-Pede | Potassium salts of fatty acids | Unclassified | 12 |
| TetraCURB MAX* | Botanical oils | Unclassified | 0 |
Continued Monitoring and Pest Management for Thrips
Although dips can help reduce incoming thrips, they aren’t a silver bullet to get rid of the problem. if you’re concerned about Parvispinus on your mums, you can add weekly applications of Lalguard M52 OD on the misting bench. This product is compatible with predatory mites for western flower thrips control, so it’s a good choice.
On-farm investigations by Plant Products and OMAFRA have demonstrated M52 to be the most effective microbial product against Thrips parvispinus (so far, anyways). Luckily, M52 is just as effective against western flower thrips as Beauveria-based products, so you’d be killing two birds with one fungus. Note that products containing the fungi Metarhizium, like M52, tend to be less stable after opening than Beauveria, so it’s best to use the bottle up right away.
Another potential tool is the is a second dip of rooted cuttings in M52 or Suffoil X before potting, to further reduce thrips in production. This is still in the research phase, but on-farm trials last year showed a reduction of 50-70% of Thrips parvispinus on rooted mandevilla. (Note, this has not yet been tested on mums re: phytotoxicity).
Although it might be tempting to apply conventional insecticides for suspected Parvispinus on your mums (e.g. Success, Pylon, Avid), these products will disrupt mite-based biocontrol programs for western flower thrips and spider mites for many weeks. This should be avoided unless you’ve had an expert make a positive identification and can help you come up with a management plan. The only exception to this might be Rimon (Novaluron), known as Pedestal in the U.S. Novaluron is effective against Parvispinus larvae but is also generally soft against predatory mites.
Fig. 4. Lobularia is an excellent indicator plant for Parvispinus, as it’s attract to the abundance of small flowers. Putting a sticky card in the pot can help you find any infestations early.
You’ll also want to continue monitoring the crop for any signs of Parvispinus through the whole crop. Thrips problems often take around 3-4 weeks to show up after sticking cuttings, once populations build up. You can read about effective strategies for monitoring for this pest in this post from 2023.
You can also put a few sweet alyssum (Lobularia) plants out if you have any left over (or grow new ones from seed), as studies by OMAFRA and Plant Products have shown they are an excellent indicator plant for Thrips parvispinus.
The Bottom Line
Keeping an eye on your mum crop, especially when getting cuttings from areas where Thrips parvispinus has become endemic, is a great idea. However, it’s important to keep in mind that we have yet to see this pest on this crop in the North East. This is true even when mums have been grown in greenhouse zones directly after a crop with Parvispinus (e.g. Mandevilla). I have also not heard reports of Parvispinus causing damage on chrysanthemum from growers in the Southern U.S.
If you are still concerned about any dark-coloured thrips you might be finding in your mum crops, stay tuned. Next week we’ll put a post up on ways to to detect low numbers of thrips in the crop and do a quick and dirty assessment to make sure you’re not dealing with Parvispinus.
Dr. Sarah Jandricic has been the Greenhouse Floriculture IPM Specialist for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) since 2014. She has previously done floriculture IPM research in the US (at Cornell University and North Carolina State) and spent two years as Director of Research at a private IPM consulting company.