Last year around this time, Judy Colley (Plant Products) and I summarized the latest research on controlling Thrips parvipsinus without conventional chemicals. Although we still don’t have biocontrol programs that are ready to be implemented, researchers in Ontario, Florida and Europe are getting closer to some answers. Or, at least, to a place where we can reduce the number of sprays needed.

Do any natural enemies look promising? 

First, the bad news.

Although predatory mites in the family Phytoseiidae form the backbone of biocontrol programs for western flower thrips, they are unlikely to be the main player for Parvispinus control, for a few reasons. Although some lab studies have shown promise with Amblyseius degenerans, A. swirskii or Amblydromalus limonicus, these have yet to be proven effective in cage and greenhouse trials. Cage trials using whole plants are incredibly important, as tropical plants, with their broad, waxy leaves, represent a different environment than we usually use mites on. 

In cage trials in Europe, and in greenhouse trials done by OMAFA, Biobee and Plant Products, investigators were not able to recover mites after repeated releases on mandevilla or anthurium, even when supplementary food was supplied, or the release method was varied (broadcast versus sachets). The only Phytoseiid product that’s shown results in cage trials is sachets of Transeius montdorensis, a species of predatory mite that’s not likely to become available in Canada.  

Also consider the high cost of these mites compared to Neoseiulus cucumeris and the large surface area of the main hosts of Parvispinus (i.e. mandevilla, anthurium, schefflera and hibiscus), and Phytoseiid mites don’t look like an economical solution. However, the biocontrol industry does have a history of rising to the occasion if a product looks promising. For example, foil mite sachets designed for use in cannabis are now cost-effective enough to use for western flower thrips in hanging baskets and other hard-to-manage crops. A product or cost-savings program may yet be developed for use in tropicals, if the mites can prove themselves useful. Mites may also be economical on less preferred hosts of Parvispinus, where damage is lower.

Similar issues exist for large predators like Orius insidiosus and Anystis bacarrum (pictured on the previous page). These species ate far more than Phytoseiid mites in lab trials by Dr. Alexandra Revynthi’s lab at the University of Florida (U of F), and also showed promise in commercial trials in Ontario. However, cost is still a concern, especially since weekly releases of Orius are needed in ornamental crops without pollen and during colder months, when Orius go into reproductive diapause. 

A solution to this would be to “front-load” predators in propagation to reduce costs, since the area is smaller. In 2024, OMAFA and Biobee Canada ran a trial using Orius, green lacewing and Dalotia coriaria for Parvispinus control in mandevilla propagation at a commercial greenhouse. Unfortunately, we didn’t recover a single natural enemy over six weeks. This may have been due to several factors, including high propagation temperatures (often >35°C in tropicals) and the pesticide residues in the cuttings, including DynoMite (pyridiben), Talstar (bifenthrin) and Scimitar (lamda-cyhalothrin) – a pyrethroid insecticide.

Given that a year-long biocontrol program for Parvispinus in Ontario using weekly releases of Orius in anthuriums also failed due to suspected pesticide residues, this is going to be a huge challenge in developing reliable biocontrol programs for this pest.  

Microbials to the rescue?

One strategy that is looking promising is the use of microbial biocontrol agents – both nematodes and fungi. Using these in concert with conventional chemicals to reduce pesticide sprays could be a good place to start.

Research from U of F showed a 60 per cent reduction in emerging Parvispinus when nematodes were added to the soil. This test was done on whole plants in research greenhouse trials, which gives me confidence in the results. Interestingly, the most effective nematode was Steinernema carpocapsae. Steinernema feltiae the species we are used to using in greenhouse ornamentals, faired so poorly in lab trials, they didn’t even bother testing it in the greenhouse. 

Similarly, research from Florida shows that products based on Beauveria bassiana, used for western flower thrips, don’t work well against Parvispinus. Lab trials show promise with Isaria fumosoroseus but a product containing this fungus is not currently sold in Canada. Good results have also been seen with Lalguard M52 (Metarhizium brunneum strain F52), both in lab trials, and in commercial trials here in Ontario.  

Specifically, a 2023 OMAFA trial at a commercial facility had great results using “sprenches” of Lalguard M52 to control a Parvispinus outbreak in mandevilla (see Figure 2). M52 sprays were done in tandem with high release rates of Anystis, which is less susceptible to microbial insecticides than other large natural enemies such as Orius or Dicyphus. Although we still needed to do conventional pesticide applications on the most susceptible variety of mandevilla, we got away with using a softer chemical (Rimon (novaluron)), which was a win in my books. Using this strategy, we were able to save harder (and more expensive) chemicals for clean-up, preventing their overuse and potential development of insecticide resistance. 

Preventing Parvispinus

One of our main strategies for managing Thrips parvispinusis likely going to be the “start clean” approach, i.e. trying to reduce the starting population arriving on cuttings. Vineland and OMAFA have been investigating several strategies.

The first of these is cutting dips, something we know and love here in Ontario. The trick with Parvispinus is that a large amount of the population is coming in as eggs. Thrips eggs are embedded within leaf tissue, making this a challenging target.  

Preliminary screenings of cutting dips at Vineland Research are looking at survival of the whole population, and of eggs, specifically, to see which works best. The other consideration is potential phytotoxicity caused by dip products, given the high value of the plants. The product that seems to be performing the best is Suffoil-X at 0.1 per cent – this can reduce the starting population of T. parvispinus by around 70 per cent. This makes sense, as oils, rather than soaps, are thought to negatively affect eggs. Stay tuned for updates at the Canadian Greenhouse Conference in October, where Dr. Rose Buitenhuis will be presenting her findings.

As dips can be time consuming, we’ve been exploring other tactics, like cooling cuttings before sticking.  Unlike native thrips species (Echinothrips), or species that have been in Ontario for a long time (onion thrips), Parvispinus has not had time to develop tolerance to low temperatures. Ashley Summerfield (Vineland), first noticed that Parvispinus did not fare well in the lab fridge, unlike western flower thrips. She is currently determining which temperatures and exposure times have the most impact on Parvispinus eggs. Preliminary data shows that exposure to 4.8°C for 72 hours resulted in a 71 per cent reduction in egg hatch from mandevilla.  

Meanwhile, to make sure this technique is safe for tropical plants, trials were run by OMAFA in 2024 in collaboration with a commercial grower to see if cooling cuttings negatively affected rooting or growth of mandevilla. 

Figures 3 and 4 show that rooting success was only slightly impacted compared to an untreated control, even when exposed to the coldest temperature (3.2°C on average) over a 48-hour period. As long as coolers didn’t dip below 1°C for any extended period of time, the cuttings were undamaged. Assessments 17 weeks later did not show significant differences in plant growth between cold-treated and untreated cuttings. Most importantly, all plants from our initial cold treatments were sold. Pre-treating cuttings with cold temperatures before sticking could ultimately provide a low-cost and low-labour solution to Parvispinus, provided a dedicated cooler can be maintained at the right temperature on-farm and monitored closely.

Where do we go from here?

Once screening trials with cutting treatments, natural enemies, and microbials are verified by cage trials in Ontario and Florida, the next step will be integrating treatments to find the most effective combinations and application timings. Just like a successful program for western flower thrips on chrysanthemums uses an entire suite of strategies, such as cutting dips, predatory mites, nematodes, etc., the same will be true for Parvispinus. This IPM “recipe” is likely to vary between plants which are moderately or highly susceptible to Parvispinus. For the latter, I suspect pesticides will remain a large part of their control, but I’m always prepared to be surprised when it comes to this particular pest.

For the more information and resources on Thrips Parvispinus, visit ONFloriculture.com.

Dr. Sarah Jandricic is the Greenhouse Floriculture IPM Specialist for Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA) and can be reached at sarah.jandricic@ontario.ca.