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Possible pesticides for pesky Thrips parvispinus

Although the invasive pest Thrips parvispinus (pictured on the right) continues to threaten tropical ornamental crops, the good news is that the sky isn’t falling after all. 

Producing crops like mandevilla, schefflera and hoya are still possible for those who want to continue growing them. Although developing a reliable biological control program for this pest is probably a few years off, a suite of pesticides is available in Canada to successfully manage T. parvispinus. As always, employing resistance management techniques and using other integrated pest management (IPM) tactics will be needed to ensure our chemical products keep working.

Lab Testing of Pesticides: An Important First Step
Dr. Alexandra Revynthi from the University of Florida Tropical Research and Education Centre has done the important first step of screening pesticides against T. parvispinus in 24-to-48-hour lab tests. This involves spraying label rates of pesticides on the leaf discs which thrips feed on and then seeing how many thrips survive. Revynthi also quantified feeding damage on these leaf discs.

Her work found several pesticides already registered in Canada that cause high levels of mortality at various T. parvispinus life stages (Table 1), and they also reduce feeding damage immediately. Some of these are not registered for thrips specifically but could be used as part of a total pest management program for other pests of tropical plants, including whitefly and spider mites.

Table 1. Products effective against Thrips parvipsinus larvae and adults, in order of efficacy. Results courtesy of trials conducted by Dr. Revynthi, University of Florida.

Success and Pylon worked the best of these products, causing 90-100 per cent mortality of both adult and larval T. parvipsinus. Avid, Kontos and Rimon caused high mortality of larvae (>70 per cent). 

Although they were not effective on adult thrips, they will still be an important part of a pesticide rotation program for this pest. Similarly, Tristar had some efficacy on adult thrips (causing around 50 per cent mortality) but was not effective on larvae. 

Ference caused reduced feeding of larvae, which is good news for plant damage, but didn’t seem to cause much mortality. However, in short lab tests like these, it’s often hard to see how products, which inhibit feeding, actually work to reduce the pest population. A longer time frame (around four to five days) is often needed.

Tests of Pesticides in Commercial Greenhouses
As important as lab tests are for initial screening purposes, they represent ideal conditions. Pesticide coverage is 100 per cent and the insects can’t leave the treated arena. Further, lab-based insect colonies are often more susceptible to insecticides than real-world populations that may have been exposed to multiple chemicals. So, the real test is how these products work in the greenhouse under actual growing conditions, as part of a total IPM program.

This past year, I worked with a grower cooperator in Ontario who grows a large amount of mandevilla. The previous year, they attempted to manage T. parvipsinus with high levels of biological control. 

This course of action was based on previous experience here in Ontario with the chemically resistant western flower thrips, and reports from Europe that suggest T. parvipsinus may be even more prone to insecticide resistance. Unfortunately, using biocontrol alone, with late-season interventions of chemicals only, was not that successful. Around 60 per cent of the crop was lost in 2021/2022. 

In the 2022/2023 growing season, plants were potted up in late July. By November, levels of T. parvispinus had built up to almost 40 thrips per pot in a red variety (thought to be the initial source of the infestation), and around 20 thrips per pot in an adjacent white variety. Atlhough initial attemps at biological control (a combination of Amblyseius cucumeris, Anystis baccarum and lacewings) along with mechanical controls (pruning growing points, regularly removing flowers and using a large amount of mass trapping cards) reduced the thrips population by an impressive 50 to 70 per cent, this wasn’t enough to stop damage from occuring.

We, therefore, turned to chemical controls in late December. As you can see in Figure 2, an initial “sprench” of high rates of Success, along with Beleaf (flonicamid), gave us control in the red variety for almost three months, while plants were mostly vegetative and growing temperatures were cool (15 to 18 C). In the white variety, which seemed to be less attractive to T. parvispinus overall, we were able to get away almost exclusively this initial application of Success/Beleaf, with 16 weeks of control between December and April. However, T. parvispinus populations started rebounding in the reds in early March, when temperatures warmed up and the plants were allowed to flower in earnest. 

Figure 2. Average T. parvispinus over time in response to different treatments. The red variety (red line) got all seven pesticide applications. The whites (purple line) only needed treatments 1, 5 and 6 to achieve the same level of control (with 6 and 7 mainly being crop clean-up for shipping purposes).

Attempts at using “softer” chemicals, like Ference and Beleaf, along with biocontrol, seemed to slow thrips population growth, but, with the population once again approaching damage thresholds, Pylon was applied. This knocked the T. parvispinus infestation back to near zero for another month, with an application of Avid made near sale just to ensure the crop was as clean as possible for shipping.

We also saw good results with applications of Kontos, Pylon, Success and Avid in a variety of white eight-inch baskets that got hit particularly hard by T. parvispinus sometime in late February. Despite sustaining considerable damage, pesticide applications ensured these plants were able to grow out of the damage in time for sale in late May (before and after photos are at the top of this article.). 

In fact, 100 per cent of the plants farm-wide were sold in 2023, which was a considerable improvement from 2022.

Managing Pesticide Resistance
One thing that likely contributed to our success with pesticides was delaying the need for applications as long as possible with biological and mechanical controls. Without knowing which pesticides were applied at the propagator’s end, the time period between July and December where we employed non-chemical techniques likely allowed the T. parvispinus population to be more susceptible to the pesticides we had at our disposal. This is much akin to how we successfully manage whitefly populations (Bemisia tabaci species) here in Canada on poinsettia crops. 

Overall, known resistence management strategies such as delaying the use of pesticides are likely going to play a large role in the successful IPM of this pest. Here are some other tips to help avoid resistence when it comes in T. parvispinus:

• Dip incoming cuttings in reduced-risk pesticides, like soaps and oils, to reduce the number of thrips coming in on product. Research from the Vineland Research and Innovation Centre has shown that dips in BotaniGard (2.5 g/L) or mineral oils (at 0.1 per cent) reduce T. parvispinus on cuttings by 70 per cent. This technique has been very successful in helping to manage resistance in Bemisia whitefly and western flower thrips IPM programs.
• Use weekly scouting and develop damage thresholds to avoid unnecessary sprays, especially early in the crop cycle. Determining a guideline early of 10 thrips per nine-inch pot as our damage threshold was incredibly helpful in this scenario. Adjust thresholds up or down as plants get bigger or closer to sale, as needed.
• Avoid spraying all plant varieties if they aren’t showing damage. Thrips parvispinus seems particularly attracted to certain varieties of mandevilla, anthurium and other host plants, compared to other pests. Additionally, some varieties just don’t seem to show T. parvispinus damage as much as others. Leaving susceptible populations of insects in small refuges is a tried-and-true method of resistance management. A pocket of susceptible (unsprayed) insects will ultimately breed with resistant insects and bring down the resistance level of the whole population.
• Always start with low label rates and wait at least five days to see if chemicals are effective. For example, in our trials, we found a half rate of Pylon to be effective against T. parvispinus, and Kontos took a while to show impacts on the population by affecting larvae. You can always move to higher rates, or a second application of pesticides from there. Blasting insects with high rates of chemicals at short intervals is how we quickly produce resistant populations in the lab.
• Make sure to couple all pesticide sprays with other IPM tactics. Our trials estimated that pruning the growing tips off the plants (where T. parvispinus likes to feed) and removing flowers (a source of nectar and pollen) reduced initial T. parvispinus infestations on plants by around 60 per cent. Mass trapping using sticky cards contributed another 17 per cent. No pesticide will give you 100 per cent control, so reducing the pest pressure in other ways is critical to lowering insect populations below damage thresholds. 

Sarah Jandricic has been the Greenhouse Floriculture IPM Specialist for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) since 2014. She also runs the popular ONFloriculture.com blog, which brings timely information to floriculture growers on pest and production issues.