Although the invasive pest Thrips parvispinus continues to threaten tropical ornamental crops, the good news is that the sky isn’t falling. Producing crops like mandevilla, schefflera and hoya is still possible, at least in a closed greenhouse setting.
Although developing a reliable biological control program for this pest is probably a few years off, a suite of pesticides is available in the U.S. and Canada to successfully manage T. parvispinus.
This post covers these pesticides, their relative efficacy and demonstrates outcomes when used in an 8 month on-farm trial in mandevilla.
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 24-48 hour lab tests. This involves spraying label rates of pesticides on the leaf discs which thrips then feed on and seeing how many survive. She also quantified feeding damage on these leaf discs compared to an untreated control.
Her work found several pesticides that are already registered in the U.S. and Canada that cause high levels of mortality of various T. parvispinus life stages (Table 1, below), and also reduced feeding damage.
Some of these products 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 lab trials conducted by Dr. Revynthi, University of Florida. GH = registration on greenhouse ornamentals; LO = registration on landscape ornamentals in Canada.
Success (Conserve), Xxpire and Pylon (Piston) worked the best of these products, causing 90-100% mortality of both adult and larval T. parvipsinus. Although Xxpire is not available for greenhouse ornamentals in Canada, Flowers Canada is pursuing the product Twinguard, which has the same actives, as a minor use label expansion for thrips in greenhouse crops.
Avid (Trimectin), Kontos and Rimon (Pedestal) caused high mortality of larvae (>70%). 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% mortality) but was not effective on larvae. Ference (Mainspring) 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 4-5 days) if often needed.
Tests of Pesticides in a Commercial Greenhouse
As important as lab tests are for initial screening purposes, they represent ideal conditions. Pesticide coverage is 100% 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 in a greenhouse facility. In the previous year, they had 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 suggested 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% of the crop was lost in 2021/2022.
In the 2022/2023 growing season, madevilla cuttings from a supplier from Guatemala were potted up in late July. Thrips parvispinus pressure was monitored in the crop using plant taps (Fig. 2).
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. At this point we used both biocontrol tactics (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).
These strategies reduced the thrips population to just 10 thrips per pot in both varieties, a reduction of 50-70% (see Figure 3). However, this wasn’t enough to stop damage from occurring. (We subsequently used this information to set our damage threshold for this pest to 10 thrips/9-inch pot).
We therefore turned to chemical controls in late December. As you can see in Figure 3, an initial “sprench” of of Success (Conserve), along with Beleaf (flonicamid – registered as Aria in the U.S.), gave us control in the red variety for almost 3 months, while plants were mostly vegetative and growing temperatures were cool (15-18 °C or 59-64 °F). 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.
Unfortunately, Thrips parvispinus populations started rebounding in the reds in early March, when temperatures warmed up and the plants were allowed to flower in earnest. Attempts at using “softer” chemicals, like Ference (Mainspring) and Beleaf (Aria), along with biocontrol, seemed to slow thrips population growth. But, with the population once again approaching damage thresholds, we decided to abandon biological control and apply Pylon (Piston) . This knocked the T. parvispinus infestation back to near zero for another month, with an application of Avid (Trimectin) 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 8-inch baskets that got hit particular hard by T. parvispinus sometime in late February (see Fig. 4 below). Despite sustaining considerable damage, pesticide applications allowed these plants to grow out of the damage in time for sale in late May.
Ultimately, 100% of the plants farm-wide were sold in 2023, which was a considerable improvement from 2022.
Figure 4. Left: A variety of mandevilla showing significant T. parvispinus damage in late February. Right: the same variety cleaned up by pesticides in time for sale; all plants flowered and were sold. Photos by OMAFRA.
Managing Pesticide Resistance: Using IPM and Resistance Management
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 akin to how we successfully manage highly-resistant whitefly populations (Bemisia tabaci species) here in Canada on poinsettia crops. Based on biologically-based IPM programs developed for Bemisia whitefly in Ontario, I’ve developed a tentative program for T. parvispinus in mandevilla and other long-term tropical crops (Figure 5). We’ll be testing out this program for the 2023/2024 growing season.
Known resistance management strategies are also likely going to play a large role in the successful chemical control of this pest. Here are some other tips to help avoid resistance when it comes to 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%) reduce T. parvispinus on cuttings by 70% (see this post). 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 9-inch pot as our damage threshold was incredibly helpful in avoiding sprays until absolutely necessary. Adjust thresholds up or down as plants get bigger or closer to sale.
- Don’t spray varieties that aren’t showing damage. Thrips parvispinus seem particularly attracted to certain varieties of mandevilla, anthurium and other host plants. 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 5 days to see if chemicals are effective. For example, in our trials, we actually 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!
- Beware of using sticky cards, instead of plant taps, to monitor chemical efficacy. Sticky card numbers give you a picture of an insect population over the period the cards were up – they do NOT tell you what’s happening on the plants right now. Often numbers of thrips on sticky cards go up immediately after a spray, due to thrips flying off the plants. This can seem as though your chemical applications didn’t work and lead to further applications, when the pest population has actually decreased dramatically on the plants themselves.
- 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%. Mass trapping using sticky cards contributed another 17%. No pesticide will give you 100% control, so reducing the pest pressure in other ways is critical to lowering insect populations below damage thresholds.
Take Home Message
Our work this past growing season demonstrated producing high-quality tropical ornamentals IS still possible, despite the presence of T. parvispinus – which is unlikely to go away anytime soon. However, two important caveats to the success of our study were a) we were working in an Ontario greenhouse facility with low production temperatures over the winter, which slowed the population growth of T. parvipsinus, and b) we weren’t dealing with new populations flying in from outside.
For growers attempting to produce susceptible crops under cover or outside in areas where T. parvispinus is established in the landscape, effective pesticide rotation and resistance management techniques will be even more important in managing incoming pest populations.
Sarah Jandricic has been the Greenhouse Floriculture IPM Specialist for the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) since 2014. She can be reached at firstname.lastname@example.org .