Got Tropicals? How to Detect and Monitor for Thrips parvispinus.

In a previous blog post, we talked about which pesticides can control Thrips parvispinus if you end up with an infestation from imported plant material.

But how do you figure out if this invasive pest is in your facility in the first place? And, if so, what’s the best way to monitor their populations? 

After working with this pest for the last 1-2 years, researchers like myself have figured out which monitoring methods are most effective. This can help you identify the problem early, begin a management plan, and monitor the efficacy of your controls.

Detection and Monitoring in Propagation:

If you’re growing your own tropical crops, inspecting imported cuttings is usually your first step to detecting unwanted pests. It can also usually give you a feel for what the pest pressure is going to be that year (think Bemisia whitefly adults or nymphs in bags of poinsettia cuttings). 

However, inspection of mandevilla cuttings, as well as the water from cutting dips, at a commercial greenhouse (with help from the Buitenhuis lab at the Vineland Research and Innovation Centre) revealed very few adults or larvae. This suggests that most of the thrips are coming in on imported plant material as eggs.

(Note: this doesn’t mean you shouldn’t dip your cuttings for T. parvipsinus! Every shipment/supplier could be different, and dips also help reduce other common pests of tropical crops, such as whitefly and spider mites. Additionally, dipping in oils can kill thrips eggs!).

So, how do you detect and count Thrips parvispinus eggs on your cuttings? Well… you don’t. Like other thrips species, T. parvipsinus embedded their eggs in leaf tissue. The eggs aren’t visible with a hand lens or even with a regular dissecting microscope.

The best monitoring method is to put a large amount of yellow sticky cards in propagation, just above the cuttings. Don’t worry about them getting wet– sticky cards can still function under overhead watering or at high humidity. Especially if the cuttings are sealed in propagation tents, any adult thrips that get stuck on the cards over the next 7-14 days are evidence of what’s coming out of the new crop.

If you have a microscope, you can then cover the cards with clear cling wrap and confirm the identify your thrips using the simple thrips key for growers developed by OMAFRA and the Vineland Research and Innovation Centre. Or, you can send them to a consultant or specialist who can do this for you.

Monitoring in Production:

The one good thing about T. parvispinus, is that it seems VERY host specific. In Canada, we have yet to see damage on crops outside of the tropical plant species listed in the table below, even when grown right next to each other. This includes plants listed as “potential” hosts, such as chrysanthemum, gerbera, bougainvillea and other foliage plants like ferns.

If you are growing any of the 8 ornamental host plants listed in Table 1, the first sign of T. parvispinus you’ll notice in production will likely be damage (unless you’re monitoring in propagation).  Damage can show up differently on different plant species, so be familiar with the symptoms on your crop.

 Table 1. Ornamental crops in North America where significant damage has been seen from Thrips parvispinus. The potential host list is much wider, but this pest seems to show a strong host preference for these plant species. Species listed in order of suspected preference.

Common Name	Species name	Damage Signs
Gardenia	Gardenia spp.	Heavy feeding scars on foliage that resembles broad-mite damage. Damage to flowers similar to regular thrips feeding.
Laceleaf	Anthurium spp.	“Cork damage” on the growing point; inhibits crop growth. Will feed on the flower and leaf stalks.
Rocktrumpet	Mandevilla & Dipladenia spp.	Heavy feeding scars on foliage that resembles broad-mite damage. Leaf buds will fail to develop. Damage to flowers similar to regular thrips feeding.
Schefflera	Schefflera arboricola	New growth will turn yellow and resemble infection with a plant virus. On older foliage, feeding scars resemble broad-mite damage.
Ornamental peppers	Capsicum spp.	Damage similar to regular thrips feeding, with more tissue distortion.
Hoya	Hoya spp.	Heavy feeding scars on foliage that resembles broad-mite damage. Leaf buds will fail to develop and some leaves may abort.
Hibiscus	Hibiscus rosa	On both foliage and flowers, damage resembles regular thrips feeding.
Frangipani	Plumeria spp.	On foliage and flowers, damage resembles regular thrips feeding at first, but can develop into more heavy scarring.

Damage can also vary by plant growth stage (Figure 3). Damage on new growth might first appear as mild tissue distortion (similar to broad mite or even foxglove aphid) but can advance to leaf or bud abortion of the entire meristem under a heavy infestation.  Damage on old growth tends to look more like heavy mechanical scarring.  Interestingly, damage seems to only appear on the upper sides of leaves.

Besides visually inspecting plants for damage, your other most important tools for detecting Thrips parvipsinus are the same as for other thrips species: plant taps and sticky cards. 

Plant Taps for T. parvispinus:

Plant taps are the most useful tool for monitoring T. parvipsinus, as they give you a more accurate read of the number of pests actually on the crop at that moment in time (and not what’s just been flying around in the general area).  Also, plant taps are generally more highly correlated with plant damage than other forms of monitoring and can help you develop action threshold for particular crops and varieties. Here’s some tips when it comes to conducting plant taps for T. parvispinus.

• • Use a white pan or this sheet developed by University of Florida. Vigorously tap plants whole plants onto a white surface, wait a few seconds for the thrips to recover, and then start looking!
  • Look for movement. As Thrips parvispinus is very small compared to Western flower thrips, they will be harder to detect during taps. But, these thrips are much more active than any other species I’ve seen. It’s easier to hone in on them by looking for movement, instead of looking for colour/shape (especially since the males and females are different colours!).
  • Make sure you aren’t confusing them with other pests. When tapping, spend a few minutes confirming their identity with a 10-15X hand lens each time you do taps.  I’ve been in a greenhouse where they also had a lot of small arthropods call springtails (or Collembola) that were similar in colour, shape, size and activity!
  • Just look for adult thrips. Unlike other thrips I’ve worked with, the larvae of T. parvipsinus don’t seem to tap out of the plants very well. Save some effort and only look for and count adult thrips.
  • Squish each thrips as you find them! Not only does this make sure you aren’t counting a thrips twice, but it’s also a form of mechanical control!
  • Standardize the number of plants you tap in each crop, variety, or area each week.  This will make detecting population increases much easier.
  • Sample at least 5% of the crop. This will make sure your sample size is big enough to give you an accurate picture.
  • Spread out your sampling. Make sure you are sampling from the front, back and middle of a row or bench, as T. parvispinus is known to have a very patchy distribution. 
  • Combine small plants into 1 sample. With small plants (4-6 inch pots) or low pressure, sometimes it’s better and faster to tap 5 small plants into 1 pan and then count. You can always divide by the number of plants sampled to convert this to “thrips per plant” later.

•  

Sticky cards for T. parvispinus:

Sticky cards can also be a useful tool for T. parvispinus because they are very active flyers compared to species like onion thrips (T. tabaci) and poinsettia thrips (Echinothrips americanus).  

However, given the quickly damaging nature of this pest, cards should not be relied on as the only form of detection. Walking the crop and looking for damage will be much more important for early detection.  However, once T. parvispinus has already been detected in the crop, cards can be a reliable  monitoring which direction your population is headed – up or down – depending on your conditions and control measures. As shown in an on-farm trial ( Fig. 5), cards gave us similar information as plant taps in terms of pesticide efficacy, but the response on cards was delayed by a week.

Here are some tips when it comes to sticky cards for T. parvispinus.

• • Blue and yellow cards seem similarly effective for this pest in preliminary trials in the summer (more on this later!) However, due to its dark colour and small size, T. parvipsinus is much easier to see and count on yellow cards.
  • Don’t rely on monitoring cards alone to determine the efficacy of control measures. Remember that monitoring cards give you a picture of what happened since you first put them up – not what’s happening right now. Also, sometimes sprays can trigger insects to fly, which can make it appear as if populations are actually going up (see Figure 5).
  • More cards is always better. T. parvipsinus seem to move in waves – from their more preferred or source host onto less preferred hosts/varieties. It’s important to have good coverage with your monitoring cards to figure out which parts of your farm are affected, and be able to detect thrips in an area even at low densities.
  • Standardize the number of cards you check weekly in each crop and variety.  It’s a lot easier to figure out if pest pressure is going up or down if the count is always X thrips/5 cards, for example. When things get busy on the farm, you may not always have time to work out an average per card.
  • Cards seem to provide more information on smaller / vegetative plants. Once large plants have begun to flower, a single sticky card at the top of the plant seems to get lost. (For what to do instead, see “Finishing”, below).

• Inspecting Finished Product:

Whether you’re growing your own product from start to finish, or bringing in finished crop from Florida, monitoring changes a bit at this stage. Plant taps would still apply to foliage-only crops like hoya and schefflera. However, T. parvipsinus is strongly attracted to pollen and nectar. So, for flowering crops, counting adult thrips inside flowers at this stage is a better approach. It will give you a sense of the pressure on the whole plant and is much faster than plant taps (and is easier than trying to tap large mandevilla plants or hibiscus “trees”).

At this point, what damage has been done to the foliage has already likely been done, and there’s not going back.  But if the issue is mostly the presence of thrips in the flowers before sale, you may want to do a few sprays with contact insecticides to eradicate thrips. A threshold of more than 2-3 thrips per flower is a good indicator sprays may be needed if the product is staying in Canada (where T. parvispinus cannot overwinter). The threshold should be considered 1 thrips/plant if shipping to the U.S., where T. parvipsinus is a “pest of concern” in all states, and is a quarantinable pest in Florida.

Final Thoughts:

Lastly, if you’re going to put in all the work of getting weekly counts to monitor T. parvispinus (or any other pest!) graphing your data is a must.  We really are visual creatures. Numbers on a page can often be too abstract, and don’t provide context of what happened previously in the crop.  Plotting your data, as in Figure 5, or using scouting applications like BugVision or IPM Scoutek that can do this for you, can help you determine the value of card counts versus plant taps, develop damage thresholds, and make more informed pest management decisions.

Dr. Sarah Jandricic has been the Greenhouse Floriculture IPM Specialist for OMAFRA since 2014. She brings over 20 years of experience in Floriculture Entomology to the role. You can follow more of her greenhouse IPM information on ONFloriculture.com.