How to effiectively monitor for Thrips parvispinus
August 23, 2023 By Dr. Sarah Jandricic
In a previous edition of this magazine, we talked about which pesticides can help control or suppress Thrips parvispinus if you end up with an infestation. But how do you figure out if this invasive pest is in your facility in the first place? And, if they are there, what’s the best way to monitor their populations?
After working with this pest for the past one to two years, researchers like myself have figured out which monitoring methods are most effective so you can identify the problem early, begin a management plan, and monitor the efficacy of your control methods.
If you’re growing your own tropical crops, inspecting imported cuttings is usually your first step to detecting unwanted pests. It can also 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, cutting washes of tropical crops conducted by a commercial greenhouse and the Buitenhuis lab at the Vineland Research and Innovation Centre revealed very few adults or larvae in the wash water. We can conclude from this that most of the thrips are coming in on imported plant material as eggs.
So, how do you detect and count Thrips parvispinus eggs on your cuttings? Well… you don’t. Like other thrips species, T. parvispinus embed their eggs in leaf tissue. The eggs aren’t visible with a hand lens or even with a regular dissecting microscope, really. The best monitoring method is to put a large amount of yellow sticky cards just above the cuttings in your propagation area. 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 to 14 days are evidence of what’s coming out of the new crop, specifically. If you have a microscope, you can then cover the cards with clear cling wrap and confirm the identity your thrips using the simple thrips key for growers developed by OMAFRA and the Vineland Research and Innovation Centre. (This can be found on ONfloriculture.com, under the “Thrips Identification” tab at the top). Or, you can send them to a consultant or specialist who can do this for you.
Using a large amount of sticky cards in propagation also has the added bonus of removing thrips from the population. By our calculations, mass trapping alone was able to reduce the T. parvispinus population by almost 20 per cent over one month in a trial in mandevilla.
If you’re growing any of the tropical plant species listed in Table 1, the first sign you have T. parvispinus 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.
Damage can also vary by plant growth stage. 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 inspecting plants for damage, your other most important tools for detecting Thrips parvispinus are the same as for other thrips species: plant taps and sticky cards. Plant taps are the most useful of the two, 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:
- Thrips parvispinus is very small compared to Western flower thrips. After tapping plants onto a white pan, wait a few seconds for the thrips to recover, and then look for movement. These thrips are much more active than any other species I’ve seen, and it’s easier to hone in on them this way, instead of looking for colour/shape (especially since the males and females are different colours!).
- Once you know you definitely have T. parvispinus on your farm, and know generally what they look like, 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! But taking a few minutes with a hand lens at the beginning of sampling helped me to able to recognize who was who.
- Unlike other thrips I’ve worked with, the larvae of T. parvispinus don’t seem to tap out of the plants very well. Save some effort and only look for and count adult thrips.
- If thrips counts are high, 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. Make sure this sample size is big enough to give you an accurate picture. A rough rule of thumb is to sample five per cent of the crop.
- Make sure you are sampling from the front, back and middle of a row or bench, as T. parvispinus can have a very patchy distribution.
- With small plants (four- to six-inch pots) or low pressure, sometimes it’s better and faster to tap five small plants into one pan and then count. You can always divide by the number of plants sampled to convert this to “thrips per plant” later.
Sticky cards can also be a really 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 tool to tell you which direction your population is headed – up or down – depending on your conditions and control measures (see Fig. 5). 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. parvispinus is much easier to see and count on yellow cards.
- Remember that monitoring cards give you a picture of what happened in the crop since you first put them up – not what’s happening right now. Don’t rely on monitoring cards alone to determine the efficacy of control measures. Sometimes sprays can trigger insects to fly, which can make it appear as if populations are actually going up (see Figure 5).
- T. parvispinus seem to move in waves – from their more preferred or source host onto less preferred hosts/varieties over time. It’s important to have good coverage with your monitoring cards to figure out which parts of your farm are affected.
- 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/five 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 “Finished product”, below).
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. For flowering crops, however, T. parvispinus is strongly attracted to pollen and nectar. Counting adult thrips inside flowers at this stage will give you a sense of the pressure on the whole plant and is much faster than plant taps (and 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 no going back. But if the issue is mostly just the presence of thrips in the flowers before sale, you may want to consider a few sprays with contact insecticides to knock back thrips. A threshold of more than two to three thrips per flower is a good indicator sprays may be needed, especially if shipping to the U.S., where T. parvispinus is a “pest of concern” in all states, and a quarantinable pest in Florida.
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.
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