It’s that time of year again where you have to make a choice with your poinsettia. Do you stay the course with natural enemies, or abandon your bio program and spray for whitefly? And spray with WHAT?
This post has tips on how to test pesticides NOW, so that when it comes down to the wire, you’ll know what is – and isn’t – working.
The Critical “Tipping Point” In Poinsettia:
Some of you may be familiar with the “whitefly tipping point” I’ve referred to in several blog posts and articles (see this issue of Greenhouse Canada Magazine). Essentially, this is when growers have to decide whether or not to switch to chemicals for whitefly control, so they can end up with a clean crop. Switching to pesticides later than the last week of September/first week of October may mean you don’t have time to get on top of your whitefly population if repeated applications are needed.
What’s Up with Whitefly in 2020?
This year has been a real mixed bag. Some people have little whitefly pressure. Others had very high starting populations, e.g. up to 60% plants infested with whitefly in August, even WITH dipping cuttings. This variation seemed to be within the same suppliers, but cuttings may have come from different source farms.
High starting pressure may mean reaching the decision to spray earlier (i.e. THIS week). Ultimately, your decision will have to be based on several factors particular to your farm, NOT just on the rough metric of percent infestation. Questions to ask include:
- Is your main red variety heavily affected? In all areas or just one?
- Are you seeing evidence of your biocontrol program working? (Again, in all areas?)
- Are you seeing whitefly colonization on new growth, or is it on old growth only (original cutting leaves)?
- Do most plants have at least 1 densely colonized leaf? Or are there only 1 or 2 adults or pupae here and there on each plant?
Ideally, to stay the course for biocontrol, we’d like to see evidence that a) the parsitoids are working, b) there are few dense clusters of whitefly colonies c) generally low whitefly density on new growth, d) less pressure in your main crop than in the colours, and e) less than 20% of the total crop infested (see here for more details on how to determine this).
If 3 out of 5 of these are NOT true for your crop, then it may be time to seriously consider pesticide application.
How To Know Which Pesticides Will Work?
This is always the $64,000 question, as Bemisia whitefly pesticide resistance can vary year-to-year, depending on which chemicals are used at the producer end. Although recent efficacy tests from the U.S. can give us clues as to what to try, they aren’t guaranteed. One thing you can do to be MORE sure is a quick pesticide trial. Here’s the basics of set up and sampling:
1.Choose benches of your most heavily infested plants – usually this will be your colours.
2. Test 1 pesticide (or pesticide combo) per bench or per half bench. You’ll need a lot of plants to look at to see what’s working if whitefly numbers are low-to-moderate, since whitefly can have a patchy distribution. Make sure to label, label, label, so you’re sure which treatment is which.
3. Include a bench or half-bench of control plants – i.e. a “do nothing” treatment, so you can see how much your whitefly numbers would have changed (or NOT changed) had you not applied pesticides.
4. Measure whitelfly pressure BEFORE you spray to get a baseline. It’s best to look at TWO life stages of whitefly, to really get a sense of control. A.) Look at adult whitefly per plant – normally this would be a bad metric for a flying insect, but Bemisia whitefly move around very little. B) Look at large nymphs. Pupae are easier to see, but are a life-stage that are less affected by pesticides, in general. You’ll need a hand lens of 15x to see if nymphs are healthy or dead looking.
Graeme Murphy showing how poinsettia monitoring is done.
5. Monitoring adult whitefly: Looking at only a few plants WON’T give you a good enough picture when it comes to adult whitefly. This is because adult whitefly per plant tend to be low (<10) under a moderate infestation. Instead, you want to get a sense of what’s happening on the WHOLE bench. To do this, scout AT LEAST 50 plants per bench, and count the adult whitefly by raising the plant and turning it slowly so you can see the undersides of most leaves. Then check the growing point a little more carefully, since adult whitefly tend to lay their eggs there. Overall, this is a MUCH faster approach than flipping over each leaf, but still gives you a fairly accurate count. Use this method BEFORE you apply your treatments, as well as 4-5 days after to look at efficacy if you’re using a drench (2 days later should be enough if you’re using a foliar spray with an insecticide that has contact activity). If you have time, take adult counts again 10-14 days after pesticide application, since some products (e.g. a Beleaf drench), can take a while to work in poinsettia.
6. To determine efficacy of pesticide on nymphs, you’ll want to have flagged heavily infested leaves on individual plants with flagging tape in Step 3 (numbering them as you go with a sharpie) and counted the healthy nymphs per flagged leaf or per plant. Try to find at least 5 heavily-infested leaves to flag per treatment. Come back to these exact same plants after 4 days and see how many nymphs have changed from white/yellow (healthy), to shriveled, flat and brown (dead). You can check them again up to 8 days after spray – after that, they will have most likely developed into pupae.
8. Calculate the percentage difference. This will help you rank how well your various pesticides are working. To do this, use the following formula:
First: work out the difference between the two numbers you are comparing.
Difference = New Number – Original Number
Then: divide the difference by the original number and multiply the answer by 100.
% Difference = Difference ÷ Original Number × 100.
If your answer is a negative number, then this is a percentage decrease. If it’s positive, then it’s a percentage increase.
Below are some example data, for both adults and nymphs. (NOTE: These are NOT actual data, and are not to be treated as pesticide recommendations). Red text indicates whitefly numbers are increasing, while green text indicates whitefly numbers are significantly dropping. Black indicates little change.
|Adult Monitoring – Pesticide Trial||Total WF Adults (4 d)||Total WF Adults (14 d)|
|Bench||Variety||Treatment||Plants Sampled||Before Spray||After Spray||% Diff||Before Spray||After Spray||% Diff|
And here’s some example of nymphal data:
|Nymph Monitoring – Pesticide Trial||# of healthy nymphs on 5 flagged Leaves||# of healthy nymphs on 5 flagged Leaves|
|Bench||Variety||Treatment||Before Spray||4 d After Spray||% Diff||Before Spray||8 d After Spray||% Diff|
Based on these (fake) data, it appears that bios are still contributing to whitefly control somewhat, as the numbers of healthy nymphs went down. However, numbers of adults were still increasing, suggesting a pesticide application is necessary to keep on top of new colonies being laid.. Here, I would use Ference as my first choice, since it knocked out the highest number of adults and nymphs. Also, Ference is thought to be soft on parasitic wasps, so you can continue to receive benefits from your bio program even after spray. Kontos/Beleaf looks like it could be a good choice for a second application (if needed), and also has less chance of knocking out bios than Intercept.
You can download the example data spreadsheet (complete with formulas) here: example pesticide trial poinsettia
Caveats (because there’s always a catch):
Unfortunately, with a single-application pesticide trial like this, all you’re REALLY doing is seeing how well the more-susceptible chunk of your whitefly population responds. Remember, BOTH B and Q biotypes of Bemisia have the capacity for resistance. A second application of the same chemical to the same crop might give you no further kill.
This is why testing several chemicals at once is a good idea – you’ll know which pesticides to include in a rotation program – your best bet for taking care of Bemisia whitefly if repeated sprays are needed.
Lastly, it’s going to be impossible to determine if a brown-shriveled whitefly nymph died from host-feeding or pesticides (see the picture near the top of this post). This is why it’s good to always include a control bench – so you can monitor how much your bio program is still playing into pest reduction. If a large reduction in healthy whitefly nymphs happens quickly in your chemical treatments, and you don’t see the same drop in the control bench, you can assume what you’re seeing is due to pesticides.