The whitefly "tipping point” and testing pesticides in poinsettia

Essentially, the tipping point 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.

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 whitefly 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.

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