The “Tipping Point” for Whitefly Control in Poinsettia

If you started out  using biological control in your Poinsettia crop this year, you’ve now reached the crucial tipping point for whitefly control decisions.

Based on the size of your whitefly population in mid-late September, your populations could end up too high by November  to effectively control.  Ultimately, this could affect sales.

So, do you keep going, and hope for the best?  Or switch horses mid-stream, and start using chemicals?  To help you determine which is the right call, keep reading.

Assessing Your Whitefly Pressure:

For those of you using biocontrol  for whitefly in poinsettia, hopefully you’ve already figured out that monitoring is your best friend when it comes to making sure your program is working.

If you’re not already doing it, now is the time to collect data on presence/absence of whitefly in your crop.  Pick up 15-20 plants per bench for at least 50% of the benches in your compartment, hold them above your head so you can see the undersides of as many leaves as possible, and score the whole plant as “With whitefly” or “No whitefly”.   Plants with “very high” whitefly numbers should also be noted, as should the variety.

Then, add up the number of infested plants and divide this by the total number of plants you sampled to get the % infested plants.  It’s often helpful to break this info down by each variety you grow. 

This technique is quick but gives you a sense of the whitefly pressure across your whole farm, including in different varieties, which can attract whitefly differently.

How Much is Too Much?

Although there’s no strict cut-off, consultants who’ve been in the business for 20+ years have developed a general rule of thumb when it comes to whitefly on Poinsettia.

• Are fewer than 20% of your pots infested mid-September? Great – then you can likely continue with your biocontrol strategy, but keep monitoring!  Pockets of higher infestation can likely be dealt with by releases of Delphastus.
• Are more than 20% of your pots infested? This is a bad sign in mid-September. If numbers per plant are generally high, AND this is affecting the major variety you grow, it’s likely time to switch to chemical controls.

If you’re riding close to the 20% line in September, then you’ll likely want to make sure you keep monitoring the crop closely.  What you’re aiming for is to keep the population level to under 20% of pots infested by Nov 1.  At these levels, that close to sale, whitefly likely won’t be a noticeable issue to customers, and the crop can be cleaned up with Dynomite if necessary.

But, if you’ve already surpassed the 20% point, AND it’s not just a handful of whitefly per plant, AND the issue is in your major variety (not just relegated to a few benches of colours), then you may want to think about insecticide drenches to bring the number of whitefly nymphs down.  Otherwise, whitefly levels come November could be unmanageable.

Success has been seen the last few years using the following chemicals to manage whitefly nymphs:

• Distance (pyriproxifen; IRAC Group 7)
• Kontos (spirotetremat; IRAC Group 23)
• Beleaf (flonicamid; IRAC group 9C): Note that this chemical may cause phytotoxicity in poinsettia if applied more than 1x per crop as a drench

The Caveat (There’s Always a Caveat…)

So far this year, a few growers using a chemical-only approach have seen their whitefly numbers rise recently, despite waiting to apply them until late August  (which usually guarantees the resistant “Q” biotype has time to switch to the pesticide susceptible “B” biotype).  There could be many reasons for this: B biotype may have finally developed resistance to these chemicals; or, the whitefly population is still predominately Q due to high starting levels; or, we just started with higher than normal whitefly numbers.

The point is that this year, more than most, you’ll want to evaluate your whitefly numbers closely before deciding to switch to chemicals, as they may not be as effective as in previous years.  And, once you spray (even as a drench), it could be hard to go back to biocontrol due to pesticide residues.

Good luck with your whitefly control, and hopefully I’ll be seeing a few of you at the Canadian Greenhouse Conference Oct 2-4!