Biocontrol of Bemisia: 7 Things to Consider Before You Turn to Chemicals.

It’s that time of year again where it’s “do or die” time in Poinsettia: either your biocontrol program is on track, or it’s time to see if pesticides work.

Here’s what to look for in your crop to help you make the decision, including acceptable infestation levels and evidence of successful biocontrol.

This post also contains information on chemical options for Bemisia whitefly in 2019.

Previous posts and magazine articles have covered how to effectively scout for Bemisia and use of a cut-off rate based on percent infestation to help with Bemisia control decisions.

But infestation rate isn’t the only thing that matters.  At this point in time, it’s important to get more information from your crop before you think about abandoning biocontrol. (Remember, it’s a crap shoot each year if pesticides will actually work or not!  See this post).  Here are details on 7 things should look at before you decide to spray.

(And, as always, a big THANK YOU to the growers, consultants and owners that are willing to share what they’re seeing in the field so we all can benefit).

1.Which varieties are affected

It’s common to see more whitefly pressure in your colours (white, pinks) compared to your reds.  These hot spots can potentially trickle over into other poinsettia varieties, but may still be controllable with releases of Delphastus.  It’s certainly worth trying hot-spot releases of predators before turning to pesticides, which may leave you with no recourse but the pesticide treadmill from now until early December.

However, >20% plants infested in your main crop (reds) is generally cause for concern at this point.  But don’t let this be your only deciding factor.  (It’s only a rough guideline, after all). Make sure to take the rest of this list into consideration first.

2. Bemisia Life Stages Present

Are you seeing mostly adults flying around? This likely means that your whitefly egg predators (like Delphastus and A. swirskii) and parasitic wasps (that parasitize and host-feed on nymphs) are working.  Remaining adults may be from a previous generation, as whitefly adults can live over 30 days.  Conversely, lots of nymphs and pupae, along with adults, may mean your biocontrol program isn’t keeping up.

Make sure to make notes of things like this when you’re scouting each compartment/bay to help bring more decision-making power to your presence/absence sampling.

3. Bemisia numbers

Although I’ve been stressing using a quick-and-dirty presence/absence sampling in order to monitor your whole crop effectively, now’s the time to actually start recording some Bemisia numbers per plant.  This will help you get a picture of what “presence” actually means in your crop. Remember, the 20% infestation “cut off” is just a guideline.  It highly depends on WHAT that infestation looks like.

Are you seeing just one or two adult whitefly present per plant, and leaves with <10 immature stages here and there? Going into the period of poinsettia cooling, when whitefly development will slow down, this kind of pressure is acceptable.

But if you’re seeing LOTS of infested leaves, or leaves with LOTS of immatures (e.g. >25 nymphs/pupae), that’s cause for concern.

4. Which pot sizes are affected

Because of the amount of plant material involved, seeing 2 or 3 adults (and possibly a few immature-infested leaves) per 8-inch plant is inevitable, and generally tolerated by buyers.  It is certainly less concerning than this same trend on 4 or 6 inch plants.  Expect higher percentages of “infested” plants using presence/absence sampling in 8 inch and larger pots, and spend more time characterizing the infestation.

You can also consider changing your presence/absence sampling to whitefly per stem (usually 3 per 8 inch pot), instead of per pot, as that may give you a better picture.

5. Evidence that Biocontrol is Working

The other critical thing to look for this time of year is successful parasitism or feeding on whitefly by the natural enemies you’ve released.

Specifically, you’re looking for at least 40-50% of the whitefly immatures you encounter to be either a) parasitized or b) fed on.  Many IPM scouts are trained to look at parasitism, but host feeding by both Encarsia and Eretmocerus on whitefly nymphs plays an equally important role, especially in bicontrol programs relying only on these parasitoids.

Here, I’ve taken pictures through a hand lens (15x) in the field to show you exactly what you should be seeing when examining leaves.

• Evidence of host feeding:  Dead nymphs/pupae of whitefly resulting from host feeding by Encarsia/Eretmocerus appear yellowish brown and are dried-out looking.  The edges often curl up away from the leaves. Successfully hatched out whitefly pupae, on the other hand, leave clear pupal cases behind.

  

• Parasitism by Eretmocerus: As these tiny wasps are entirely yellow, their parasitized whitefly hosts also appear as yellow.  To me, they look sort of like aphid mummies: i.e. puffy yellow versions of whitefly pupae.  As with aphid mummies, you can easily see round exit holes once the Eretmocerus adults emerge.  Conversely, healthy whitefly pupae will be more white-yellow, less puffy and have red eyespots visible through the pupal case.

  

• Parasitism by Encarsia:  Although greenhouse whitefly parasitized by Encarsia appear black, Bemisia parasitised by Encarsia appear brown.  This is probably because Bemisia immatures are yellow, while greenhouse whitefly nymphs/puape are more clear/white. When they are ready to hatch out, you’ll  be able to see the more distinct yellow abdomen and black head/thorax of the wasps through the whitefly case (see below). As with Eretmocerus parasitism and host feeding, whitefly parasitized by Encarsia tend to occur in patches.

  

Because of their distinct brown colour, whitefly parasitized by Encarsia are also fairly distinguishable on whole leaves without a hand lens (see below).  You’ll definitely need to use a hand lens to figure out Eretmocerus parasitism rates, though.

If you’re not seeing these 3 things, or if parasitism rates appear low (<30%), then your bios may not be able to play catch up over this next month to prevent credits.

6.  Testing Pesticides in Hot Spots

You may remember this post on recent U.S. efficacy tests of pesticides against Bemisia whitefly in poinsettia.  Although efficacy studies can point us in the right direction, trials with Bemisia, specifically, need to be taken with a grain of salt.

This is because a) efficacy tests don’t always accurately represent the mix of B and Q Bemisia species we usually see in real life and b) pesticide results are going to vary year by year depending on what chemicals are used at the producer end (i.e. what your Bemisia have already been exposed to and how often they were exposed to it).

So, before you decide to apply something that has toxic effects on your whitefly parasitoids or Delphastus to your whole farm, the better bet is to trial it first in your bad areas now, while you still have enough time to play around before sale.  This could be in a specific compartment, a specific variety, or a specific bench or two that seem to be worse than others.

To see how the chemical performs, you’ll want to a) flag 5 plants with particularly bad whitefly issues and b) perform more accurate counts of whitefly numbers and life stages on these plants.  Compare numbers before and after application, giving at least a week for drench products to fully translocate through the plant under these temperatures before the “after” assessment. (Products like Beleaf and Versys, that work through feeding cessation in insects, may need up to 2 weeks to show results).  Products that kill through contact (instead of feeding) can be assessed 24-48 h after application.

Currently, there are few definitive reports of which pesticides are working (or not working) against Bemisia whitefly.   The general exception is Altus (flupyridifurone), since it’s reported to be more effective if applied earlier in the crop.  Not only did it perform poorly in U.S. efficacy trials, but it also doesn’t appear to be working for several Ontario growers this year either.  Older chemistries like Dynomite (pyridiben) and Avid (abamectin) also may not be working. Other newer chemistries, like Versys (afidopyripen) and/or Beleaf (flonicamid) may still provide control.

7. Looking at Whole Picture

Once you’ve done 1-6, you’ll want to put all the information together for your farm.  For example, you may see something like the following:

1. Varieties effected: 30% average infestation rate in colours across all sizes; ~20% in the reds.
2. Bemisia life stages present: Most infested plants with 1-2 adults (especially in 8 inch), but few plants with nymphs/pupae
3. Bemisia numbers: Infested leaves in reds generally <10 immature whitefly/leaf; some infested leaves in the colours with 20-30 immatures/leaf
4. Pot sizes affected: 80% infestation rate in 8 inch (but mostly 2-3 adults per plant); ~20% in 6 and 4 inch
5. Evidence of Biocontrol: ~40% parasitism/host feeding on heavily infested leaves
6. Pesticide trials: Small trial of colours demonstrated that Dynomite did not work; trying Beleaf next.

Although individual metrics may be concerning (i.e. general % infestation rates,  rates in the 8 inch), put together, there’s a lot of evidence that biocontrol is still working for this hypothetical grower (life stages present, parasitism rates).  In this case, I would still delay spraying.  Only by figuring out your own metrics will you be able to decide if spraying is right for you.