Whitefly Control in Poinsettias

Table of Contents

• Using pesticides?
• Using biocontrol?
• Final words of wisdom

It seems as though I am writing about poinsettias and whiteflies more and more often lately. Perhaps it is because of recent concerns about new strains of whitefly; perhaps because I have been involved in whitefly projects in poinsettia for the last couple of years and it is on my mind more often. Whatever the reason, given that this will be printed at the start of another poinsettia season, I think it is appropriate to refresh for growers, pest control strategies that will minimize their whitefly-induced sleepless nights this year.

I am sure that most growers are aware of the new Q biotype strain of silverleaf (also called sweetpotato) whitefly which I alluded to above, and the fact that in one year in the US it has spread from its initial find in Arizona to more than 20 states by the end of the 2005 poinsettia year. Most of that spread has occurred on poinsettia cuttings, although Q biotype has also been found on other crops. We don't know whether it found its way into Canada last year, but it would not be surprising considering the number of poinsettia cuttings that are imported. We hope to answer that question this year by collecting whiteflies and checking their species through DNA analysis.

In making control recommendations, it is probably easiest to separate them into pesticide control options and biological control. However, no matter what your choice of control strategy, it is critical to start early and monitor well (very well - make sure you know exactly what is happening in the crop every week). And don't be fooled by apparently clean crops through August and September. Often growers are surprised by a sudden increase in numbers in early-mid October that coincides with the 2nd generation of whiteflies after the cuttings were received. This 2nd generation is the one that results in the sudden increase to really noticeable numbers.

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Using pesticides?

For those growers planning to use pesticides, carefully check cuttings when they come in. Check with your cutting supplier to see if you can get a pesticide history of the cuttings. The more you can stay away from pesticides that were used in the stock, the better. The chances are good that most international propagators are placing a heavy reliance on recently registered products that are not available in Canada yet. That can be a good thing from a resistance management perspective, despite every grower's fondest wish to be able to use some of these same products.

So what pesticides are most appropriate to use? Perhaps before we answer that question, we need to distinguish between silverleaf whitefly and greenhouse whitefly. If you are not sure which species you have then it is important to find out. For greenhouse whitefly, pesticides are in general more effective than they are for silverleaf. Pesticides in the family that includes Intercept and Tristar should still work well and provide long-term control. Other registered products such as Enstar II, Dyno-Mite, Thiodan and to a lesser extent Endeavor should work well.

For silverleaf whitefly, which is much more likely to be the species that poinsettia growers are battling, it is a different story. It is probably not that helpful at this stage to worry about whether we have the B or Q biotype. Most of the resistance concerns around "Q" in the US are for products that we don't have yet anyway. For the past several years, Dyno-Mite has been consistently the most effective product against this whitefly. It has been especially effective later in the crop where it can be used safely on the bracts after they are coloured. Products such Intercept and Tristar have produced very mixed results for the last few years. I know of a number of growers who don't even bother with these any more. However, if you decide to use one of these products (and only use one, not both), make sure you follow the directions carefully to achieve the best results. If it doesn't work, don't use a second application, or increase the rate. The other products listed above are best used earlier in the crop before the whitefly populations build up to too high a level. Use indicator plants where you find whitefly to assess how well the pesticide applications work.

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Using biocontrol?

There is increasing interest among growers in the use of biological control in poinsettias. This has been fuelled by a number of developments:

• Reduced effectiveness of pesticides in controlling whitefly in recent years
  
• The emergence of Q biotype
  
• The recent availability of new and effective whitefly natural enemies
  
• Trials in Ontario in commercial and research crops that suggest it can be very effective

It is expected that a number of Ontario growers will be using biological control this year, allowing us to further refine recommendations and introduction rates. The most important natural enemy in a biocontrol program is the parasitic wasp Eretmocerus mundus which is specific to silverleaf whitefly. If greenhouse whitefly is present then another wasp Encarsia formosa is also introduced. A very recent new product is the predatory mite Amblyseius swirskii which could be used in hot spot applications. It is expected that the cost of a biocontrol program should be in the region of $0.10 per pot, which would make it competitive with pesticide control.

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Final words of wisdom

At the start of each new poinsettia crop, we are faced with the philosophy "hope for the best, expect the worst". This year is no exception. However, whatever strategy you choose, being well prepared is the best way to come out on top, no matter what you are met with.

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