Controlling Whiteflies On Poinsettias

Table of Contents

• The 3 growers we'll call A, B and C
• Results - The poinsettias
• Results - Trap Plants
• Costs
• Other issues
• And what did we learn?

The trouble that many growers have had in controlling whitefly on their poinsettias in recent years, coupled with concerns about the newly arrived "Q-biotype" of sweetpotato whitefly, have led to increased interest in biological control. This interest has been further piqued by the recent availability of a new biocontrol agent, Eretmocerus mundus which is specific to Bemisia whiteflies (i.e. silverleaf/sweetpotato whiteflies).

A collaborative project was set up with 3 commercial growers all of whom were interested in using biocontrol. As an added twist, trap plants (plants known to be attractive to whiteflies) were used at a rate of one per 1000ft2 with the purpose of attracting the whiteflies so that larger populations would establish on them than on the poinsettias. The idea was that the biocontrol agents would then be released onto the trap plants, creating a rearing system for the biocontrols within the greenhouse.

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The 3 growers we'll call A, B and C

• Grower A had about 150,000 ft2 of poinsettias. The trap plants used were primarily eggplants and tomato plants (side by side), although there were some others used as well such as zucchini, okra and sweet potato. Cuttings were purchased unrooted and biocontrol agents were introduced after the cuttings had been on the rooting bench for about 2 weeks.
  
• Grower B had 15,000 ft2 of finished crop. Trap plants were only tomato. The poinsettia plants were purchased as rooted cuttings and biocontrols introduced after all the plants were potted up.
  
• Grower C grew 100,000 ft2 of finished crop. He purchased unrooted cuttings and began introductions when the plants were removed from the rooting benches and had been placed in the greenhouse. The trap plants used were eggplant and tomato plants side by side for comparison (Figure 1).

Figure 1. Tomato and eggplant used as trap plants in poinsettia

All 3 growers used Eretmocerus mundus for the silverleaf whitefly, but also Encarsia formosa for any greenhouse whitefly that may have been present. Greenhouse whitefly does not usually come in on poinsettia cuttings, but is often already present in the greenhouse on weeds or other plant material.

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Results - The poinsettias

The following graphs show the sticky card weekly whitefly counts for all 3 growers and the percentage of poinsettia plants each week that were found to have whitefly present (very rarely was there more than 1-2 whiteflies found per plant).

The main point of interest for Grower A is the spike in whitefly numbers on the sticky cards at week 37. Normally, these sorts of numbers would result in the biocontrol program being terminated. What kept us from abandoning ship was that despite the sticky card numbers and the fact that adults were becoming easier to find on the plants, we found very little evidence of reproduction. No egg-laying, no sign of immature whiteflies. From week 37 onwards, we were able to breathe a lot more easily as the adult numbers declined and there was still no sign of whitefly eggs, larvae or pupae on the poinsettias. This crop was shipped without any whitefly problems.

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Grower B purchased 2 varieties of poinsettia that from the outset were infested with whiteflies. The biocontrol agents did not give effective control early on, but there was some suggestion that the cuttings were treated with pesticides on the rooting bench before the grower received them. If this was the case, then the lack of parasitism makes sense. The case for pesticide residues being the culprit is supported by the fact that a few weeks after the trial was stopped and pesticides were used to control the whitefly, we actually started to see significant numbers (up to 50% ) of parasitized whitefly (the parasitism would have taken place before the pesticides were used and the wasps were then protected inside the whitefly nymph.). As the initial pesticide residues declined to levels that were not toxic to the Eretmocerus, we started to see parasitism taking place. If this is the case, it emphasizes how whitefly management by propagators can affect the biocontrol program in the end crop.

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Grower C seemed to be following the same trend as Grower A with increasing numbers of adults being seen, but initially without significant reproduction. However, at about week 40, it started to become obvious that there was colonization taking place with increasing numbers of immatures starting to be seen. The biggest difference in this crop, compared to the other growers was a lower introduction rate, about one third that of the other two growers. The pesticide history of the cuttings is also unknown, but may have played a role in the lack of success. Both these factors will be taken into account in developing a program for next year's crop.

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Results - Trap Plants

The trap plants gave mixed results. The zucchini, okra and sweet potato used by Grower A were only occasionally found infested by whiteflies and were used sparingly. Figure 3 shows the percentage of eggplants and tomatoes found to be infested with whiteflies compared to the poinsettia crop. Eggplant was by far the most attractive of the three and it would appear from the graph, that it shows great promise. The problem however, is that the same trend was seen on these plants as on the poinsettias. Adults were commonly found, but very little reproduction and colonization, which meant that the biocontrols were not able to build up on the trap plants either. However, as a monitoring tool and a release point for biocontrols they were very useful. Similar observations were made at Grower C.

The tomato trap plants used by Grower B worked very successfully with greenhouse whitefly. There was good establishment on the plants and the Encarsia established well on the whitefly colonies. However, they were ineffective in attracting large populations of silverleaf whitefly. Very few adults were found and no reproduction or establishment.

Infested trap plants in Greenhouse A showing the higher numbers caught on eggplant compared to poinsettia and tomato.

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Costs

For Greenhouses A and B, the cost for the biocontrols was between $0.10 - 0.11 per pot. Greenhouse B had some extra pesticide costs after the biocontrols were stopped, but probably less than $0.01/pot. For Greenhouse C, the cost for biocontrol agents was a little more than $0.03/pot, reflecting the lower introduction rates. Both Grower A and B felt that $0.10/pot was a reasonable cost for whitefly control if consistent, successful results could be obtained. For Grower A, this was lower than the pesticide costs for whitefly control the previous year, and with much better control achieved.

Other issues

Using eggplants as a trap plant in poinsettias has other challenges apart from whether or not it actually works as planned.

• The planting or purchase of the plants has to be scheduled so that they arrive as young plants at the same time as the poinsettia cuttings.
  
• The plants have to be managed somewhat differently from the poinsettias. Water requirements will be different. Obviously the height of the plants is very different compared to the poinsettias. The 3 growers in this trial treated their trap plants differently. One used growth regulators to keep the plants at a manageable height. This had the effect of hardening the growth, possibly making it less attractive as a whitefly egg-laying site. Another grower pinched off the top growth to control height. This worked well, but made the plant bushier, creating potential problems with the surrounding poinsettias. Also, by removing the new growth which is favoured by the whiteflies for egg-laying, it may have had a negative effect on the establishment of whitefly colonies on the trap plants. Grower B who used tomato plants only, grew them using the same technique as a commercial tomato grower, supported by polypropylene twine. Although the tomato plants were not effective in attracting Bemisia whiteflies, there were considerable numbers of greenhouse whiteflies reproducing on the plants.
  
• Perhaps the issue of most importance is that eggplant works as a trap plant for more than just whitefly. Thrips, mites, aphids all seem to have selected eggplant as one of their favourites and it is almost inevitable that they will find their way onto the trap plants at some stage during the poinsettia crop. As long as you are aware of this, it is not as big a problem as it might seem. Preventitive releases of biocontrol agents against these other pests keep them well under control and the cost is minimal. It is critical however, that these releases be done.

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So, what was the final analysis of the trials? For one grower, the results exceeded his expectations and he shipped out one of his cleanest ever crops. The other two growers, despite not maintaining biocontrol through to the end were reasonably satisfied with the end result, and were able to successfully clean up with pesticides over the last month of the crop. Both have indicated that they are willing to try the strategy again next year.

And what did we learn?

1. We need to pay more attention to introduction rates and ensure that adequate numbers are released as soon as possible
   
2. Eggplant was the most attractive trap plant used. However, we now need to better understand what is happening with whitefly reproduction on these plants
   
3. Pesticide residues are likely to play an important role in the effectiveness of biocontrol in poinsettia. Growers should be talking to propagators to better understand the pesticide history of the cuttings they buy.
   
4. Biocontrol programs in poinsettia stock plant crops may result in cuttings that will allow for more effective biocontrol in the finished crop.

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