Biological Control Of Whitefly In Poinsettia
- Can Trap Plants Help?-

Before you start to read this, I should warn you that this is quite a bit longer than most articles, so it may not be the sort of thing that you scan quickly during coffee break. It describes whitefly biocontrol trials in poinsettia crops in 2006; trials that occupied most of my time between August and November. In keeping with the amount of time I spent in poinsettia crops, it seemed that they only way I could do the trials justice, was with a similarly lengthy article. So pace yourself, and I hope that there is enough in here to interest not only poinsettia growers, but growers of other crops who can also see potential possibilities.

About this time last year I wrote an article describing biocontrol in 3 poinsettia crops in 2005, where eggplants and tomato plants were used to try to attract whiteflies and act as a breeding site for biocontrol agents (BCAs) feeding on the whitefly. As a result of that earlier work we wanted to look further at this idea and pitched the idea to a group of poinsettia growers in June, 2006. You'll notice I have used the plural pronoun "we". The "we" includes a broad cross-section of people and companies involved in the biocontrol industry in Ontario:

• Mike Short from EcoHabitat (pest management consultants)
  
• Ann Marie Cooper from Plant Products,
  
• Margarethe Fast from Global Horticultural
  
• David Neal from Koppert Canada
  

The "we" should also include the 12 growers who agreed trust us and work with us in the 2006 season to see if we could better understand what is happening with biocontrol in this crop and what merit if any the trap plant concept has.

The idea was to establish a standardized procedure for using biocontrol in poinsettia. As such, we tried to be consistent with all growers in as many aspects of the program as possible.

Trap Plants

Eggplants were used as the trap plant of choice, based on the 2005 work which showed that they were much more effective than tomato at attracting whitefly. Nine out of the 12 growers used the trap plants which were interplanted in the poinsettia crop at 1/1,000ft2 (final crop spacing). A dwarf variety, "Baby Bell" from Stokes Seeds was used. This was more easily managed than the commercial varieties used the previous year.

Biocontrol Agents Used

The parasitic wasps Encarsia formosa (for greenhouse whitefly) and Eretmocerus mundus (for silverleaf whitefly) were the primary BCAs introduced. Two other BCAs were used only if needed:

• the parasitic wasp Eretmocerus eremicus which preys on both GWF and SWF
  
• and the recently available predatory mite Amblyseius swirskii.

Introduction Rates

Introduction rates of BCAs were based on results from the 2005 work. Both wasps were introduced as soon as possible after the poinsettias were planted. For greenhouses that purchased unrooted cuttings, this was about 1 week after they were put on the rooting bench. For growers with rooted cuttings, introductions were started the same week the plants were potted.

• Encarsia was introduced at 0.15 adults/ft2/week for 6 weeks, and reduced to 0.1/ft2/week for the second 6 weeks.
  
• For Eretmocerus, the introduction rates were twice that of the Encarsia (0.3/ft2/week for 6 weeks and 0.2/ft2 for the second 6 weeks).
  

One grower used higher numbers than those given here and one used slightly fewer.

Cost

The cost of biocontrol is critical if it is to be widely accepted and ideally shouldn't cost much more than pesticides. In speaking with a number of growers, it was felt that $0.10/ft2 was a reasonable cost to expect for pesticide control of whiteflies, so we tried to stay below that. At the introduction rates described above, the cost worked out to about $0.08/ft2, giving us some room to adjust the program if needed.

Monitoring

The crops (and the trap plants) were monitored by careful inspection every week with the number of adult whiteflies and older immatures being counted. The whole poinsettia plant was checked and with even one whitefly (adult or immature) found, it was counted as infested. For the eggplants, 3 leaves and the growing point were checked. The monitoring program was discontinued in Week 43 (the end of October), by which time we felt comfortable with the outcome of the control program.

Results

Out of the 12 crops, biocontrol was completely successful in 9, with no pesticides being used for whitefly control. Of the 3 greenhouses where some pesticides were used, 2 had to do some minor cleanup sprays in November. The third greenhouse ran into whitefly trouble in early October and reverted to pesticides at that time.

Figure 1 shows the percentage of poinsettias with whitefly in each greenhouse during Weeks 31-43. I know the graph looks a bit complex with 12 greenhouses represented by a mass of lines, but it can be simplified (in the mind anyway) by just looking at the red circle which indicates the final counts of the 9 greenhouses that were successful without using pesticides. The 20 % infestation level under which they all fell, seems to represent a level of whitefly that growers can tolerate and although this may seem like a high number of infested plants, in reality it was difficult to find whitefly in these crops. This result supports some work done at Cornell University back in the mid-1990s that found that a 10% infestation level was essentially non-detectable (they looked at just 6 leaves per plant whereas we looked at the whole plant).

 

Figure 1. Percentage of poinsettia plants infested with whitefly each week

Text equivalent of graph

What happened to the 3 greenhouses where some pesticides had to be used?

• Firstly, if we look at the two greenhouses within the yellow circle in Figure 1, it should be realized that their biocontrol programs were not exactly failures. Both had to apply some pesticides in November to clean up.
  
  • in one case, a single application of DynoMite was required although it was more for insurance purposes, as good parasitism of both GWF and SWF was being achieved
    
  • in the other, one variety, MarbleStar became quite heavily infested and a number of applications were needed to clean it up. For some reason (perhaps pesticide residues), the BCAs did not control the whitefly in that variety.
    
• For the third greenhouse, a combination of factors came into play. Perhaps the most crucial of these, demonstrated the double-edged sword of using trap plants. They must be well-maintained, and well-watered because they can support a large population of whitefly (especially GWF), and if not maintained in good health, they can become more of a problem than a solution. That is what happened.

The other point that is relevant to each of these 3 greenhouses is their starting population of whitefly. Figure 2 shows the percentage of poinsettias infested with whitefly for all 12 greenhouses in the first week of monitoring. Greenhouses 1, 2 and 3, were the ones needing some pesticide treatment at the end of the crop. Although there is not yet enough information to be certain, it may be that there is an early threshold that we can use to decide the likelihood of success for a biocontrol program in poinsettias. That is something we would like to look more closely at in 2007.

Figure 2. Percentage of infested poinsettia plants in each greenhouse in the first week.

Text equivalent of graph

What about the trap plants? Well in case you were wondering, the 3 greenhouses that did not use trap plants, were all successful in producing a poinsettia crop without using pesticides. For the growers using trap plants, there are a number of observations:

• The eggplants are extraordinarily attractive to GWF. In several greenhouses it was not uncommon to see hundreds of GWF (adults and immatures) on the eggplants (Figure 3). Surrounding (and touching) these eggplants were poinsettia plants with virtually no whiteflies. Parasitism of GWF on the eggplants was excellent and they served the function of not just a trap plant, but also a banker plant, increasing the number of Encarsia that were introduced. One word of caution, the eggplants if not monitored and maintained well, could almost be too attractive, resulting in high populations that could spill over into the poinsettias. In a couple of greenhouses, some extra BCAs including Eretmocerus eremicus and A. swirskii were used, to keep numbers from becoming too high.

 

Figure 3. GWF adults (top) and parasitized pupae on eggplants used as trap plants

• For SWF (Bemisia) the results were less encouraging. More adult SWF were found on eggplants than on the poinsettia (especially early in the crop), but there was little reproduction and immatures were seldom found. One possibility is that SWF are acclimated to poinsettia, having spent many generations on the crop at propagators (which is where our SWF populations come from each year). Under those circumstances, it may take some time for their reproductive preference to shift to eggplant.
  
• The other point to note about the trap plants, is that even though they were not shown to actually improve the success of the biocontrol programs (3 growers who did not use trap plants were equally successful), a number of the growers are likely to continue using them as a monitoring tool, an indicator plant and a very effective educational tool for employees.

How effective was whitefly control in biocontrol greenhouses compared to those who didn't use biocontrol? This was a question that we were thinking about from the start, especially since 2006 was not a particularly bad year for whitefly problems. In Week 44 after the monitoring had concluded in all the bio greenhouses, I spent a week monitoring 10 greenhouses that had relied on pesticides for control. The results can be seen in Figure 4, which is the same graph as shown in Figure 1, but with one extra week added.

Figure 4. Percentage of whitefly-infested poinsettias in crops not using biocontrol (Week 44) compared to those using biocontrol.

Text equivalent of graph

The red crosses in Week 44 show the pattern of whitefly problems in non-biocontrol greenhouses. The similarity between Week 43 in biocontrol greenhouses and Week 44 shows that control using biocontrol in 2006 was comparable to the use of pesticides.

So what are the implications of this work for growers? For growers of poinsettia where SWF is the major pest, there is still much we need to understand and it is too soon to think of recommending this to growers on a general basis. For growers of other crops where GWF is the major whitefly pest, I think there is reason to believe that this could be a very valuable strategy if used carefully.

There are obviously questions that need to be answered, some of these we hope to address in further commercial trials in 2007. Other questions however, are much better placed in the hands of researchers, working at the lab level to better understand what is actually happening. With this in mind, it is exciting that the work has attracted the attention of researchers and there are two PhD projects, one at Cornell (studying the use of eggplants against SWF and GWF in poinsettia) and the other at the University of British Columbia (looking at GWF in pepper crops), that have arisen out of this work.. Although results from these projects may take several years to find their way to growers, they promise to provide a much better understanding of the potential of trap plants as a pest management tool in greenhouse production.