Greenhouse Grower notes: August 2010

July 31, 2010
Written by Graeme Murphy
As we start another poinsettia season, the subject of whitefly is never far from the surface of any conversation. Control programs for whitefly have evolved considerably in the 22 years I have been working in greenhouse IPM and that evolution has never been faster than in the last four to five years.

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Figure 1. Finished poinsettia crop grown using biocontrol.
PHOTO COURTESY GRAEME MURPHY, OMAFRA
 
For a number of years in the late 1990s and early 2000s, growers were becoming concerned by the inconsistent levels of control they were seeing and the possibility of resistance to registered pesticides. These concerns were reinforced in early 2005 with news of the discovery in the U.S. of Q biotype silverleaf whitefly on poinsettia, and the subsequent realization in the following years, that this pest was here to stay and likely to be a regular part of the poinsettia environment. Q biotype has also been documented in poinsettia in Canada and it is probably reasonable to assume that we will be dealing with “Q” every year from now on.

TWO MAJOR SPECIES OF WHITEFLIES FOUND AS PESTS OF POINSETTIAS

Perhaps it is worthwhile to give a quick recap on the poinsettia/whitefly relationship. There are two major species of whiteflies that can be found as pests of poinsettia – greenhouse whitefly and silverleaf (or sweet potato) whitefly.

Greenhouse whitefly (GWF) has been a pest of greenhouse production for as long as there has been greenhouse production. It has a wide host range including all the greenhouse vegetable crops, plus ornamentals such as gerbera and many spring crops. It will also survive quite happily on weeds. Although resistance to pesticides has been documented, it has not been an issue in Ontario poinsettia production. Biological control can be very effective using the parasitic wasp Encarsia formosa. It should be noted that when GWF becomes established in poinsettias in Ontario, it is almost always as a result of local whitefly populations moving into the crop after the cuttings are planted, perhaps from other crops already present in the greenhouse, or from outside sources.

Silverleaf whitefly (SWF) is also called sweet potato whitefly and is also often known just by its scientific name Bemisia. Any of these three names is likely referring to the same insect (especially if poinsettia is mentioned in the same sentence). In Ontario (and probably most of Canada and the northern U.S.), this insect is primarily a pest of poinsettia. I have seen it cross over onto other crops, but not very often. It arrives on poinsettia cuttings in the summer, and disappears from the greenhouse after the crops are shipped at the end of the year. That describes its current behaviour, but it is not to say that at some point it won’t develop a taste for other crops.

BREEDERS AND STOCK PLANT PRODUCERS WORKING HARD ON WHITEFLY CONTROL
The poinsettia breeders and stock plant producers in general do an excellent job controlling whiteflies; however, as any grower knows, it is almost impossible to eliminate pests completely. The few whiteflies that arrive on the cuttings are usually in very low numbers and present as eggs or young immature whiteflies that are difficult to detect. Unless you are looking for them very carefully, it is unlikely that they will be noticed, at least until they have developed through a couple of generations and their numbers have built up. As a result, it is often in the first half of October that I start to get calls from growers who mistakenly think that the whiteflies have come in from outside after the first frost. In fact, they have just built up to a level where they are easily noticed.

It is SWF that includes the Q biotype, which is resistant to many of our registered pesticides (and many others that are registered in the U.S. but not here). The neonicotinoids (Intercept, Tristar), pyrethroids, growth regulators such as Distance (and Talus in the U.S.) and other products such as DynoMite (Sanmite in the U.S.) are all ineffective against “Q.” The only product currently registered in Canada that is still effective (as of 2009) against this pest is Forbid (Judo in the U.S.). It is “Q” that has resulted in a greater willingness of growers to look for alternatives and a massive shift in control strategies in Ontario and elsewhere in the last few years.

GROWING NUMBER OF FARMS ARE UTILIZING BIOCONTROLS
Between 2006 and 2009, an increasing number of Ontario growers have successfully been using biocontrol for whitefly management in poinsettias. In each year, 10 to 15 growers have been part of a closely monitored project to assess the effectiveness of biocontrol. An increasing number of other growers (working with consultants from the biocontrol suppliers) have been using biocontrol outside of the monitored project.

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Figure 2. Silverleaf whitefly pupa on poinsettia leaf.

 
Each of the four years could be considered successful. In 2008, none of the monitored growers used any insecticides. In each of the other three years, all but two or three growers produced a successful poinsettia crop without the use of insecticides. Bemisia was present in all crops each year and was the major whitefly species, but in most crops was kept well in check. GWF was often present but never developed into populations of concern. One of the most interesting aspects of the project was that it was possible to predict with a fair degree of confidence (usually within the first two to three weeks) whether the control program would be successful. The presence of whitefly in the young plants in the first few weeks after sticking is the key factor.

CROP MONITORING INVOLVED ‘WHOLE PLANT OBSERVATION’
Monitoring of the crops was carried out using whole plant observation, with the underside of all leaves being carefully inspected for the presence of late instar immature whiteflies or adults. There was no monitoring with yellow sticky cards. Whiteflies caught on sticky cards are almost impossible to identify and this information is important. That is not to say that yellow sticky cards should not be used by growers. They can be very useful in understanding where to look more closely in the crop. However, in this program, whole plant monitoring provided more useful information such as:
  • Species present.
  • Life stages.
  • Location in the greenhouse.
  • Varieties affected.
  • Per cent parasitism.
  • Host feeding.
The above information was recorded as part of the project, but from a grower’s perspective, the basic monitoring information needed is the percentage of plants on which live whitefly are found. Even a single whitefly is recorded as an infested plant, so in theory, once a whitefly is detected, that plant does not need to be further inspected. At least 100 plants are inspected in every crop, although in larger crops, that number is often higher.

The time taken to monitor was similar to that needed to change and count sticky cards (especially in the all-important first few weeks when there are only a few leaves on the plant). It is important that plants from each variety being grown are monitored. Often it is noted that whitefly may be present in greater numbers on one variety from very early on. That variety remains a focal point of infestation throughout the duration of the crop.

As would be expected, low infestation levels were the norm in the first few weeks of the crop. It was not unusual to find no infested plants and the majority of crops (80 per cent during the four-year project) had less than five per cent infestation levels by the fourth week after planting. These low levels are testament to the excellent control being achieved by stock plant producers.

Almost always at the levels noted above, it was possible to predict a successful outcome of the biocontrol program. Occasionally there were instances of infestations higher than 10 per cent after four weeks (or in some cases earlier), and it was almost equally easy to predict that such crops would present difficulties. The grey area was in between those two levels. Depending on a number of factors (e.g., infestation evenly spread throughout the crop or focused heavily in one to two varieties), there were successes and failures in such cases.

MONITORING USUALLY WRAPPED UP THE FIRST WEEK OF NOVEMBER
And what levels of infestation could be expected by the time the crop was shipped? The monitoring program generally finished in the first week of November. At that time, any crop with less than 20 per cent infested plants never had to have insecticides used against whitefly. That may sound like a lot of infested plants, but in most instances that meant just one or two whiteflies on a fully grown poinsettia – and those were only found with careful inspection. To the untrained eye, whitefly were very difficult to detect. Any crop with more than 40 per cent infested plants had to have insecticides applied. Again, there is a grey area between these two levels, but from observation, most crops between 20 to 30 per cent infestation are unlikely to need whitefly control. Above 30 per cent and it becomes more problematic.

THE ‘WILD CARD’ OF PESTICIDE RESIDUES
One wild card in using biocontrol in poinsettias is the presence of pesticide residues on the cuttings, which could potentially have a devastating effect on the biocontrol agents (BCAs) introduced. Information on the pesticides that have been used on the stock plants prior to cuttings being taken can be difficult to get; however, growers should continue to ask their supplier for that information, since it may be a deciding factor in whether or not their biocontrol is successful. From the perspective of the stock plant producer, it should not be difficult to develop a pesticide program that minimizes the potential for harmful residues on the cuttings.

So after all this discussion of results, what program were the growers using?

It should be noted that at the start of the project, field technical specialists from each of the three major biocontrol producers were part of the planning process in developing recommendations for introductions. The primary BCA for control of SWF was the parasitic wasp Eretmocerus mundus introduced at 3/m2/week for the first six weeks and at
2/m2/week for the next six weeks.

At the end of that period the program was assessed to decide whether further introductions were needed. For GWF, Encarsia formosa was introduced at half the rates of E. mundus and for the same duration. In 2009, E. mundus was no longer available from two of the major producers and growers working with those companies switched to the related BCA Eretmocerus eremicus. There was no evidence of any difference in the level of control achieved with the two Eretmocerus species. The level of whitefly present on the cuttings is the prime consideration in the potential for success of biocontrol in poinsettias. And the cost? At the above introduction rates, the cost works out to about $0.08 per plant, a cost comparable to that of a pesticide program.

ONE OTHER POINT
I get frequent questions about the benefits of eggplant as a trap plant for whiteflies, the idea being that the whiteflies are drawn to the trap plant, which is then used as a release point for BCAs. These questions are based on some work done in 2006 and 2007 with some of the same growers who participated in these trials.

However, the results were not as promising as first hoped. They were excellent as a trap for GWF, attracting large numbers, and Encarsia released onto the plants resulted in a significant buildup of beneficials. However, SWF was not as attracted, and since that is the primary whitefly pest on poinsettia, they could not be considered successful. As well, they need careful management since they also attract insects such aphids and thrips.

Some growers still use them, but in general I don’t recommend them for poinsettia. (However, they can be useful for other crops where GWF is the major pest).

IN CONCLUSION
So, where are we in 2010? There are a number of Ontario growers who have now successfully used biocontrol in their poinsettia crop for four years and new growers are using the strategy successfully each year. It should be remembered, however, that these biocontrol programs were carried out under Ontario conditions and while there has been consistency over four years, growers in other regions should develop their own programs carefully and in consultation with biocontrol consultants. Diligent monitoring is critical for success, especially in the early stages of the crop. ■

Graeme Murphy is the greenhouse floriculture IPM specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs at Vineland.
• 905-562-4141, ext. 106, or This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

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