More About Trap/Banker Plants - Poinsettia and Tomato - an odd partnership

Last year I wrote an article about the use of trap plants and banker plants in biocontrol programs. Just to quickly refresh:

• trap plants are more attractive to certain pests and are used to attract insect pests away from the main crop
  
• banker plants are used to develop a rearing system for biocontrol agents in the greenhouse.

Combining these two functions into one system was the subject of the previous article, and it used the example of tomato plants being used as an aid to the control of whitefly in fuchsia. The whitefly found the tomato plants more attractive than fuchsia; and the biocontrol agents were released just onto the tomatoes where they reproduced and multiplied, providing excellent control of whitefly throughout the whole greenhouse.

This concept has been taken one step further by Mike Short of Eco Habitat, a pest management consulting firm, working with a Niagara poinsettia grower. They wanted to see if the trap plant idea (also using tomato) would work in a crop of poinsettia. The big unknown was which of these two plants would be the more attractive. We know that both crops can suffer from heavy infestations of whitefly. But what if the whiteflies are given a choice? We are lucky to have innovative growers, willing to take a risk to help answer this type of question.

The grower in question had tried biocontrol previously in poinsettia crops without long term success. Infested cuttings and early season pressure from outdoors caused rapid buildups of whitefly populations that often led to inefficient parasitism. Eventually chemical sprays were used to control increasing populations of whitefly. If biocontrol was to succeed, then clearly a method was needed to establish parasitic wasp populations early in the season without increasing introduction rates so that control became uneconomic. They used the work done with fuchsia and tomato as the starting point for the 2004 poinsettia crop.

What did they do?

• Poinsettia cuttings were rooted in early August and were relatively free of whitefly when they arrived
  
• At the same time, beefsteak tomato plants were seeded (in hindsight, tomato plants should be seeded a couple of weeks before the poinsettia cuttings arrive, or alternatively seedlings purchased from a commercial propagator)
  
• When the poinsettia crop was rooted and placed in the main production area (10,000 m2), the tomato plants were potted into 15 cm pots and positioned approx. 10 m spacing (1 plant per 100 m2). This was in week 38. The tomatoes were positioned beside greenhouse support posts and tied back to these posts as they grew. (If not tied to posts, plants would have to be pinched on a regular basis and would bush out, shading surrounding poinsettias).
  
• Plants received the same fertilizer treatment as the poinsettias, but required extra (hand) watering above that they received from the trough irrigation system.
  
• Biocontrol introductions started with an initial introduction of the parasitic wasp Eretmocerus eremicus at a rate of 1/m2 (this was done throughout the poinsettia crop in potential hot spots such as warmer areas, along walls etc.). From week 41 onwards another parasitic wasp, Encarsia formosa was released just on the tomato plants at a rate of 100 per plant per week.
  
• The crop was scouted weekly and recommendations made to the grower.

What happened?

• An infestation of greenhouse whitefly started on the tomatoes within weeks of setting out the plants and parasitism by Encarsia soon followed.
  
• Most poinsettia plants showed no significant whitefly problems
  
• In one small area where whitefly did move onto poinsettia, the Encarsia followed, parasitism was evident and complete control of the outbreak was achieved. Encarsia were found at points mid-way between the tomato plants, so they obviously distribute themselves well at this density of plants
  
• No chemical applications for whitefly were applied to the crop

What did this cost?

Mike Short kept some interesting data on the costs of this control strategy and compared them to the previous year's crop when conventional pesticide control had been used.

Savings were made in a number of areas. Reduced chemical costs is the obvious one. However, in 2003, the grower had also used large quantities of sticky tape to control adult whiteflies. He did not have to use sticky tape in 2004. And there were labour savings. The other point to note, is that the system that they used was also considerably cheaper than if they had used biocontrol in the conventional way, distributing the biocontrol agents throughout the whole poinsettia crop. By concentrating just on 100 tomato plants, labour costs and introduction rates were greatly reduced.

Is this the answer then to whitefly control in poinsettia?

Before we get too carried away, there are some words of caution.

• 2004 was a year of low outdoor pest pressure. Temperatures were moderate and there was little build up of whitefly populations outside. What would happen if we had larger numbers of whitefly moving in through the vents?
  
• The predominant whitefly species on the crop was greenhouse whitefly. That in itself was unusual. Silverleaf (a.k.a. sweet potato) whitefly is usually the more common species on poinsettia, and it is more difficult to control biologically. Encarsia which was used so successfully in this trial, does not control silverleaf whitefly well.
  
• Greenhouse whitefly worked beautifully with tomato as the trap plant. Will tomato work as well with silverleaf whitefly?
  
• Eretmocerus is an excellent natural enemy of silverleaf whitefly, but will it establish as well on tomato as the Encarsia did (if the whitefly are attracted to tomato in the first place)?
  
• Questions, questions, questions! Research often finishes up asking more questions than it answers, but bit by bit, we get to the bottom of it.

So, is it premature to plan for this type of approach in your poinsettia crop in 2005? Perhaps not, but I would still be treating it as your own research trial. Seek help from someone experienced in biocontrol. If you have the option of trying it in a small area of your greenhouse that can be isolated from the rest of the crop, then that is always a good means of improving your comfort level. Working with stock plants where you have a little more margin for error, is also a good idea. Just remember, no matter how you control whiteflies, monitor the crop very closely. Even if things do not work as well as you'd like, you will be able to make adjustments before things gets totally out of hand.

More work will be done by Mike and his colleagues this year, and every trial in every greenhouse adds a little more to our bank of information.

• Trap Crops and Banker Plants - thinking outside the pest management tool box