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

Most growers have known (probably from the first crop they ever grew), that insects and mites show obvious preferences for some crops over others, and some varieties over others. And to some extent, growers have always made use of this knowledge in various ways. For example:

• It is common for growers to use such crops or varieties as monitoring aids to provide early detection of pest problems
  
• Grouping susceptible varieties or crops together can limit the spread of a particular pest or disease to a smaller area of the greenhouse
  
• Some growers target these varieties for more rigorous pesticide or biocontrol applications than they do for others
  
• In extreme situations, a grower may simply stop growing a particular crop or variety in which a pest or disease problem has become unmanageable

All of the above are perfectly legitimate IPM strategies; an additional step in concert with other control options to reduce pest populations. However, in some ways they are simply reactive strategies to deal with a situation that has occurred because of a grower's established production practices.

A change in thinking occurs when a grower deliberately changes production practices so that he can make use of varietal or crop differences in pest susceptibility. This is where trap crops and banker plants enter the picture. Perhaps some definitions are in order.

• A trap plant (or crop) is one that is more attractive to a particular pest than the main crop. It is planted either as an aid to early detection and monitoring of a pest (or disease), or as a focus for control strategies.
  
• A banker plant is more specifically associated with biological control. Plants are brought into the greenhouse and allowed to develop pest infestations. Biological control agents are released onto the banker plants and as they reproduce and increase in numbers, they spread out into the rest of the greenhouse. It is like a mini-rearing system for the biocontrol agents. This strategy has come into popularity in recent years with their introduction by biological control producers, particularly for aphid control. In the most commonly used situation, a cereal grass (wheat, oats, barley) is sold in rockwool cubes, pre-infested with an aphid that is very specific to feeding on these types of grasses. The rockwool cube is potted up into a larger pot size or hanging basket and grows along with the rest of the crop. Aphid parasitic wasps are released onto these banker plants and as new adults emerge they fly out into the main crop. The system gives excellent control of aphids and has the advantage of continuous introduction of newly emerging wasps that have not had to undergo the trauma of being shipped half-way around the world before they reach their final destination.

Some innovative growers are now starting to look at how they can make use of the principles described here, adapting them to their own particular situation, and combining the trap plant and banker plant ideas.

The key obviously is that the trap plant used must be more attractive to the pest than the main crop is. It follows, then that for some crops, this won't work, since they seem to be the most attractive plant in the whole world for certain pests. For example, is there a more attractive plant to mites than roses?; or gerbera to whitefly?; or cineraria to aphids? Maybe not, but plants such as these (or others equally attractive) could be used as the trap plant.

There are many crops that suffer from whitefly infestations, perhaps not in a totally out-of-control way, but enough to cause on-going headaches. Crops such as alstroemeria, roses and some spring crops come to mind. A local Niagara grower of fuchsia has had to deal with whitefly problems every year he has been growing the crop. He has been working with biocontrol for several years and prior to Christmas 2003 to try the trap plant/banker plant approach. The trap plant of choice was tomato, known to be a whitefly magnet, but equally important, known to work very well in biocontrol programs. He seeded in December and released the parasitic wasp Encarsia formosa directly onto the tomatoes. It was probably not until February or early March before the plants were large enough to see results. By the time he shipped his fuchsia in May, all tomato plants were heavily infested with parasitized whitefly. The fuchsia? For the first time since he has been growing them, they left the greenhouse clean and without a pesticide being applied.

There are probably many opportunities to make use of out knowledge about the food preferences of different pests. Sometimes all it needs is a little imagination and a willingness to think outside the traditional pest management tool box.

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