Using trap plants successfully

A trap plant is a plant that is different from the crop being grown and is more attractive to one or more pests than the crop itself. A trap plant can also be called an indicator plant.

Trap plants are not limited to growers who use biological controls. Trap plants can be used by growers who manage pests with traditional pesticide programs. These plants can be used as an early detection system for pest problems.

There are different trap plant systems available. The following are two common trap plants being used by commercial greenhouse growers.

Bean trap plants usually show signs of two-spotted spider mite damage much sooner than most other ornamental crops like New Guinea impatiens.Bean plants, two-spotted spider mite
One of the most commonly used trap plants is for the early detection of two-spotted spider mite (Tetranychus urticae). Since mites are not able to fly, they are often missed by scouts who are focused on using sticky cards for monitoring pest populations. Two-spotted spider mite is often not detected until there is a considerable amount of damage to a crop.

Bean plants are most commonly used as trap plants for two-spotted spider mite. Bean plants are one of the mite’s favorite food sources. The bean plants in many cases are more attractive then the ornamental crop itself.

Regular bush bean plants do very well at attracting mites. Bean seed is cheap and most ornamental plant growers have containers and growing media available so it is easy to implement a detection system.

Bean plants usually show signs of two-spotted spider mite damage much sooner than most other crops. For example, three or four adult mites on a bean leaf show the characteristic yellow speckling feeding damage on top of the leaf.

The same three to four mites on a New Guinea impatiens plant don’t show any initial signs of damage. It is not until there are many more mites on the plant that damage symptoms appear on the leaves. By that time it is much harder to control the overall population. In some cases the damage is so severe that the plants can’t be salvaged. Bean plants can be used as indicator plants in traditional pest management programs that incorporate pesticide applications.

For growers who are using biological controls, bean plants can act as both trap and banker plants. Banker plants are plants that are different from the primary crop being grown. Banker plants can assist in establishing, supporting and maintaining a population of one or more biological control agents. Predatory mites such as Phytoseiulus persimilis and Amblyseius californicus, and the predatory midge Feltiella acarisuga can be released as soon as spider mites are detected on bean plants.

Another benefit of trap plants is that for many crops the spider mites don’t migrate to surrounding plants until the bean plants show signs of over population. By the time this occurs, the bean plants have started to deteriorate from the damage.

Bean plant considerations
When using bean plants as trap plants growers need to manage the spider mite population. Otherwise the bean plants can become excellent mite producers. This is why biological control producers often use bean plants for the production of biological control agents. If bean plants are used to monitor spider mites and the plants are not managed properly, they can produce large numbers of mites. It is important to use bean plants as part of a monitoring system. The plants need to be checked on a regular basis.

Bean plants won’t work as trap plants for all ornamental crops. Thunbergia is extremely attractive to two-spotted spider mites and comparable to bean plants in their attraction. In these situations where the ornamental plant is very attractive to spider mites, growers should consider implementing preventive releases of biological control agents.

Bean plants are also very attractive to thrips and possibly aphids. Growers who are using biological control agents should release the biologicals on both the bean plants as well as on the ornamental crop being produced.

The number of bean plants placed in a greenhouse can vary. It depends on the crop and how much production space a grower is willing to give up in order to detect the spider mites earlier in the crop cycle. The earlier the detection of spider mites occurs, the better the chance of control and the less chance of plant loss.

In spider mite susceptible crops 40 bean plants per acre is considered the minimum. Better detection results usually occur when 60-80 bean plants per acre are used.

Gerbera, eggplant and whitefly
Both gerbera and poinsettia are very susceptible to whitefly. But they are not as attractive to whitefly as eggplant (Solanum melongena). Eggplants are also extremely good host plants for the reproduction of greenhouse whitefly (Trialeurodes vaporariorum).

On gerbera one female adult greenhouse whitefly produces approximately 200 eggs in its life time. On eggplant that number increases to around 800 eggs.

Both cut and potted gerbera growers are using eggplants as trap plants. In most cases the plants are part of a pest management program that includes biological controls. In these situations eggplant is a trap plant for greenhouse whitefly and a banker plant for biological control agents.

During the past two years gerbera growers, particularly those producing potted gerbera, have had a difficult time controlling thrips. Part of the problem has been associated with the development of pesticide-resistant thrips populations. It is, however, not possible to implement biological controls for thrips and to continue applying traditional pesticides for whitefly control.

Using biological controls on gerbera can only be successful if a pest management strategy is in place for all common pest problems, including whitefly, thrips, two-spotted spider mite and aphid. Eggplants placed in a gerbera crop are used first as trap plants, but very soon after whitefly is detected, the whitefly pupa are parasitized by the parasitic wasps Encarsia formosa and Eretmocerus spp. Detection of whitefly on the eggplants usually triggers the release of the whitefly predator Delphastus pusillus. This small beetle can eat up to 160 whitefly eggs per day, almost the same amount that one adult female whitefly produces in her life span on gerbera.

Both cut and potted gerbera growers are using eggplant (back left) as a trap plant.Poinsettia, eggplant and whitefly
Eggplants do a very good job of attracting greenhouse whitefly away from poinsettias. Whitefly also reproduce very well on eggplant, which can act as a banker plant for Encarsia formosa, the parasitoid for greenhouse whitefly.

For sweet potato whitefly (Bemisia tabaci, B- and Q-biotype), eggplant does not have the same effect. This whitefly is attracted to eggplant, but not to the same degree as greenhouse whitefly. Also, sweet potato whitefly does not reproduce well on eggplant.

For poinsettia crops, the number of growers using eggplants as trap and banker plants has decreased. This is especially true for growers who have been using biological control agents for a few years and have become experienced and successful without eggplants. However, for starting growers, eggplant can be an excellent educational tool to see pest and biological control agents in action.

Using eggplant is not limited to only those growers who incorporate biological controls. Eggplant can also help determine if pesticide applications are effective.

Ronald Valentin is IPM and biological control specialist, Biobest USA Inc., (519) 322-2178; RValentin@biobest.ca; www.biobest.ca.