The key to a successful biological control program is reducing pest insects while maintaining the beneficial insect population. As the beneficial insects are successful in parasitizing or preying on pest insects, their food supply becomes limited, which results in the plummet of the beneficial insect population. How can a grower prevent this phenomenon? Providing beneficial insects with alternative food sources with banker plants is one common method to maintaining the beneficial insect population. It also helps to reduce the cost and inconvenience of buying beneficial insects each week.

A banker plant provides alternative food source for your beneficial insects and, in some cases, a reproduction site for them. In an ornamental cropping system, there are a few commonly used banker plants: oat and wheat for aphids, ornamental peppers or castor beans for thrips, and mullein for whitefly (Photo 1). Oat or wheat grass is commonly used as a banker plant system for aphids. These cereal plants are hosts to the bird cherry oat aphid (Rhopalosiphon padi), which will not move to your ornamental crop unless you are growing ornamental grasses or other monocots. Bird cherry oat aphids are a supplemental food source for Aphidius colemani, a parasitoid for green peach and black melon aphids.

In order for this system to work, you will need to introduce new cereal plants infested with the bird cherry oat aphids amongst your crop every other week. To do so, you could routinely buy infested cereal plants from a supplier or learn to maintain the bird cherry oat aphid population on plants that you grow yourself in a cooler or cages (Photos 2-3). The cooler or cages protect the bird cherry oat aphids from parasitoids in the greenhouse. Michigan State University Extension recommends you initially have a minimum of two aphid banker plants per acre in the beginning of the production season and then increase to a minimum of one per acre.

 
Photos 2-3. Growers can rear bird cherry oat aphids on cereal banker plants in a cooler (left) and then disperse then amongst their crop every other week (right) to provide a supplemental food source for Aphidius colemani, a parasitoid for green peach and black melon aphid. Photo credits: Heidi Wollaeger, MSU Extension.

A common banker plant for thrips is ‘Purple Flash’ pepper plants. The pollen of the ornamental pepper plants is a food source for the beneficial insect the minute pirate bug (Orius insidiosus). Orius banker plants should be started extremely early as it takes around two months to build up an adequate population. We recommend about 100 pepper banker plants per acre of production space. In order to minimize the thrips population on your pepper banker plants, place an A. cucumeris sachet on each pepper plant. One challenge with Orius is they become less effective biological control agents as temperatures dip below 65 degrees Fahrenheit or the daylight is shorter than 11 to 13 hours because Orius will go into diapause. Diapause is a resting stage where insects remain inactive until a critical temperature or an adequately long day. An alternative to ornamental peppers, castor bean plants are also used as banker plants for thrips. However, they are less commonly used because their fruit (castor beans) contain one of the most poisonous naturally-occurring compounds known: ricin.

Mullein is another commonly used banker plant in commercial production systems. It is used in whitefly biological control and often implemented in poinsettia or greenhouse tomato or cucumber cropping systems. The mirid bug (Dicyphus hesperus) feeds on the pollen of the mullein plants. Similar to Orius, Dicyphus also requires about two months to build up a sufficient population. Forty mullein plants should be placed per acre of production space.

Using banker plants requires a grower diligently scouting them and the crop for both beneficial and pest insects. Growers should never spray banker plants with an insecticide as it will kill all beneficial insects. In “Applying pollen as an alternative food source for predatory mites,” I will address how applying pollen to crops as an alternative food source for predatory mites is also effective in some cropping systems.

If you are a Michigan grower interested in learning more about these biological control strategies, consider attending the Ontario Bus Tour of Floriculture Greenhouse Insect Biocontrol Programs, Oct. 28-30, 2015. More information will be available mid-July on the MSU Extension Floriculture Events page, and registration will open to growers on Aug. 1, 2015. On this tour, we will visit four wholesale greenhouses, one retail garden center and the Vineland Research Station. While the focus of our tour will be implementing an insect biological control program, others topics will include greenhouse automation to save on labor expenses, retail marketing, various greenhouse structures and other production practices.