Aphids parasitized by A. colemani on chrysanthemum.
Photos by Tracey Payton Miller
Cotton melon aphids on ornamental pepper.

Biological control is one of the pillars of IPM, or integrated pest management. Integrated pest management combines biological, cultural, mechanical, physical and chemical control in a manner that makes the crop environment unsuitable for arthropod pests, plant pathogens and weeds. In many cases, IPM methods are used prior to treating with pesticides. These methods may include altering the planting date of crops, removing diseased plants or plant parts, removing pests by hand, applying a biologically based pesticide, enhancing the crop habitat to attract beneficial insects or releasing parasitic or predatory insects and mites to target and control specific pests (i.e., augmentative biological control). One strategy of IPM is biological control, which exploits the relationship between an arthropod pest and its natural enemies: predators, parasitoids or pathogens. Predators include, but are not limited to, lady beetles, lacewings and rove beetles, which attack and consume target pests. Parasitoids lay eggs in or on a target insect host. The eggs hatch into larvae that devour the host and ultimately kill it. Pathogens include specific fungi, viruses, bacteria or protozoa that infect and kill target pests. The success of a biological control program hinges on enhanced reproduction, survival and/or longevity of natural enemies. Banker plants consist of a non-crop host plant harboring a natural enemy, and in most cases an alternate non-pest prey. Some banker plants only consist of a natural enemy and an alternative plant that may provide pollen, an egg-laying site or other required resource that enhances natural enemy survival and reproduction. Banker plant systems have been used mainly to target aphids, but they are also used against spider mites and thrips. In this unique system, pest populations may be reduced and maintained at densities below economic and/or aesthetic thresholds, the level at which plants would suffer significant damage from feeding and other pest activities. Banker plants provide a preemptive means of biological control via improved crop habitat, alternative food and/or alternate host insects to sustain natural enemies. Biological control works well in controlled environments because enclosed production spaces decrease the likelihood of natural enemies escaping and provide favorable climates that promote continuous reproduction of natural enemies. If using grain aphids such as bird cherry-oat aphids as alternative prey, use banker plants with caution where greater than 10 percent of the greenhouse space consists of ornamental grasses, Easter lily, spring flowering bulbs, onions, garlic, leeks and cole crops. In these growing situations, the alternative prey has the potential to become a pest. Scouting should still be performed regularly to maintain the quality and efficacy of biological control agents as well as detecting pest outbreaks as they occur.

Benefits of using biological control

Biological control can provide many benefits to growers as part of a comprehensive IPM program. First, traditional pesticide applications may be reduced, thereby minimizing the likelihood of pesticide resistance developing in a target pest population. Reduced reliance on pesticides can improve environmental quality and decrease operating costs. Biological control can be user friendly and more easily implemented than conventional control options, saving time and labor costs. Banker plants provide a continuous source of natural enemies that are maintained in the crop, reducing the potential for insect outbreaks. Compared to augmentative biological control, banker plants can reduce costs by minimizing the need to purchase replacement biological control agents. Also, banker plants effectively maintain colonies of natural enemies on site, decreasing the lag time between pest detection and receiving shipments from the biological control supplier. Implementing biological control with or without banker plants poses no threat to employees and requires no pre- or post-harvest intervals nor re-entry intervals. This permits workers immediate access to plants following release of natural enemies, thereby facilitating maintenance, shipping and sale of plants. Lastly, growers have a chance to convey their choice to use more natural methods of pest control. With the demand for organic or more sustainable practices by consumers, biological control gives the grower a chance to market plants produced in an environmentally friendly manner.

Types of banker plant systems

Many types of banker plant systems can be implemented with commercially available biological control agents. Alternate prey are typically nonpests in the greenhouse, so risk of an outbreak is minimized. In addition, non-pest prey may be reared on banker plants that are not typically present in the greenhouse, such as cereal grains. This reduces or eliminates the risk of introducing an additional pest into the greenhouse.

Wheat banker plants providing A. colemani to a pepper crop.
Photos by Tracey Payton Miller

Parasitic wasps

An example of a commercially available banker plant system includes a parasitoid and aphid prey for controlling the green peach aphid, Myzus persicae and cotton melon aphids, Aphis gossypii. These are two of the most commonly encountered pest aphids in greenhouse production systems. The system targeting these pests consists of bird cherry-oat aphids, Rhopalosiphum padi reared on winter wheat or barley, serving as alternative prey for the beneficial parasitoid, Aphidius colemani. This parasitic wasp seeks out and attacks individual aphids, parasitizing them by laying an egg within their bodies. Upon hatching, the wasp larva consumes its aphid host from within and eventually completes its development. When the larva pupates, the aphid swells in size, hardens and turns brown, resulting in what is called a “mummy.” The adult wasp eventually emerges from the mummy through a smooth, round exit hole. When pest aphids aren’t present in the greenhouse, A. colemani is sustained on the bird cherry-oat aphid. However, the parasitoid has a preference for green peach and cotton/ melon aphids over bird cherry-oat aphids. When populations of green peach aphid or melon aphid increase, A. colemani switches prey and begins attacking pest aphids within the crop. This system works best if established slightly before or at the start of the crop cycle. Different species of parasitic wasps (Table 1) that use alternate aphid prey on cereal grains may be reared in the same fashion.

Predatory mites

Predatory mites can be reared on pollen plants in banker plant systems designed to control pest mites, thrips and other pests. Banker plants used in these systems include castor bean, corn and ornamental peppers. Because mites cannot fly, banker plant foliage must overlap or touch crop foliage to facilitate their movement among infested crop plants. Colonies of predatory mites can be maintained by the grower, and banker plants should be populated with mites a few days before or at the start of the crop cycle. This may mean infesting banker plants with mites weeks or months before moving the plants into the greenhouse alongside crop plants.

An example of a banker plant set up, note the wheat banker plant alongside the pepper crop.
Photos by Tracey Payton Miller
‘Black Pearl’ ornamental pepper crop, aphid control provided by bankers in the background.

Predatory bugs

Other banker plant systems may use pollen as an alternate food source for a predatory insect or mite. Certain alternate plant cultivars have been identified that show improved reproductive benefits for natural enemies and are included in Table 1. For example, the minute pirate bug, Orius insidiosus, is a predator of western flower thrips and two-spotted spider mites. Predators provided with pollen from ornamental pepper, Capsicum annuum ‘Purple Flash,’ or common mullein, Verbascum thapsus, have increased predator longevity and reproduction compared to those that are simply purchased and released in an augmentation biological control program. Unlike the A. colemani-bird cherry oat aphid system, the minute pirate bug system must be started three to four weeks prior to the beginning of the crop cycle. The recommended density of banker plants is approximately one pepper plant per 500 square feet or one mullein plant per 1,000 square feet of production space. Even when banker plants are successfully implemented, insecticide and fungicide applications may be necessary to control non-target pests and diseases. When spraying pesticides, it is best to choose narrow-spectrum products that target the specific pest and minimize impacts on natural enemies. The method of pesticide delivery, such as a spray, dip, or drench, should be considered in a way that will least impact biological control agents. Fortunately, new pesticide formulations and modes of action may be “softer” on predators and parasitoids. However, banker plants should be moved out of the greenhouse when using any pesticide until sufficient time has passed to reintroduce the natural enemies.

Erik Rebek and Mike Schnelle contributed to this story. Tracey Payton Miller is a horticulture extension educator, Eric Rebek is an associate professor and state extension specialist, and Mike Schnelle is a professor and extension ornamentals floriculture specialist at Oklahoma State University.