To manage aphid infestations in greenhouses, growers can introduce banker plants that draw different species of aphids that don’t feed on the main crop but that, crucially, serve as attractive hosts for parasitoid wasps, which will then also parastize the pest aphids on the main crop. Banker plants can be grown in pots and transported easily to be placed adjacent to plots of crops in need of aphid management. (Photo credit: Tracey L. Payton Miller, Ph.D.)

By Edward Ricciuti

Undeniably, a relatively new and promising strategy of biological pest control called “banker plants” has a name that lends itself to painfully obvious puns. Be that as it may, you can bank on it—or, if you prefer, take it to the bank—that banker plant systems have a key role to play in integrated pest management (IPM), especially when employed in greenhouses. Also known, more formidably, as “open-rearing systems,” banker plants are mobile habitats that supply growers with predators and parasites which are natural enemies of insects that attack cultivated plants.

Ed Ricciuti

“As a biological control strategy, banker plants offer a novel non-chemical approach to managing commonly encountered pests in the greenhouse,” write the Oklahoma State University-based authors of a new paper published Monday in the open-access Journal of Iintegrated pest Management. The paper reviews existing research and examines use of banker plants as a tool for integrated pest management (IPM) in closed growing environments such as greenhouses and hoop houses and on its effectiveness against aphids, in particular.

It may sound contradictory, but one way to rid a greenhouse of aphids that suck the life out of daisies, pansies, and other ornamental flowers is to introduce more aphids: bird cherry-oat aphids (Rhopalosiphum padi), that is, whose menu is heavy on cereal grains but not ornamentals and vegetables grown in greenhouses. The paper describes how, raised in containers, or “banks,” of cereals such as oats and barley, bird cherry-oat aphids are used as an alternate host for the wasp Aphidius colemani. The tiny wasp lays its eggs in both the bird cherry-oat aphid and two other species, the green peach aphid (Myzus persicae) and melon aphid (Aphis gossypii), both of which can wreak havoc in greenhouses

The female wasp is attracted by the smell of both aphid honeydew and odiferous distress signals—i.e., scent emissions—given off by plants under attack. She injects a single egg in a host aphid per sitting but may visit hundreds of aphids in a few days. As the wasp larvae develop into adults, the host aphids become moribund, virtually mummies. Even the presence of a wasp can alarm aphids so much that they drop off plants to the ground, where they usually die.

The parasitoid wasp Aphidius colemani lays its eggs in several species of aphid, making it a useful natural enemy for deployment in integrated pest management of aphids in greenhouses. (Photo credit: David Cappaert, Bugwood.org)

Wasps for release by growers are available commercially, often shipped as adults or within the aphid mummies. Raising them in banker plants is much more efficient, providing a standing army of wasps that can be stationed in the greenhouse even before the aphids invade, a huge plus because these hard-to-see insects often go unnoticed until infestation is full blown.

The wasps that emerge from aphid mummies on the banker plants spread to the aphids on cash crops and parasitize them as well, providing a self-reproducing colony of protectors that supplies a continual flow of reinforcements. It is a much cheaper process than multiple releases of wasps to combat long-term infestations, says co-author Tracey L. Payton Miller, Ph.D. Easy to move, moreover, banker plants are compact and readily replaceable.

The use of banker plants in greenhouses is a fairly new concept in IPM, first described in the late 1970s for use on tomatoes. Banker plants can be used alone or as an effective first step in pest management with little or no negative environmental impact.  Combined with other biological control agents or pesticides, they can solve many knotty pest issues. They reduce pesticide applications, and thus they also reduce cost and pests’ development of resistance to chemical controls.

Wheat grown as a banker plant hosts bird cherry-oat aphids (Rhopalosiphum padi), which serve as a host for the parasitoid wasp Aphidius colemani. The banker plant can be placed near a crop infested with other species of aphids that A. colemani also targets. Such use of banker plants is a useful—and green—tool for integrated pest management in greenhouses. (Photo credit: Tracey L. Payton Miller, Ph.D.)

With a banker plant nearby providing a stock of parasitoid wasps, the green peach aphids (Myzus persicae) on this pepper plant begin to show signs of being parasitized. (Photo credit: Tracey L. Payton Miller, Ph.D.)

Banker plants can be placed among a crop or even in hanging baskets or on the floor. (Photo credit: Tracey L. Payton Miller, Ph.D.)

In this setup, Black Pearl ornamental pepper plants are the crop targeted by aphids, while a wheat plant (center) is the banker plant that hosts an alternative species of aphid that doesn’t feed on the pepper plants but does serve as an alternative host for parasitoid wasps—and those wasps will also target the aphids on the pepper plants. (Photo credit: Tracey L. Payton Miller, Ph.D.)

The green peach aphid is extremely resistant to many pesticides, making it a prime candidate for control by banker plants. It feeds on more than 800 plant species, including ornamentals, vegetables, and fruits. In one study, the A. colemani-R.padi system on oat banker plants reduced numbers of green peach aphids so effectively that pesticides were unnecessary to keep their density below the economic threat level.

Says Miller, “I think banker plants are the future of aphid pest control in greenhouse situations. With just a little knowledge of how to replace and install them, the grower can manage these tricky pests with no need for personal protective equipment or a waiting period before the greenhouse can be safely reentered. Banker plants, says Miller, are “a great option for growers.”

There is another advantage to using banker plants that has nothing to do with the control of pests but everything to do with sales. Growers who use banker plants, says Miller, can take advantage of the appetite of so many consumers for buying and consuming food produced naturally. Says Miller, “Banker plants are not only a useful way to manage aphids in the greenhouse, but growers can market their products as greener.”

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.