How to Set Up a Banker Plant System

A banker plant system is the most elegant solution to the chicken-and-egg problem of greenhouse biological control: you need predators present before pests arrive, but predators starve and disappear if there is nothing to eat. Banker plants maintain a resident population of beneficial insects year-round by hosting a non-pest prey species that feeds the predators between pest outbreaks on your actual crops.

How Banker Plants Work

The concept is simple. You grow a non-crop plant (the banker plant) that hosts a non-pest herbivore (the alternative prey). A beneficial predator or parasitoid feeds on the alternative prey and builds a self-sustaining population on the banker plant. When pest species appear on your crops, the beneficials are already present in the greenhouse and immediately begin attacking the pests.

The critical design principle is that the alternative prey must be unable to survive on your greenhouse crops. If the alternative prey can colonize your crops, you have introduced a new pest problem rather than solving one. The classic banker plant systems are carefully designed to avoid this — the cereal aphid used as alternative prey can only survive on grass, not on broadleaf vegetables or ornamentals.

The Cereal Aphid Banker Plant System

The most widely used and best-documented system. Grow winter wheat or barley in a 6-inch pot. When the grass is 4 to 6 inches tall, introduce bird cherry-oat aphid (Rhopalosiphum padi), a cereal-specialist aphid that cannot colonize any broadleaf crop. Once the aphid colony establishes (1 to 2 weeks), introduce Aphidius colemani parasitic wasps. The wasps parasitize the cereal aphids, building a resident population on the banker plant.

When green peach aphids, melon aphids, or other crop-damaging species appear on your vegetables, the Aphidius wasps detect them immediately and begin parasitizing them. Because the wasp population is already established and reproducing, the response is immediate — there is no 7 to 10 day shipping delay while you order beneficial insects from a supplier.

The Pepper Banker Plant System

Purple Flash ornamental pepper plants produce pollen-rich flowers that sustain Orius insidiosus (minute pirate bug), a voracious predator of thrips. Orius feeds on both thrips and pollen — the pollen sustains the predator population when thrips are scarce. When thrips appear on your crops (tomatoes, peppers, ornamentals), Orius is already present and hunting.

This system is particularly valuable for growers who struggle with thrips pressure. Western flower thrips are among the most difficult greenhouse pests to control chemically (rapid resistance development), and Orius is one of the few predators that attacks both thrips larvae and adults effectively.

Setting Up and Maintaining Banker Plants

Place banker plants on a central bench or shelf where the emerging beneficials have easy access to the entire greenhouse. One or two banker plant pots per 200 square feet of greenhouse space is sufficient. Replace cereal grass every 6 to 8 weeks as it matures, yellows, and becomes less attractive to the aphid colony. Start a new pot of grass 2 weeks before removing the old one to ensure continuous aphid and wasp populations.

Do not spray banker plants or the area immediately around them. Any insecticide — including most organic options — kills the beneficial population you are cultivating. Mark banker plants clearly so that anyone working in the greenhouse (family members, helpers) knows not to disturb or spray them.

Sourcing Alternative Prey

Bird cherry-oat aphids and Aphidius colemani wasps are available from the same biological control suppliers that sell predatory mites and nematodes: Koppert, Biobest, Arbico Organics, and Nature's Good Guys. Some suppliers sell complete banker plant kits that include the grass seed, aphids, and parasitic wasps in a single package — the easiest starting point for growers new to this approach.

Advanced Banker Plant Systems

Beyond the two standard systems (cereal aphid for Aphidius wasps, pepper for Orius pirate bugs), researchers and commercial growers have developed banker plant systems for other pest-predator combinations. Mullein (Verbascum thapsus) hosts the mullein bug (Campylomma verbasci), a predator of whitefly eggs and spider mites. Castor bean plants host the whitefly Trialeurodes ricini, which sustains Macrolophus pygmaeus — a generalist predator of whitefly, spider mites, and moth eggs used in commercial tomato greenhouses.

These advanced systems are less well-documented for hobby use and may not be available from standard biological control suppliers. If you are interested in experimenting with non-standard banker plant systems, start with the two proven systems described above and add advanced combinations once you have a working biological control program established.

Banker Plants and the Broader IPM Context

Banker plants are one component of a comprehensive integrated pest management program. They work best when combined with preventive releases of predatory mites (Amblyseius cucumeris for thrips, Phytoseiulus persimilis for spider mites), nematode soil drenches for fungus gnats, and thorough scouting with yellow sticky traps. No single tool controls all pests — the strength of IPM is the layered defense where each component covers a gap left by the others. The banker plant provides year-round wasp presence for aphid control; the sachets provide year-round mite presence for thrips control; the nematodes handle soil pests; and the sticky traps tell you whether the system is working or needs reinforcement.