Banker plants are plants that are different from the primary crop being grown. They assist in establishing, supporting and maintaining a population of one or more biological control agents that are released on the banker plants. Banker plants support the biological control agents with pollen or other plant material, or a host pest that doesn’t affect the primary crop grown. In the last few years, two banker plant techniques related to aphids and thrips have received increased interest from ornamental plant growers.
The aphid banker plant system is based on introducing cereal aphids into a greenhouse on cereal plants (i.e., rye, barley, wheat). Cereal aphids serve as food source for the parasitic wasp Aphidius colemani. Photos courtesty of Biobest, CanadaAphid banker plants
The aphid banker plant system was first developed in the Netherlands for supporting biological control systems in hydroponic sweet pepper greenhouses. The system is based on introducing cereal aphids into a greenhouse on cereal plants (i.e., rye, barley, wheat). Cereal aphids only reproduce on monocotyledonous plants. Cereal aphids serve as a food source for the parasitic wasp Aphidius colemani, which reproduces on the cereal aphids.
The aphid banker plant system is an open rearing system. For it to be most effective, the banker plants need to be introduced as early in the crop as possible before aphids appear.
- Start the aphid banker plant system as early as possible in the crop cycle. Once the first banker plants are out in the greenhouse and inoculated with A. colemani, it takes another four weeks for the wasps to become reproductive.
- Introduce a minimum of one banker plant per acre per week or two every other week. The system is only successful when new banker plants are brought into the greenhouse. This supports continuous production of A. colemani, instead of having peaks and valleys in the number of wasps produced in the crop.
- Having more banker plants doesn’t hurt. Many growers use higher rates, especially in April to ensure a high population of Aphidius in May when aphids are very active.
- Do not remove older banker plants until they start to die off. Rye and barley plants can last for eight to 10 weeks.
- Monitor the banker plants and make sure they are watered. Drip irrigation works the best rather than overhead watering.
Producing banker plants Banker plants can be purchased or growers can produce their own. Producing your own allows growers to increase the number of plants that can be put into the greenhouses and keeps the banker plants system affordable. Production of banker plants needs to be done in A. colemani exclusion cages. Otherwise it is impossible to produce sufficient amounts of cereal aphids on the banker plants that are grown in-house.
Banker plant considerations
Growers who exclusively produce monocotyledonous plants, such as ornamental grasses, should not use the aphid banker plant system, as cereal aphids can become established in these crops. Experience has shown that growers who produce only a small percentage of monocotyledonous plants do not need to be concerned about using this system.
When using banker plants, it is important to identify the aphid pests to ensure good control is achieved. A. colemani is an excellent parasite for green peach aphid (Myzus persicae) and black melon aphid (Aphis gossypii). However, A. colemani does a poor job of controlling foxglove aphid (Aulacorthum solani) and potato aphid (Macrosyphum euphorbiae). For these two aphid species, use aphid parasite Aphidius ervi.
Some growers who produce their own banker plants use A. colemani excluusion cages to raise cereal aphids on cereal plants. Another benefit of the aphid banker plant system is the plants attract outside naturally occurring aphid predators such as lady bugs, lace wings, syrphid flies and gall midges — especially during the spring. These predators are attracted by the aphids on the banker plants, even if there are no aphid pests present on the greenhouse crops. In traditional pest management programs, these naturally occurring predators would not survive pesticide applications.
Pepper banker plants
Sweet pepper greenhouses worldwide are using Orius species as a predator to control thrips. This very aggressive predator, also named minute pirate bug, is an excellent hunter. It can kill up to 80 thrips per day, even though it only feeds on a few. It also doesn’t shy away from other pests such as two-spotted spider mite and moth eggs. It can feed and establish itself on plant pollen. Pepper pollen is a favorite food source.
Orius has five nymph stages and completes its life cycle from egg to adult in 30–35 days. Use in greenhouse pepper production has shown it takes two generations to get Orius established in a crop. After its release, it takes eight to 10 weeks before the Orius population is high enough to have an impact on a thrips population. Once Orius is established, the thrips numbers drop very quickly.
Orius usually lays its eggs in growing tips and buds, which can be a problem in ornamental plant production when finished ready-for-sale flowers and plants are being shipped to retailers. Under short-day conditions, Orius diapauses and becomes inactive.
Greenhouse pepper crops usually stay in production for nearly a year. Orius is introduced early in the crop, and once established, it usually remains active for the better part of the crop. Even though Orius is effective once it’s established, pepper growers still use other biological control agents (Amblyseius cu-
cumeris and Amblyseius swirskii) at the start of a crop to combat thrips. Using Orius alone, thrips populations can reach levels that cause serious damage before the Orius population is established. After Orius is established, the pepper crop itself sustains the Orius population.
The compact ‘Black Pearl’ ornamental pepper has become the Orius banker plant of choice. Since pepper plants germinate and grow slowly, plants need to be started well in advance to be used as banker plants. When sown in October, pepper plants flower in late January. Its many flowers provide pollen for Orius to feed on in the absence of thrips. Orius can be introduced as early as late February under natural day length conditions. Extension of the day length with supplemental lighting allows Orius to be released earlier.
After Orius is released on pepper plants, nymphs can usually be found three to four weeks later. Once established in the greenhouse, it is not unusual to count dozens of Orius on yellow sticky cards each week and virtually no thrips.
The system has been used with a variety of crops including cyclamen, African violets, both cut and potted mums and gerbera, alstroemeria, snapdragon and a variety of spring crops.
Ronald Valentin is IPM and biological control specialist, Biobest USA Inc., (519) 322-2178; RValentin@biobest.ca; www.biobest.ca.