Rearing parasitic wasps

©Aves | adobestock

Several species of aphid attack greenhouse grown vegetables, spring bedding plants and ornamentals. Some can be managed effectively with parasitic wasps using an aphid banker plant system. Banker plants are a self-contained sustainable system that supplies a non-pest prey species to support a continual source of natural enemies that disperse into the crop in search of other pests.

For the aphid banker plant system, Rhopalosiphum padi, a cereal grass pest that doesn’t attack most greenhouse plants, is raised on grain plants. Aphidius colemani is a parasite commonly used to manage several aphid pest species found in greenhouses. This parasite can be released onto the banker plant when the population of the cereal aphid is high enough to sustain the parasite population. Over time, the parasite population will increase sufficiently so that it will disperse into your crop in search of pest aphids. The banker plant will continue to produce more parasites as long as there are cereal aphids.

Most spring bedding plants and vegetables such as greenhouse-grown tomatoes and cucumbers, particularly those that are dicotyledonous plants are appropriate for this system. Do not use this system with monocotyledon plants, including Easter lilies, Alstroemeria, ornamental grasses, orchids, day lilies, iris, spring bulbs, palms, sweet corn, onions and garlic.

How many banker plants do you need?

Rates can be variable based on several factors (crop, parasites/predators supported, time of year, etc.). Ask your biocontrol supplier to find which rate is the most appropriate for your operation. Biobest recommends starting out with at least two banker plants per acre of greenhouse. For operations with less than an acre, start with two banker plants. Each week after first introducing the banker plants, add one more banker plant per greenhouse or acre. For this system to work effectively, it is critical to continue to produce and add new banker plants throughout the season, until around July 1. Then you should have enough banker plants to provide management against most of your aphid pests until late August when the system is usually abandoned.

When should I start to produce banker plants?

Banker plants are used as a preventive IPM tool. They should be put into greenhouses as soon as they are opened up and production plants are added. It takes at least three to four weeks before a banker plant is producing adult parasites, so do not wait until you see the first aphid in your crop to start your banker plants.

Materials needed

1. An extra-large (24-inch) white polypropylene hairnet/bouffant or a dedicated aphid banker cage. Cages are available commercially or you can make your own.
2. Plastic plant pots or hanging baskets (12-inch diameter)
3. Potting soil (whatever you commonly use for your crop plants)
4. Grain seed: Winter barley, oats or winter wheat. Winter wheat is ideal because the grass blade is particularly wide, which supports a larger population of aphids.
5. Starter population of bird cherry oat aphid (Rhopalosiphum padi)
6. Aphidius colemani (the parasite to release on the banker plant).

Step-by-step procedure

Start aphid banker plants six weeks before you plan to start plants in a greenhouse in the spring. It takes that long to get the system up and running and ready to use in a clean greenhouse. Banker plants should be put into the greenhouse on the same day you warm up the house and start to fill it with crop plants.

Week 1

1. When the aphid starter populations arrive, fill four plant pots with potting soil and make the soil moist (enough for up to two banker plants per greenhouse).
2. Plant one cube of the aphid starter population per pot directly into the moist potting soil.
3. Scatter half a cup of grain seed per pot directly on the moist potting soil around the starter.
4. Immediately cover each pot with a hairnet or place in containment cage. If you don’t, stray parasites can get in and destroy the cereal aphid population before it is able to build up on the grass.
5. Continue to water pots as needed, and lift up the edge of the hairnet, making sure you reseal the pot. Don’t over water. The greenhouse temperature should be 70-75°F.
6. Place an order to receive 500 A. colemani each week for three to four weeks, with the first shipment to arrive the following week.

Week 2

1. Fill four plant pots with potting soil and make the soil moist.
2. Scatter half a cup of grain seed per pot directly on the moist potting soil.
3. Immediately cover each pot with a hairnet or place in containment cage.
4. Using scissors, trim off three to five sprigs of grass blades containing a few cereal aphids from pots started week 1. Lift up the edge of the hairnet on the newly seeded pots and place these grass sprigs on the moist potting soil and grain seed. When the seeds germinate, the cereal aphids will move to the new plants. Don’t lift the hairnet until you are ready to add the infested grass so parasites don’t get into your banker plants.
5. Release 100 A. colemani wasp mummies evenly into each of the pots you started on week 1. If there are very few aphids on the banker plant, you may want to wait a week before introducing the parasites. Continue to water pots as needed. Do not over water. Hanging baskets can be put on a drip irrigation system.
6. Important note: Make sure you release the parasites after infesting week 2 pots with cereal aphids (step 4) so you don’t accidentally let parasites in those pots.

Week 3

1. Repeat steps 1-3 from week 2.
2. Follow step 4 above, taking sprigs of infested grass from week 2 pots.
3. Release 100 A. colemani wasp mummies into each of the pots you started on week 2, as described in step 5 above. You should see a few aphid mummies on the grass.
4. Continue to water pots as needed. Plants from week 3 and 4 are covered with hairnets; week 1 and 2 and are uncovered to allow parasites to disperse into the crop.

Week 4

1. Repeat steps 1-3 from week 2.
2. Follow step 4 above, taking sprigs of infested grass from week 3 pots and then infesting week 3 pots with the parasites.
3. Remove hairnet from week 1 pots then place the pots around the greenhouse. You should see parasites flying around the plants and aphid mummies with exit holes.
4. Continue to water pots as needed.

Week 5

1. Repeat steps 1-3 from week 2.
2. Follow step 4 above, taking sprigs of infested grass from week 4 pots.
3. By now parasites should be established and additional releases should not be needed.
4. Remove the hairnet from week 2 pots and place them near the week 1 pots in the greenhouses.
5. Continue to water pots as needed.

Week 6

1. Repeat steps 1-3 from week 2.
2. Follow step 4 above, taking sprigs of infested grass from week 5 pots to put in week 6 pots.
3. Remove the hairnet from plants started in week 3 and place them around the greenhouse.
4. Move week 1 pots to other greenhouses you are starting to fill with crop plants.
5. Continue to water pots as needed.

Future weeks

Continue to start new banker plants weekly until July. Before discarding old banker plants, place under a bench for 7-10 days to let remaining parasites emerge. Keep an eye out for an infestation of hyperparasites, tiny wasps that lay an egg in the A. colemani which has parasitized an aphid. Watch for a jagged and uneven parasite exit hole in the aphid mummy. Hyperparasites can wipe out your A. colemani population if allowed to reproduce. Destroy any remaining aphid banker plants at the end of the season to avoid the buildup of hyperparasites.

Margaret Skinner, Ph.D., is an extension entomologist and research professor (mskinner@uvm.edu) and Cheryl Frank Sullivan, M.S., is an IPM research technician and Ph.D. candidate (cfrank@uvm.edu) at the University of Vermont’s Entomology Research Laboratory. Ronald Valentin is the director of technical business at BioWorks (rvalentin@bioworksinc.com).