Guardian plants: the ultimate Trojan Horses of pest management

Cew-GR-MR-2-Guardianby Sanne Kure-Jensen

Guardian plants offer simple, non-chemical pest management in field crop and greenhouse settings. Guardian plants include indicator plants, trap crops and banker plants. Their use can dramatically reduce the need for pesticides. Jack Manix, of Walker Farm in Vermont, said the key is to know your likely pests and plan ahead. Keep and consult last year’s records of pest outbreaks to anticipate when to expect repeat outbreaks of pests such as aphids, thrips, whitefly or spider mites. Establish your guardian plants before you are likely to have a pest problem.

Indicator Plants

Europeans have used indicator plants for decades in the same way American growers use yellow sticky cards. Indicator plants should be easy to grow and highly attractive to target pests. Be sure the indicator plants are not virus carriers.

Some pests seek shoots and leaves, others only flowers. Start indicator plants early enough so they are at the right growth stage when the target pest outbreak is likely. For example, spider mites are attracted to buds and new growth on bush beans.

Select varieties that will bloom during your target period. Not all crops bloom during short, winter days. Plan to reseed and replace the indicator plants at least once per season for maximum efficacy.
Position a few indicator plants throughout the crop you wish to protect. Place a few indicator plants near vents if the pests can enter the greenhouse that way. If possible, use fine screens to limit pest access. Consider fan capacity as fine screens may reduce airflow and lead to fungal problems. Fine screens may also produce more drag on the fan motors causing overheating or early fatigue.

Indicator plants should attract pests in a few days to a week. Be sure to scout daily for pests and natural pest enemies on indicator plants. Remove highly infested plants immediately.

Use peppers, Gerber daisies, sunflowers and blooming marigolds to attract thrips.

URI’s Dr. Rebecca Nelson Brown has seen success with indicator and banker plants. Growers must have the right plant species for the target insect species. She gave this example: “Petunias may work for flower thrips, but they are useless as an indicator of onion thrips, which are not attracted to the flowers.”

Trap Plants

Growers use trap plants to attract pests. The trap crop must be more attractive to the target pest than the crop growers want to protect. Growers treat the trap plants or remove and destroy the infested plants. Plan to replace the guardian plants at least once per season for maximum efficacy.

Beans, eggplants, peppers and marigolds work well as trap plants for drawing thrips from herb plants. Beans and tomatoes also attract spider mites.

Andy Radin of URI Extension did his master’s thesis on control of striped cucumber beetle using trap crops. He found that several cultivars of Cucurbita maxima (which includes Hubbard, Turk’s Turban, buttercups and kabocha squashes) are far more attractive to striped cucumber beetle than cucumbers, melons (Cucumis spp.), butternut (Cucurbita moschata), summer squashes and pumpkins (Cucurbita pepo). Radin emphasized that timing matters. He recommended putting out a few rows of sacrificial squash transplants of Cucurbita maxima prior to planting cucumber or melon. Another important part of this strategy is that once striped cucumber beetles have collected on trap crop plants, they must be killed with insecticide.

Banker Plants

Using banker plants can be simpler, cheaper and safer than spraying. These plants provide habitat for the natural enemies of target pests. Growers may purchase the natural enemies as larvae or adults and release them into banker plants.

Be sure to start the banker plants early and match the natural enemy to target pests.

Use wheat grass infested with a predatory aphid (Aphilodoletes aphidimyza) against destructive aphids to protect tomatoes. Use marigolds to host Cucumeris, which attacks thrips.

Biological Controls

“Using biologicals, growers can reduce their pesticide use as much as to 90 percent,” said Manix. Natural enemies of aphids include parasites such as Aphidius colemani. Aphidius ervi attacks foxglove or potato aphids Encarsia formosa attacks whitefly. Steinernema feltae and Cucumeris attacks thrips. Persimillis attacks two-spotted mites.

Install biological control insects gently. They may be gently shaken from a container or allowed to crawl out of a sachet placed on host plants. Fine mesh hairnets may be placed over plants to contain predatory insects while they build their numbers or to contain a pest infestation. Be sure to install host plants with predators in or near known pest ‘hot spots.’

Another method of reducing pest pressure is to thoroughly empty and clean out a greenhouse each fall. Turn on the heat for a few days to encourage pests to hatch. Then let the house freeze to kill pests and reduce the risk or severity of pest infestations the following year.

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