I found the following information to be incredibly useful, and have since used this tactic in my gardens. They talk more about orchard management, but it works for any garden plant prone to aphids - this year I've had trouble with hops and brassica already. Bugger! The article, “Flowers Promote Aphid Suppression in Apple Orchards,” was published in the July 2013 issue of Biological Control, and is available online at http://bit.ly/165spKS. WSU entomologist William Snyder was also a co-author.
Washington State University researchers have found they can control one of fruit growers’ more severe pests -- aphids -- with a remarkably benign tool: flowers.
The researchers recently published their study in the journal Biological Control. They found that plantings of sweet alyssum, a popular annual with small, white, sweet-smelling flowers, attracted a host of spiders and predator bugs that, in turn, preyed on woolly apple aphids, a pest that growers often control with chemical sprays.
“The results were striking,” said lead researcher Lessando Gontijo, a doctoral student in the WSU Department of Entomology. “After one week aphid densities were significantly lower on trees adjacent to flowers than on control plots, and these differences were maintained for several weeks.”
To select an appropriate flower for the study, the researchers screened six candidates, including marigolds and zinnias. They chose sweet alyssum because it attracted the greatest number of hoverflies, or syrphids, which have larvae that often feed on aphids. Hoverflies and other insects are attracted to flowers because they can find food in the form of both pollen and nectar.
Alyssum attracts beneficial insects that eat and help control wooly aphids.
Researchers compared plots of apple trees with interplantings of sweet alyssum to tree plots without flowers. While the sweet alyssum attracted hoverflies as desired, Gontijo and colleagues found few hoverfly larvae, indicating that the hoverflies were not the primary biological agent of control for aphids in the test orchard.
The mystery of the disappearing aphids seemed solved when the researchers found a diverse community of spiders and predatory bugs in the plots with sweet alyssum. But was it really the flowers that attracted these aphid predators? The scientists sprayed protein markers on the sweet alyssum and later captured bugs at a distance from the flower plots. Many of the creepy-crawlies tested positive for the proteins, proving that they had visited the flowers.
Betsy Beers, WSU Research/Extension Entomologist, mentor of Gontijo and co-author on the paper, notes that “the woolly apple aphid is surprisingly damaging for an aphid, attacking tree shoots and roots. The aphids also secrete a sticky liquid called honeydew, which can coat the apples, causing much annoyance during harvest.”
The aphids were previously kept at bay when orchardists sprayed pesticides to control codling moths. Since the phase-out of organophosphate insecticides used for codling moth, the woolly apple aphid has been making a comeback in central Washington and elsewhere.
The researchers say that the use of sweet alyssum for biological control can be easily integrated with standard orchard-management practices, but should be even more appealing to organic growers who have fewer insecticide options.