Fruit growers may be able to control one of their more severe insect pests, aphids, with a remarkably benign tool: flowers. Researchers from Washington State University said their discovery is a boon for organic as well as conventional tree fruit growers.

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The researchers found that plantings of sweet alyssum 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. Sweet alyssum, a member of the Brassicacea family, which also contains broccoli, cabbage and Brussels sprouts, is native to the Mediterranean region around the Bay of Biscay, though is widely grown elsewhere in the world.   

Lessando Gontijo, who led the research project said, "The results were striking. 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, the larvae of which will feed on aphids. Hoverflies and other insects are attracted to flowers because they can find food in the form of pollen and nectar.

Researchers compared plots of apple trees with sweet alyssum to plots without flowers. While the sweet alyssum attracted hoverflies, as desired, Gontijo and colleagues found few hoverfly larvae, suggesting that the hoverflies had only a marginal effect on the aphid population.

However, the team also found a diverse community of spiders and other predatory insects in the plots with sweet alyssum and concluded that the plots grown with flowers had attracted a wider range of aphid predators. In fact, testing of insects captured in and around the flower plots used in the experiment revealed that they had indeed visited the flowers.

Study co-author Dr Betsy Beers added, "The woolly apple aphid is surprisingly damaging for an aphid, attacking tree shoots and roots. These aphids also secrete a sticky liquid called honeydew, which can coat the apples, causing much annoyance during harvest." She said aphids were previously controlled by organophosphate insecticides sprayed by orchardists, but that since these chemicals began to be phased out the pest has made a comeback.

The researchers state that the use of sweet alyssum for biological control can be easily integrated with standard orchard-management practices and should be especially appealing to organic growers, who have fewer insecticide options.