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Lessando Gontijo collects syrphid flies from potted alyssum.

Lessando Gontijo collects syrphid flies from potted alyssum.

Syrphid flies are by far the most important predator of woolly apple aphid, researchers at Washington State University have found. And a new study suggests that planting alyssum in the orchard could be a way to attract more of the beneficial flies.

Dr. Elizabeth Beers, entomologist at WSU’s Tree Fruit Research and Extension Center in Wenatchee, said data accumulated over recent years by researchers studying biological control of the woolly apple aphid showed that syrphid fly larvae were by far the most important predator. This was a surprise, she said, because a couple of decades ago, the flies were not considered a major factor in biological control of the aphid.

"Back then, we were using pesticides that were just so devastating to syrphids that they could not survive at all," she said.

Beers said it’s possible that the newer pesticides are less devastating to the flies than the broad-spectrum pesticides used in the past. Or, it may be that other predators are being negatively impacted nowadays, and syrphids are the best able to survive.

Another recent change has been the status of the woolly apple aphid as a pest. "Just in the past five years, it’s gone from nobody ever talked to me about woolly apple aphids to everybody’s calling me about woolly apple aphids," said Beers, who believes that also is related to changes in pest management programs.

"Organophosphates were pretty effective against woolly apple aphid, and they probably knocked it out at a critical point in the season. We were spraying for something else—probably leafrollers—but we knocked out the woollies. We never had to put on specific sprays."

It’s possible, she said, that the old pesticides killed both the pest and beneficial insects, whereas the new, somewhat softer pesticides are selectively killing off natural enemies but not the pests. Next year, she intends to study the nontarget effects of new pesticides on the woolly apple aphid parasite Aphelinus mali.

Woolly apple aphids, which are purple but concealed by a white, cottony substance, feed on the bark and roots of apple trees, weakening the trees and causing galls that may be infected by perennial canker. In extreme cases, the aphids can kill the roots. Woolly apple aphids produce honeydew that can drip onto the fruit, resulting in sooty mold. High populations can create unpleasant working conditions for harvest crews.

One method often used to enhance biological control of a pest is to plant cover crops that provide a source of nectar for beneficial insects. However, there’s no guarantee those extra insects will effect the pest population in the orchard.

"I’m a little bit of a cover crop skeptic," Beers said. "Most of the studies seem to be a little too general."

Beers and doctoral student Lessando Gontijo began looking for plants that would specifically attract syrphid flies. Since the adult flies feed on nectar, a flowering plant seemed like the answer—but it needed to be one that would be easy to grow, would flower over a long period, and would not be difficult to manage if planted within the orchard. It seemed likely that an annual plant would bloom longer and more profusely than a perennial.

After consulting with Master Gardeners, they chose six common annuals to test for attractiveness to syrphids: marigold, buckwheat, cosmos, mustard, zinnia, and sweet alyssum. The plants were grown in a greenhouse, potted, and placed in pots in a former apple orchard at the research center last August. The site was adjacent to orchards and a large tract of unmanaged ground with native plants.

The scientists counted visiting syrphid flies on the various plants on five different days last September and found that alyssum attracted far more flies than the other plants. Mustard was the next most attractive plant, but it was also extremely attractive to honeybees, which are much larger than syrphids and jostled them around. Zinnia and cosmos were the least attractive plants tested.

Alyssum has other attributes that make it a good candidate for the job. It is low growing, with an average height of less than 9 inches, compared with 32 inches for mustard. It survives well in hot, dry weather but can tolerate some shade also.

Beers is not sure why alyssum attracts the flies. It might have something to do with the quality of the nectar, the structure of the flower, or the density of flowers.

The researchers also captured some of the flies visiting the plants to identify the insect species. Results are not yet available. There are many species of syrphid flies whose larvae pray on aphids and other soft-bodied insects. About six have been found in orchards. One particular species, Eupeodes fumipennis, seems dominant in woolly apple aphid colonies, Beers said, so it’s important to attract the right species.

The next step will be to test alyssum in a commercial orchard and work with growers to find out how the plant can be used in an orchard without interfering with practices such as mowing or weed control. She envisions that seeds will be sown in the orchard.

The alyssum needs to be planted within the orchard so that the flies are in close proximity to the problem, rather than on the edge of the orchard, Beers believes. "If you’ve just got a tiny patch, you might not be able to get distribution of parasites or natural enemies over 100 acres of contiguous orchard, because that’s a lot of area to cover," she said.

Beers doesn’t see syrphid flies as the total answer to woolly apple aphid problems, however. "I think we should always look at biological control as supplemental," she said. "At the very least, it would be in combination with other biocontrol measures, including other predators and parasites. There may be other IPM [integrated pest management] strategies and tactics we will have to employ in order to do this."

She expects that use of alyssum will be of greatest interest to organic growers, who are accustomed to managing different cover crops.