Oblique streaktail hoverflies zip from bloom to bloom wearing a wasp costume to avoid getting eaten. But it’s all for show – they don’t even have stingers! Their fierce maggots, on the other hand, devour hundreds of insect pests. As they gorge, they help keep orange trees safe from disease.

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Entomologist Nic Irvin, at the University of California, Riverside, has found that the maggots of oblique streaktail hoverflies eat more than 400 Asian citrus psyllids in the week before they transform into pupae.

Orange growers despise Asian citrus psyllids, which spread a destructive bacterium when they feed on the sap of citrus trees.

The bacterium, Candidatus Liberibacter asiaticus, causes a disease known as citrus greening or huanglongbing. Infected citrus trees make green, bitter fruit and eventually die.

Irvin has planted alyssum, a plant with fragrant white flowers, in orange groves near Riverside. The alyssum attracts adult oblique streaktail hoverflies that feed on its pollen and then lay their eggs on orange trees under attack by Asian citrus psyllids. When hoverfly maggots hatch out of the eggs, they devour the psyllids. In one experiment, Irvin found that having alyssum near orange trees reduced by 10% the number of Asian citrus psyllids on them.

Are hoverflies good for the garden?

Yes, hoverflies help backyard gardeners too. They pollinate flowering plants. And their maggots feed on aphids, a common pest of vegetables.

How do hoverflies fly?

Hoverflies fly like tiny helicopters – they can hover, fly straight up and down and backward and turn in almost every direction, said Karin Nordström, who studies hoverflies at Flinders University in Australia.

“Seeing them come in and land on a flower, it’s really beautiful,” she said, “because it’s such a controlled landing.”

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