Researchers at Washington State University have found that flowers attract predatory bugs and spiders whose offspring feed on woolly apple aphids, helping to rid fruit trees of a significant pest without using pesticides.
“The attraction to flowers is by the adult bugs probably due to an abundance of nectar and pollen,” said Elizabeth Beers, professor of entomology at the Tree Fruit Research and Extension Center.
“They are not the ones feeding on aphids. We are hoping that they will lay eggs in aphid colonies after they are well fed (as) they need nutrition from nectar and pollen to lay eggs. The eggs hatch into the larvae that will feed on the aphids.”
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The woolly apple aphid is native to North America and found in all apple growing regions in Canada and the United States. It feeds mainly on apple but also attacks pear, quince, mountain ash and hawthorn.
Cotton-white fluffy colonies are found on new season’s growth, or unhealed pruning wounds. Underground, they are found throughout the root system which, in severe cases, can stunt or kill young trees, although mature trees can manage to weather infestations.
The aphids can cause mould on the fruit and galls on the plant parts. They can also transmit apple canker. They secrete a sticky residue called honeydew that drips onto fruit and leaves, causing russet spots on the fruit itself.
The stickiness is a nuisance to harvesters and the substance stains clothing on contact.
The research team at first focused on six flowering plants for the study.
“The flowers we tested were based on reports from the scientific literature as being attractive to syrphids,” said Beers. “They were mustard, buckwheat, cosmos, zinnia, alyssum, and marigold.”
The precise attractant is still unknown but, according to Beers, it could be a chemical or it could be a long bloom period with lots of nectar or pollen.
In the report, Flowers promote aphid suppression in apple orchards, recently published in the journal Biological Control, Beers and co-author Lessando Gontijo wrote that predators and parasitoids often benefit from feeding on nectar and/or pollen to the extent that there is direct value in planting flowering plants close to vulnerable crops to stimulate biological control.
The team selected sweet alyssum flowers because aphid predators appeared particularly attracted to it.
Sweet alyssum is a low-growing, cool season plant with tiny flowers that give a subtle, sweet scent. It grows from early spring through to the fall and even winter in frost-free climates. It is a great cover plant and very popular with gardeners.
In two field experiments, researchers placed apple trees adjacent to, or far from the flowers. Those further away were the control plots.
They recorded aphid densities at the outset then compared densities a week later. The results showed that aphid populations dropped significantly in the nearby trees compared to the control trees further away and those differences continued for several weeks.
Researchers saw that a significant population of spiders and a variety of bugs including hoverflies, or syrphids, had larvae that feed on the aphids. The predators were drawn to the alyssum then moved from the flowers into the orchards.
The researchers were able to track the movement of the predatory insects by spraying protein markers on the flowers. When the bugs were later trapped in the orchard, re-searchers could identify that the bugs had visited the flowers before moving to the trees.
Beers expects that this same biological control approach could work for other tree fruits and possibly berry bushes.
“The groups we are targeting (syrphids) are generalist predators on aphids and virtually every plant has an aphid that attacks it,” she said.
“Organic growers would be most keenly interested in this approach because it fits well with their objectives of using natural, rather than chemical means of control, but it could be used by any grower.”
A biological control for aphids to replace at least some chemicals would also likely be a welcome alternative to the consumer, she said.
