WENATCHEE, Wash. — Apple orchards are intricate webs involving a delicate balance between trees, soils, water, insects and more. To manage the pests that can potentially damage a crop of apples, a grower might apply a pesticide. There are several problems associated with pesticide use, though, including risks to environmental and human health; the costs of the chemicals; and the equipment, labor and expense needed to apply them. That’s why Washington State University researchers have been pioneering alternatives to pesticide use for managing pests.
One of the effects of using a particular pesticide is that, in addition to controlling a target insect, it may also kill populations of insects beneficial to orchard health. WSU researchers have countered that problem by developing and advocating for a broad group of strategies called Integrated Pest Management. IPM tries to minimize the use of harsh chemicals in favor of ones that are highly selective for target pests. WSU scientists have also helped develop methods of biocontrol, in which a good insect is encouraged by various means to prey upon the bad insects that cause damage to trees and fruit.
The sensitive balance of orchard ecology is such that, if one thing is changed (for instance, a pest-control regime), other things may change as well (like the proportion of bad pests to their good natural enemies). For the last decade, one of those changes in Washington apple orcahrds has been an expanding woolly aphid population.
“Growers began noticing an increase in the number of woolly aphids in apple orchards starting in the early 2000s,” said WSU entomologist Betsy Beers. Beers is based at WSU’s Tree Fruit Research and Extension Center in Wenatchee. “There is some evidence that suggests it was a change in pesticide programs that led to an increase in woolly aphids. The question became, what could we do about it?”
Beers said that woolly aphid debilitates trees over the long term by sucking the sugar out of plants. “They are pernicious little beasts. They use the plant’s sap (phloem) as food. They not only suck sap from leaves, but also from the plant’s woody parts. They also infect the roots, reducing the plant’s ability to take in nutrients and water. They cause hypertrophic galls, which are tumor-like protuberances that can kill fruit buds. Dead fruit buds mean lowered productivity, a serious concern for apple growers. “This is one of the meaner aphids.”
Beers said that a survey of woolly aphid’s natural enemies indicated that syrphids are the most commonly seen predator in woolly apple aphid colonies. “We predicted syrphids could control the aphids–the trick is getting more syrphids into the orchard, and making sure they are happy there.”
Syrphids are more commonly known as flower flies or hoverflies. As their name suggests, they like to hang out among flowers, as the adult syrphid’s primary food is nectar. The syrphid larvae, however, grow and thrive on a diet of aphids–and woolly aphids are just dandy for maturing syrphid larvae. “It’s wonderful to watch them chew their way through an aphid colony,” said Beers.
Enter doctoral student Lessando Gontijo, who came to WSU from his native Brazil to study biocontrol. Funded in part by a grant from WSU’s BioAg program, and working with his mentors, Beers and WSU biocontrol expert Bill Snyder, Gontijo set up a series of experiments to test the idea that syrphids could be used to control woolly aphid populations. The team first examined six different flowering plants for attractiveness to syrphids, and sweet alyssum won hands down. This plant is low-growing and flowers from a few weeks after planting until frost, making it an ideal planting for use in orchards. The next step was to plant small plots in orchards, in the lanes between rows of apple trees (normally occupied by grass sod). While all the details aren’t sorted out, the aphids disappeared faster next to the alyssum plots than they did in the grassy control plots. Gontijo used a sophisticated method known as “immunomarking” to prove that the syrphids found in the apple trees had visited the alyssum flowers, further evidence that alyssum could promote biological control.
The results of the small-scale experiments are promising, Beers said. “These experiment are the first steps in a much longer process,” she said. “We needed to pass the initial proof-of-concept tests: do these flowers attract beneficial insects, are they manageable in an orchard, and do we see a decrease in the pest populations. We passed all those tests.”
The next step will be to run experiments in commercial orchards. Beers said she is looking for funding to expand the project, adding “It’ll be a great opportunity for future graduate students to do good science that has a real-world impact on the way apple growers manage pests.”