With a score of hosts available to spotted wing drosophila, cherry growers must manage their orchards to keep ahead of the pest throughout the growing season.
This is why non-crop hosts of the pest are a major focus of Dr. Howard Thistlewood, entomologist with Agriculture and Agri-food Canada at the Pacific Agri-food Research Centre in Summerland, British Columbia, Canada.
“It builds year-round, so we look at what it does outside the crop, in the rest of the environment,” he said.
The fly, native to Asia, was identified in strawberry fields around Watsonville, California, in 2008 and in Oregon’s Willamette Valley and British Columbia’s lower mainland the following year.
It is now widespread across fruit growing regions of Canada and the United States and is a particular challenge for berry and cherry growers.
Unlike other fruit flies, which typically infest damaged fruit, spotted wing drosophila is equipped with a saw-like ovipositor that allows it to deposit eggs in undamaged fruit.
But if spraying can provide effective fly control, it’s also among the least favored options because many export markets have low tolerances for pesticide residues.
Understanding how the pest behaves is key to determining if sprays are even required.
Among the fascinating discoveries that Thistlewood and colleagues in the United States, including Dr. Peter Shearer, entomologist with Oregon State University, have made is that the fly transforms into a different, tougher critter as temperatures drop and daylight diminishes.
“It’s actually a different kind of fly in the winter,” Thistlewood said. “It’s a different color, different size.”
But as the days lengthen and temperatures rise, it emerges from its overwintering locations in search of the nourishment that allows it to reproduce and multiply.
“In the coastal situation, that’s going to be the berries. In our situation in the interior, that’s going to be the cherries,” Thistlewood said.
The fly then moves onto currants in July and honeysuckle in August, as well as later-season cherries, elderberries, and chokecherries—a total of nine different plants in all.
“It’s continuously in all these other fruits as late as the middle of October, and when it’s in all those plants, it’s exponentially multiplying,” Thistlewood said.
When populations boom and non-crop hosts are insufficient to accommodate them, the flies will gladly move back into late-season cherries as well as grapes—anything that provides nourishment.
This is why spraying is both helpful in controlling the pest, yet of little benefit in areas where there are several alternative hosts.
Thistlewood notes that a trap adjacent to a research orchard that was regularly sprayed to control pests managed to rack up 1,600 flies one October.
“During the season it’s pretty well sprayed, yet the insects can build up in the non-crop plants around that block, in the bush, and then later in the year, they’re back, blowing through in the hundreds or the thousands.”
The good news, however, is that growing conditions often allow early-season cultivars, such as Santina, to mature before the fly has gained a foothold in alternative hosts, reducing the need for early-season sprays.
“You probably don’t need to spray your early cultivars if you’re in a certain location and you know that you can get them away in time,” Thistlewood said. “If you’re an organic grower, for example, it might be a good way to minimize spraying.”
However, later season cultivars such as Lapins, Sonata, and Staccato face greater risks because even if sprays are used, the pest might be taking refuge in non-crop hosts outside the spray area.
“By the time you get to mid-August, you can start to see bigger issues,” said Erin Carlson, manager at Carcajou Fruit Company Ltd. in Summerland. “You start to see them more, and they start to become a problem.”
This leaves growers without a simple solution.
While area-wide control programs are effective against codling moth and cherry fruit fly (two pests with which Thistlewood has previous experience), spotted wing drosophila resists such an approach.
“With this, there’s no way,” he said. “It has a wide range of crop hosts that it attacks, and so it’s difficult to get that area-wide effect going.”
But if growers do their part by limiting hosts for the pest, maintaining regular spray schedules, and removing ripe fruit, they stand a chance of keeping ahead of pest populations.
“We’re going to have to live with it,” Thistlewood said. “If growers do their job, it’s not an impossible pest to manage.” •
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