Male spotted wing Drosophila. (Courtesy Matt Bertone, North Carolina State University Extension)
Spotted wing drosophila populations are now so well established in Michigan that tart cherry growers would benefit from adopting a new mindset for managing the destructive pest: plan on it being a seasonal threat based on fruit phenology, like powdery mildew or cherry leaf spot.
“We need to realize that trap catch numbers are not a tool to base our sprays on. Whether we catch one or 100, our cherries are at risk until harvest has ended,” said Dave Jones, Michigan State University Extension educator. “If cherries are yellow, we’re at risk. We spray. Period.”
Unlike other pests, where growers check traps in their orchards to decide if population levels warrant spraying, that system isn’t reliable for SWD, Jones said. There’s just too much variability in the tiny fruit flies’ populations early in the season when growers make critical management decisions, he said. Scouts look for males in traps that have the characteristic spotted wings, but females can far outnumber males and go unnoticed.
That means early in the season, just as the cherries are starting to yellow in late June, a grower could decide not to spray based on finding very few flies in a trap and still end up with contaminated fruit.
Moreover, unlike insect pests where populations spike and drop with each generation, SWD reproduce so fast the population only goes up and up in pressure, more like a fungal pathogen during favorable conditions than a typical pest, he said. That’s why, as the pest naturalizes in the state and populations grow, it’s increasingly important for all tart cherry growers to prioritize SWD management.
“It doesn’t make any of us happy from the management perspective, but this fly is just so numerous that it is established everywhere we have tart cherries or sweet cherries planted in the state of Michigan,” Jones said. MSU will continue to track populations across the state to alert growers to when the pest has emerged in their region and management needs to begin.
Last year, populations in Michigan surged several weeks earlier than normal, in early July, posing greater risk to berries and stone fruit. The pest also arrived early in New York, with many trapped in June, catching tart cherry and berry growers off guard, according to Juliet Carroll, fruit IPM coordinator for the New York State IPM program.
“In early August, reports of tart cherry loads being rejected at the processor due to worms and mold underlined that SWD had found its mark in cherries. Tons of tart cherries were dumped on the ground,” she wrote in a season report, posing a danger to ripening fruit nearby.
Jones agreed that the pest caused the most problems last year where growers weren’t expecting it. Growers with good spray programs were able to protect their crop.
“This was the first year for a lot of folks in Southern and West Central Michigan that they noticed SWD was a problem and they had contamination,” he said. “Anything less than an outstanding management program will result in contaminated fruit.”
The threat posed by SWD and the best control strategies were a hot topic at the Great Lakes Fruit, Vegetable, and Farm Market Expo in December.
Risk to other crops
So far, sweet cherry growers in the East have largely been spared, as most of the crop is harvested before SWD populations really take off. However recent trends show the population growing earlier, said Nikki Rothwell, MSU extension educator.
“We have earlier first catch and a population that’s rising sooner and sooner,” she said in a talk at the Expo. “In the past, I thought we’d be done and get out of harvest before the pest rose, but now I’m not so sure with what we’ve seen the past couple of years.”
And SWD like sweet cherries. In laboratory tests, SWD seem to prefer under-ripe or just ripening sweet cherries, while they prefer Montmorency cherries that are already infested. But there was no clear relationship between larvae and fruit firmness or brix, Rothwell said.
In contrast, it appears that SWD only infest softening peaches and plums — which is interesting because sweet and tart cherries become susceptible the moment they turn yellow, said Jones, who presented on MSU’s peach and plum findings at the Expo.
“For plums and peaches, you shouldn’t be starting management the moment that color creeps in,” he said. “If you initiate a spray program around 4 to 5 pounds of firmness, you should be able to catch them.”
For plums, that’s just a week before harvest. For processing peaches, which are harvested pretty firm, there was no SWD infestation of intact peaches. Even with fresh peaches, growers mostly pick well above the 3.5 pound-firmness level that SWD seem to prefer.
“All reports of spotted wing drosophila infestation in peaches are if they are left on the tree to full softness or overripe,” Jones said. “If you leave peaches on until they are practically dripping, that’s where you may have concerns with this pest.”
Control in tart cherries
Given the high SWD pressure now commonplace in Michigan tart cherries, experts say there is no room for error in spray plans. Growers need to select only products rated as excellent for SWD and stick tightly to recommended intervals, not going past seven days.
Top options include: Danitol (fenpropathrin); Assail (acetamiprid); Mustang Maxx (zeta-cypermethrin); Imidan (phosmet); and Exirel (cyantraniliprole). MSU entomologist Larry Gut compared spray programs to fruit quality outcomes at farms around the state to make his recommendations at the Expo.
“It’s extremely important to get an excellent product on 10 to 14 days before harvest. There is no room to be using a weak product at a time when populations are building,” Gut said, adding that Imidan is a common and reliable choice for that key middle spray. “Orchards that had problems with detectable larvae at harvest had extended spray intervals or used permethrin.”
On average, growers in the Northwestern part of the state made 4.4 sprays while growers in the West Central and Southwest regions made 3.3 sprays. The best programs used a product with a three-day pre-harvest interval right before harvest. Gut acknowledged the challenge growers face paying for these chemicals at a time of low prices for tart cherries, but urged caution when using cheaper pyrethroids.
“You need to carefully consider the tradeoff between efficacy and cost,” he said, adding that the best time to use pyrethroids is early or right before harvest.
There is some good news too, Gut said. A new diamide product that’s been used in vegetables is expected to be registered under the trade name Verdepryn for tree fruit use by this season. It would also give some competition to Exirel, he said, which might cause manufacturers to lower the price, because currently at $60 an acre, it isn’t economically viable for SWD.
Rotating products is also especially important with the pest, because it reproduces so fast it’s more likely to develop resistance, Jones said. •
SWD in the West
Spotted wing drosophila pose a serious threat to Western sweet cherries as well. Spray recommendations are still a work in progress in the Northwest and there are not yet established guidelines to use traps to determine spray timing, so most growers spray prophylactically, said Betsy Beers, entomologist with Washington State University.
“Quite a few growers have traps in their orchards to help gauge the amount of pressure, but still keep covered for the most part,” she said in an email.
Pressure in the Northwest does vary significantly from season to season, depending on over-winter survival, but in California, the invasive pest is active all year round and may have up to 10 generations. California growers are advised to begin insecticidal control if any SWD are found in orchard traps and fruit is straw color or beyond.
Trap options have expanded in recent years and growers can now choose from a variety of jar or sticky card traps, thanks to the development of synthetic SWD lures in addition to liquid baits. There’s pro and cons of each, but last year, WSU recommended combining a commercial lure, such as Scentry or AlphaScents, with a sticky card trap that makes it easier to spot the male SWD as the most user-friendly option.