Entomologists in Michigan are still trying to determine whether spotted wing drosophila will be a significant tart cherry pest.
“We were hoping it would not be an issue,” Dr. Nikki Rothwell said. She is the coordinator of the Northwest Michigan Horticultural Research Center north of Traverse City, right in the heart of the nation’s largest tart cherry production area.
When SWD was first found in Michigan, it appeared that early harvested fruit, such as tart cherries, would be off the trees by the time SWD became active in orchards. But, Rothwell said, the arrival time of SWD in orchards each season is variable and so is the harvest date of tart cherries.
Tart cherry harvest can start as early as the first half of July, but in some years may continue well past mid-August. SWD numbers start building rapidly in July. The pest has been a scourge to growers of soft fruits and berries, such as blueberries and raspberries, that ripen in the late summer and fall.
“Tart cherries appear to be at high risk in some years,” Rothwell said. “They are very vulnerable just before harvest. And there is zero tolerance for insect larvae in harvested fruit.”
Other discoveries of the last year or so are that insecticides that are effective against cherry fruit fly are not effective against SWD. So presence of the insect could result in growers having to apply from 1.5 to 3 more sprays per season, she said, using different materials.
Another issue is Maximum Residue Limits (MRLs). Since processed tart cherries do move into export markets, it is important that insecticide residue levels be low enough to meet the lowest standards they may encounter in other countries. SWD means more sprays directed at ripening fruit and greater risk of residues.
Rothwell tested materials like Sevin (carbaryl) and malathion, which have three-day preharvest intervals, and found they did not adequately control SWD. “In our laboratory trials, the labeled insecticides with short preharvest intervals on tart cherries are not as effective as some other materials with longer PHIs,” she said.
A number of insecticides did work well, mostly those with longer preharvest intervals, and, she found, good coverage is essential to protect the fruit and meet the zero tolerance level.
In tests conducted across the country, researchers found that the insecticides that worked best against SWD included Imidan (phosmet), Delegate (spinetoram), Mustang Max (zeta-cypermethrin), Danitol (fenpropathrin), and Lannate (methomyl).
In fighting cherry fruit fly, growers found they could reduce pressure the following year by using a postharvest spray. That prevented flies from developing high overwintering populations after infesting the fruit that remain on the tree after mechanical harvest with shakers.
“Postharvest sprays worked for lowering cherry fruit fly populations” she said. “But spotted wing drosophila has so many alternative hosts, we don’t think postharvest sprays will work.
“This insect could be a game changer for tart cherry growers,” she said.
Monitoring to discover its first appearance will be essential. Unlike ordinary fruit flies, SWD females can lay eggs into unripe fruit with a saw-like ovipositor. They don’t need to wait for the fruit to get overripe and soft. Females live about nine weeks and lay up to 300 eggs that hatch into small, white worms.
“Because SWD reproduces so quickly under optimum conditions, the first catch information is vital to activate pest management programs to prevent rapid population increases and potential infestations in a region,” Rothwell said.
The best years for growers will be when the fruit ripens early and the insect population builds late.