Spotted wing drosophila (SWD) began showing up in traps in North Carolina strawberry fields in late April.
The threat seemed pressing enough that the Extension service recommended preventive measures, said Hannah Burrack, North Carolina Extension entomologist.
“Ripening and ripe fruit are susceptible to attack by SWD attack,” said Burrack, “but the insects do not appear to be attracted to unripe fruit.”
If adult SWD are present on your farm, aggressive management is warranted, she said. That would include the following steps:
• Do an excellent job of sanitation. Fruit should be harvested frequently and completely. “Any unmarketable fruit should be removed from the field and either frozen, ‘baked’ in clear plastic bags placed in the sun or disposed of offsite. This will either kill any larvae present or remove them from your farm.”
• Prune to maintain an open canopy. This may make plantings less attractive to SWD and will facilitate pesticide applications. Leaking drip lines should be repaired, and overhead irrigation should be minimized.
• Apply insecticide treatments at least every seven days and repeat in the event of rain. Select effective insecticides with pre-harvest intervals amenable to picking schedules, and rotate insecticide modes of action between each treatment.
But don’t exceed the maximum applications per season. Some insecticides that are effective against SWD are Brigade (bifenthrin), Radiant and Malathion. Danitol is effective but has a longer pre-harvest interval, and carbaryl longer still.
• Sample your fruit regularly. At least 100 fruit per block per harvest should be observed for infestation.
SWD had been observed only on strawberries in North Carolina in the early spring, but it can attack other berry crops. “Last year, it affected some commercial blueberries, and two seasons ago, it caused significant damage in blackberries,” Burrack said.
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The caneberry crops — blackberries and raspberries — are probably the most vulnerable to SWD, said Doug Pfeiffer, Virginia research and Extension entomologist.
“After that might come blueberries and cherries, followed possibly by grapes,” he said. “Apple and pear are on the host list, but we don’t consider them to be at much risk because the skin is relatively hard.”
Soft skinned fruit susceptible
Peaches and nectarines could be vulnerable as well, he added. “Any fruit with soft skin can be susceptible.”
Because of the life cycle of SWD, there is a danger of resistance developing to any specific insecticide. “So managing to avoid resistance is very important,” said Pfeiffer. “Insecticides with different mode of actions should be rotated.”
And pay special attention to pre-harvest intervals. They can range widely from one type of berry to another. “It will be critical to check the label of alternative products for PHI for the crop needing protection,” he said.
As in North Carolina, intense sanitation is strongly recommended in Virginia.
“Harvest fruit promptly to eliminate breeding sites,” Pfeiffer said. “Any over-ripe or rotten fruit nearby should be destroyed. If a crop is found to be infested with SWD, especially if not established in the area, it should be destroyed after samples are taken for proper identification.”
SWD is still a relative newcomer in the southeastern states. After its first discovery in the continental United States in California in 2008, it was found in Florida in 2009, in the Carolinas in 2010 and in Virginia in 2011.
As if flies with spotted wings weren’t bad enough, Tar Heel strawberry growers had to deal this season with the strawberry “clipper,” a small weevil that lays its eggs in developing flower buds.
The small, snout-nosed beetles most often appear in fields with wooded edges, where they spend the summer and the winter.
“They are not great dispersers,” said Burrack. “Damage is worse in rows bordering the woods, and it decreases as you move inward.”
Should you treat for this insect? Strawberry clipper activity typically lasts for only a few weeks, and strawberries seem able to compensate for clipper damage, at least in matted row plantings.
“So I often don’t recommend aggressive management for strawberry clippers,” said Burrack. “The materials that are effective against them are broad-spectrum insecticides that can be detrimental to bees.”
If you apply any pesticide to plants during bloom, be sure to avoid bee exposure, she said. Treat at dusk or dark, when bees are not foraging, and you allow for maximum “dry time.”
“Also, wait to treat your plants until after bloom is complete,” she added. “The latter is not an option for strawberries, but can be employed for some caneberries and crops.”
Select the least toxic material to bees that is effective against the target pest, she said.
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