NEW HOLLAND, Pa. — The spotted wing drosophila is a tiny fruit fly with a major sweet tooth.
It causes big problems for berry growers, but scientists are hoping they can turn the pest’s love for sugar against it.
“There’s some really interesting and cool research going on right now,” said Maria Cramer, a Penn State grad student who is studying pest issues in high tunnels.
Cramer spoke at New Holland Vegetable Day on Monday at Yoder’s Restaurant.
Alternative sweeteners like stevia are looking like quite effective controls. In some research, all of the drosophila died after feeding on the sweeteners.
It’s not clear if the flies are missing out on the nutrients they need by eating the product or if the sweetener itself is poisonous to the insects.
Still, “this is really both the thing that lures them and that kills them at the same time,” Cramer said.
Spotted wing drosophila have their taste receptors in their legs, so when they land on something they perceive as sweet, they start feeding reflexively.
The flies may not be able to differentiate between different kinds of sweets, said Kathy Demchak, a Penn State senior Extension associate.
The next step will be to see whether the flies prefer the sweetener over raspberries when offered both, Cramer said.
Attracticidal spheres offer another sweet but deadly approach. They’re red, sticky balls that resemble the traps used for apple maggots.
A cap on the ball contains sugar and pesticide. Rain and humidity make the concoction melt over the orb.
“Spotted wing drosophila are attracted to both the color and the sugar,” Cramer said.
Trials suggest the balls could be as effective as spraying pesticide, though it’s not clear how well they will work in a high tunnel where there’s no rain.
Growers will need to place a lot of red balls, but the balls can last for several weeks.
Although not yet on the market, the balls have been tested on some farms in West Virginia, Cramer said.
A longer term, if less sugary, control measure involves releasing sterile male drosophila.
The insects are reared in labs and irradiated to become infertile. The female drosophila will still mate and lay eggs in the fruit, but the eggs will be duds.
This technique has been used successfully against the Mediterranean fruit fly, Cramer said.
For this year, however, growers will still need to rely on cultural controls and insecticides.
Opening up the canopy, for example, increases spray penetration and reduces the shady, humid areas the drosophila prefer.
Michigan researchers have found that picking every day or every other day can significantly reduce drosophila populations.
Harvesting every other day results in larger berries, but there’s also a greater potential for bigger, more noticeable larvae in the fruit, Cramer said.
High tunnels facilitate frequent picking because they block the rain.
Refrigerating the berries soon after harvest could slow down biological activity and prevent some eggs from hatching.
Holding the berries near 32 degrees is best, though growers risk freezing their fruit, Demchak said.
Infested fruit should be discarded in plastic, not composted, Cramer said.
Many insecticides remain effective against spotted wing drosophila, but because the fly produces eight generations a year, resistance will always be a concern.
A 2011 report suggested drosophila were resistant to pyrethrins, but it’s not clear if the problem was really resistance or just a low-efficacy pesticide.
The research was conducted by a private company that did not publish its data, Cramer said.
Spotted wing drosophila has a number of wild hosts, so some populations never come in contact with insecticides. That could slow resistance development, though it also means there’s always more drosophila that can come onto the farm, Cramer said.
Spotted wing drosophila should not be confused with the spotted lanternfly, another invasive species.
Spotted wing drosophila are tiny and lay their eggs mainly in ripening berries. They are found in Pennsylvania and around the country.
Spotted lanternflies are larger, feed on a wide variety of plants and douse their surroundings in a sticky waste.