Spotted winged drosophila poses risk to soft-bodied fruits

Female has toothed organ to saw through fruit skin

Writer: Robert Burns, 903-834-6191, rd-burns@tamu.edu

OVERTON – Early awareness of a tiny insect could save Texas fruit growers a lot of money, perhaps even their entire crop, according to a Texas A&M AgriLife Extension Service entomologist.

The potential risk is from the spotted wing drosophila, a member of the vinegar fly genus, said Erfan Vafaie, AgriLife Extension integrated pest management specialist, Overton.

At first glance, vinegar flies look very similar to common fruit flies, he said. Crops at risk include apples, Asian pears, Asian plums, blackberries, blueberries, boysenberries, cherries, cold hardy kiwis, elderberries, grapes, Italian plums, mulberries, nectarines, peaches, persimmons, plumcots, raspberries, strawberries and tomatoes, he said.

Most vinegar fly species are nothing more than a nuisance, Vafaie said. They are attracted to rotting fruit, but have nothing to do with the fruit initially spoiling. This is because ordinary fruit flies don’t have the ability to break through the skin of berries, grapes, pears and other thin-skinned fruit.

But the female spotted wing drosophila has a unique attribute: a saw-like tubular organ, called the ovipositor, which allows them to hack into fresh fruit to deposit their eggs, he said.

The eggs and the resulting larvae are microscopic and pose no risk to anyone consuming them, but by creating small tears in the skin, the spotted wing drosophila opens up the fruit to invasion by bacteria and mold, which accelerates spoilage, Vafaie said.

“They do pose risk to the fruit on their own as well,” he said. “By consuming the fruit, they can cause it to soften from the inside-out.”

Vafaie is relatively new to Texas and to AgriLife Extension. He was hired in December. His previous position was as a research technician at the Vineland Research and Innovation Center, Ontario, Canada, where he worked on biological control of whiteflies and monitoring spotted wing drosophila populations.

In June, he found the insects in traps set as part of a high tunnel strawberry project he was working on with Dr. Russ Wallace, AgriLife Extension vegetable specialist, Lubbock.

Vafaie said he was not surprised to find the insect, which was introduced into the U.S. via California about 2008. He was “somewhat” surprised, however, to find relatively high numbers of the insect because there had been, to his knowledge, no previous reports by Texas growers of the insect or its damage.

“One trap had 26 female and three male SWD, out of about 60 flies total caught in the trap, which is a high proportion of SWD,” he said.

In Canada, as in some U.S. states, there is awareness of the pest because of the damage it can cause.

“I was surprised that no one had really noticed them here,” he said. “I thought it might have been because no one was looking for them.”

That no one noticed them is not unusual, because the spotted wing drosophila looks much like any other fruit fly to the naked eye, Vafaie hypothesized. Also, the holes the ovipositors make in the fruit of the skin are nearly microscopic, and the resulting fruit spoilage does not look any different than normal spoilage, except it may occur days earlier than expected.

The exception is when fly populations erupt in very high numbers, he said. In that case, a grower can lose very large amounts of his or her crop on the vine, sometimes even an entire crop.

But is the spotted wing drosophila really a big risk to Texas fruit growers?

Vafaie said the first step is to more widely monitor for the insect. Fruit fly traps are easy and inexpensive to construct. See the YouTube video at http://bit.ly/1nIczx1 for how to build a trap.

Distinguishing the spotted wing drosophila from other fruit flies is a little more difficult, he said.

People with a trained eye and excellent vision will be able to see the black spot on the male fly’s first wing vein, Vafaie said. The females are distinguishable by the serrated ovipositor, but only by using a microscope.

Suspect insects can also be delivered to Vafaie for identification.

“Place suspect flies into a sealable plastic container with 70 percent or higher isopropyl alcohol, which will help preserve the flies,” he said. “No plastic bags, as they may leak. No live insects as it’s against U.S. Postal Service rules to do so.

“Include as much information as possible with the container, such as your name, date, contact information, where you found it and what are some fruit crops nearby.”

Mail the package to: Erfan Vafaie, Texas A&M AgriLife Research and Extension Center, 1710 Farm-to-Market Road 3053 North, Overton, TX 75684.

For more information about shipping or identification of specimens, contact Vafaie at 903-834-6191 or erfan.vafaie@ag.tamu.edu .

There is also more information on Vafaie’s blog at http://sixleggedaggie.com/news .

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