A Fly in the Ointment

Fruit Growers Face a New Pest

Spotted wing drosophila
Spotted wing drosophila

Written By

Vern Grubinger

Written on

July 03 , 2013

There’s a small insect causing big damage to soft fruits that ripen late in the season. It’s new to our area, and spreading fast. Spotted wing drosophila (SWD) has been buzzing across the country for the past few years. First, it was found in California in 2008; then in 2009 it moved to Florida, Oregon, and Washington. From Florida, it moved up the East Coast to arrive in New England in 2011, and last year it was found across much of Vermont. If it hasn’t arrived in your area yet, this year it probably will.

SWD is related to the common fruit fly (more properly called a vinegar fly), which is known for laying its eggs in rotting fruit. What makes this new fly different is its ability to lay eggs in firm, ripening fruit. The females have a saw-like egg-laying appendage (a serrated ovipositor) that lets them cut small holes in the skin of fruit while depositing an egg in each hole; these soon hatch into tiny white larvae that feed in the fruit’s flesh. The larvae develop for 10 to 14 days before emerging as adults, ready to mate and lay more eggs. Females can lay 300 eggs in their lifetime, so the SWD population can increase extremely fast when conditions are right. It’s only the male adults that give this species of fly its common name; they have a single dark spot on the front edge of their wings. Females lack the spots on their wings.

Crops most susceptible to SWD damage are fall raspberries, cherries, blueberries, and day-neutral strawberries. (Unlike June-bearing berries, day-neutral strawberry varieties keep fruiting all season long.) Peaches and grapes are also among the vulnerable cultivated crops. Once attacked, damage to fruit is not immediately visible. The fruit can look perfectly ripe and firm but then rot just before, or soon after, harvest. This is because SWD makes only a small pin-prick on the fruit during egg-laying. By itself, the hole does no harm but egg-laying introduces microbes into the fruit so, within a few days, fruit flesh starts to break down, leading to discoloration and eventual collapse. Usually by this point the small white larvae can easily be seen inside the fruit if you look closely.

Crops most susceptible to SWD damage are fall raspberries, cherries, blueberries, and day-neutral strawberries.

However, all is not lost when it comes to fruit growing. SWD has a preference for softer-fleshed fruit with thin skins, so crops like apples and pears and thick-skinned grape varieties do not appear to be at risk. In theory SWD can also attack some vegetables, like tomatoes, but their skins seem to be thick enough to prevent attack, except when a fruit has cracks.

So far SWD has not been found in large numbers until late summer (mid-August in most locations) and early fall. Therefore, earlier-ripening varieties of fruits, like summer-bearing raspberries and strawberries, are at less risk than their relatives that ripen in the late summer and fall. Early-ripening blueberry varieties have also been observed to have less SWD damage than those that ripen at the end of the blueberry season, even though the difference in timing may just be a couple of weeks.

Timely and complete harvesting is one way to minimize SWD damage. Picking fruits as soon as they start to ripen and keeping the crop picked clean can avoid the build up of SWD in a fruit planting. Prompt refrigeration or freezing of fruit upon harvest will reduce losses to disease and slow the development of any eggs or larvae present. Note that there is no known risk to human health posed by ingesting SWD, so as gross as it sounds, eating a few eggs or larvae is not hazardous, and they’re so small you won’t even know they’re there.

It may be possible to use netting to exclude SWD from vulnerable fruit plantings. Floating row covers (often called remay) or agricultural insect screening with a mesh opening smaller than 1 mm square can keep the flies from getting to the crop. Obviously, to be effective the netting needs to be over the crop before any fruit start to ripen, and it must be securely sealed along the ground and at any entrances to prevent flies from getting in. Netting offers an additional benefit because it can also keep birds from eating ripe fruit. Care must be taken to avoid ripping the netting, and in larger plantings a structure may be needed to hold the netting up off the plants so that people can easily get in to pick the fruit.

Research and Extension personnel have been using traps to monitor the arrival and population buildup of SWD. This pest is attracted to several kinds of bait, including vinegar, alcohol, and yeast. Simple traps made from red plastic beverage cups can be placed within a crop to capture adult SWDs. The challenge is to develop a bait formulation for the traps that will be attractive enough to SWD to compete with ripening fruit and thus provide an “early warning’”system. Very effective baits would offer the potential to trap out enough of the fly population to actually reduce crop damage.

Insecticides can also be used as part of the management strategy for SWD. For most homeowners, this does not make much sense because of the cost of materials and the need to use proper equipment to get good coverage of all surfaces on the crop, especially leaf undersides. One must start spraying when SWD are in the area and fruit are present and just starting to change color. Spinosad (available in forms allowed for organic use) and pyrethroids (not organically acceptable) can be effective. They must be applied weekly as a crop starts to ripen and pesticide types must be rotated to prevent SWD from developing resistance. If spraying an insecticide, be sure to carefully follow the instructions on the label.

Prompt refrigeration or freezing of fruit upon harvest will reduce losses to disease and slow the development of any eggs or larvae present. 

Commercial fruit growers are taking many of the steps above to manage this new pest, along with the other insects and diseases twith which hey must already deal. Growers are actively collaborating with research and extension personnel across the country to learn which combination of methods will be sustainable for managing SWD over the long term, since it appears that it is here to stay.

For links to more information on SWD see uvm.edu/vtvegandberry/SWDInfo.html

Photo courtesy of Hannah Burrack North, Carolina State University

About the Author

Vern Grubinger

Vern Grubinger

Vern Grubinger is the vegetable and berry specialist for University of Vermont Extension and the coordinator of the Northeast Sustainable Agriculture Research and Education program of the USDA.

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