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Spotted wing drosophila attacking fruit crops in Pennsylvania and surrounding states

Posted: September 22, 2011

UNIVERSITY PARK, Pa. – A new fruit pest has been trapped in seven southern and central counties in Pennsylvania, and maggot-infested fruit are also being reported, threatening grapes, berries, and tree fruit crops.

Spotted Wing Drosophila (SWD) is a small vinegar fly that is damaging tree fruit and berry crops in the eastern United States, and can potentially attack many fruit species such as cherries, plums, peaches, some apple varieties and Asian pears, says Dr. David Biddinger, entomologist at the Pennsylvania State University Fruit Research and Extension Center. The greatest potential for damage, however, is probably to the many types of berry crops, especially strawberries, cranberries and grapes.



Native to Southeast Asia, the fly was first detected in the western United States in 2008. Recently, Biddinger was scouting in a five-acre site of late season yellow and red raspberries in southern PA and discovered that between 30 and 50 percent of the fruit was infested with SWD larvae.   “Unlike other vinegar flies that target damaged or overripe fruit, SWD females will attack any soft-skinned healthy fruit to lay its eggs,” Biddinger explains. He reported it appeared the fruit was being infested with larvae just before the fruit ripened.

A grower from Berks County reports significant losses in blackberries, raspberries and white peaches, but the damage has not been confirmed as all coming from SWD since damage from stink bugs look similar. In addition to Pennsylvania, entomologists in Connecticut, North Carolina, Virginia, West Virginia and New Jersey are reporting SWD in vinegar traps, and some report significant fruit loss. The sustained, cool, wet weather in the eastern part of the country this season has delayed harvest of many late season crops leading to more over-ripe fruit than usual and consequently more fruit flies of all types. “With over 180 species of vinegar flies in the US, it is difficult to determine if the larvae are from SWD unless they are reared to adults or traps are used to monitor SWD populations,” says Biddinger.

Biddinger explains it is important for growers to be able to identify the pest and to learn about monitoring and management of SWD. 

”Identification is key, but because of its small size and because of several similar fruit flies in our region look very similar, it can be difficult. Over a thousand fruit flies may come to a single vinegar trap with only a small percentage being SWD since many types of flies are attracted to vinegar.”

SWD are approximately two to three mm long with yellow-brown bodies and red eyes. Adult males have two distinctive dots on the wings and brown bands on the abdomen. The females look similar but do not have the wing dots or bands and have large, saw-like ovipositor for inserting eggs into fruit. Biddinger says identification of SWD should be confirmed by him or other experts. “Now that Pennsylvania has confirmed the presence of SWD, identification can be made quickly by Sven Spichiger, entomology program manager at the Pennsylvania Department of Agriculture, or can be forwarded through local extension offices to myself or Kathy Demchek at Penn State.



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SWD adults thrive at cool temperatures in the spring and fall, but growth and reproduction are greatly slowed during hot summer weather. Females live two to nine weeks, lay two to three eggs per fruit and can lay more than 300 eggs total, showing high potential for large-spread fruit infestation if not controlled. During egg-laying, rot and fungal diseases can also affect the fruit, further contaminating the fruit at harvest. Infected fruit are difficult for growers to detect, since the only symptoms at first seem to be a small pin-prick from egg-laying. These turn into small scars and indented soft spots and bruises before the fruit eventually collapses from the internal feeding of the larvae or disease. SWD larvae are white, without a distinctive head and easier to detect against darker fruit, such as cherries or red raspberries.  Up to 13 generations have been found in parts of Japan, but detections were not found in PA this year until after cherry harvest, so we may have fewer generations.  Trap captures have continued to build since then with flies moving to new crops as the fruit began to ripen.



Biddinger suggests growers use integrated pest management (IPM) methods of monitoring using apple cider vinegar traps suggested at http://extension.psu.edu/ipm/agriculture/fruits/spotted-wing-drosophila or Michigan State University http://www.ipm.msu.edu/SWD/SWD-monitor.htm. Biddinger has not found the yellow sticky cards recommended to be placed in some of the trap designs to be worth the cost and effort and often just gum up the flies making identification more difficult. Flies in the traps will drown in the vinegar bait and can be sieved out with a tea bag strainer or cheese cloth and then stored in alcohol for identification.

Regardless of the crop, Biddinger says control of this pest will be dependent controlling the flies before they lay eggs and sanitation of infested or left over fruit on the crop.  Selection of varieties or management practices such as mowing of raspberries that could give earlier harvest dates might avoid the rapid increases of SWD that comes with the cool weather of the fall. “The biggest difference between SWD and other vinegar flies is that they can attack the crop much earlier with their impressive switch-blade of an ovipositor, but like other flies they can continue to develop on rotting fruit on the ground for months after harvest. Wild raspberries, blackberries, cherries, and grapes are likely to be reservoirs of SWD populations that will move into crops from the edges in the future.”



SWD trap catch thresholds for spraying and management plans for this new pest have not been established anywhere yet. Insecticides labeled for use on specific crops may list fruit flies as pests they control, but generally these will mean fruit flies of another family such as apple maggot, cherry fruit flies and blueberry maggot. Many of the currently registered insecticides labeled for these other fruit flies should also control SWD, but care must be taken to stay within the pre-harvest limitations of the pesticide used. Check with a product representative or your local horticultural extension agent or entomologist for further information.



For more information on spotted wing Drosophila, please visit http://extension.psu.edu/ipm/agriculture/fruits/spotted-wing-drosophila or http://sites.google.com/site/spottedwingdrosophila/.
Penn State researchers will provide updates on the SWD situation in Pennsylvania as its presence is confirmed in various counties throughout Pennsylvania and as research continues.