UNIVERSITY PARK, Pa. — Grapes, berries and tree fruit may be threatened if a new pest makes its way into Pennsylvania this year, Penn State researchers say. The Spotted Wing Drosophila is a small vinegar fly with the potential to damage many tree fruit crops such as cherries, plums, peaches, some apple varieties and Asian pears, says 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, and to grapes.
Native to Southeast Asia, the fly was first detected in the western U.S. in 2008 and was discovered in Florida on strawberries in spring of 2010, and then later in the Carolinas and Michigan.
“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. “One Pennsylvania fruit grower complained of having ‘apple maggot’ larvae in his peaches this winter, with symptoms remarkably similar SWD damage, so we may already have it in the state.”
Biddinger says that because the flies are only a few millimeters long and cannot fly very far, human-assisted transport is the most likely cause of the recent rapid spread.
“It is important for growers to be able to identify the pest and to learn about monitoring and management of SWD,” says Biddinger.
Identification of the adults is difficult because of its small size and because of several similar fruit flies in our region that also have spots, including Scaptomyza sp., which are common in commercial apple orchards in Pennsylvania.
“SWD look similar to the other 175 species of fruit flies (also known as vinegar flies) in the U.S., approximately two to three millimeters 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 explains.
Identification of SWD should be confirmed by experts. Sven Spichiger, entomology program manager at the Pennsylvania Department of Agriculture, and his staff can assist with proper identifications.
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.
Biddinger suggests growers use integrated pest management methods of monitoring using baits and traps suggested at http://extension.psu.edu/ipm/agriculture/fruits/spotted-wing-drosophila. Regardless of the crop, control of this pest will be dependent on controlling the flies before they lay eggs and sanitation of infested or left over fruit on the crop.
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.
Penn State researchers will provide updates on the SWD situation in Pennsylvania once its presence is confirmed in Pennsylvania.