Spotted wing drosophila (Drosophila suzukii) is a fruit fly native to Japan and Southeast Asia that was first discovered in the USA in 2008. This invasive insect pest attacks a wide range of berry crops including blackberries, blueberries, boysenberries, raspberries, strawberries, and grapes. Wild berry crops, and mulberries and elderberries may also be attacked. In addition to berries, spotted wing drosophila will attack fruit trees such as apples, nectarines, peaches, and plums. Adult populations increase in mid-to-late summer and may remain abundant throughout early fall as berries and fruits ripen. Late-season crops are especially susceptible to attack. Spotted wing drosophila is not a quarantined or regulated pest.
Identification and Biology
The male spotted wing drosophila is on the right, and the female is on the left.
Spotted wing drosophila looks like other native fruit flies. Adults are 2.0 to 3.0 mm (1/8 inch) long with red eyes, a yellow-brown thorax, and black stripes on the abdomen. Adults are active during the day, preferring moist conditions and temperatures between 20 and 21ºC (68 and 70ºF). Spotted wing drosophila males can be distinguished from native fruit flies by the presence of a spot near the tip (outer edge) of each wing and two dark bands on the front legs (Figure 1). Female spotted wing drosophila do not have a spot on the wing but can also be distinguished from other native fruit flies by the large, saw-like ovipositor used for laying eggs, with two rows of serrations (Figure 2). This allows females to pierce the skin ofunripened, immature fruit to deposit eggs. More than one egg may be laid into each berry or fruit.
A closeup of the serrated ovipositor on the female. Photos by by Elizabeth Everman of Kansas State University.
Adult females live about two weeks and may deposit between 100 to 300 eggs from spring through fall. The life stages are an egg, larvae, pupa, and adult. The time required to complete the life cycle depends on temperature with warmer temperatures tending to reduce the time needed to complete the life cycle. The larvae typically pupate inside fruit, but they may also pupate outside the fruit unlike native fruit flies. Also, unlike native fruit flies, spotted wing drosophila females attack healthy, ripening berries or fruit as well as damaged or split fruit. They are less likely to attack damaged or rotting fruit. Darker colored fruit, in general, is more susceptible to attack. Moreover, females prefer to lay eggs in thin-skinned berries or fruits. Spotted wing drosophila females may attack berries and fruits throughout the growing season. Spotted wing drosophila overwinters as an adult, and the severity of winter conditions may influence survival.
Traps can be used to monitor spotted wing drosophila adult populations and help time insecticide applications. Because spotted wing drosophila is attracted to ripe, not overripe, berries and fruits, traps should be placed into fields when fruit starts to color. Once berries or fruit ripen, they become less attractive, and native fruit flies may be captured instead. Contact your local or state extension specialist regarding the type of trap that may be used to monitor for adult spotted wing drosophila. In order to detect larvae in berries conduct a salt test weekly. This involves placing a fruit sample into a plastic bag with warm salty water and waiting approximately 15 minutes to assess if any larvae are present.
Spotted wing drosophila populations can be managed by implementing cultural and sanitation practices throughout the growing season. This includes harvesting berry and fruit crops early, and removing and destroying overripe, infested, or culled fruit. It is important to prevent infestations by removing wild host plants such as grapes, blackberries, raspberries, American pokeweed, crabapples, dogwood, and Japanese yew from nearby locations in order to keep the fruit of these plants from serving as reservoirs for spotted wing drosophila populations during the growing season.
Insecticides can be used to suppress spotted wing drosophila adult populations during the growing season. Insecticides must be applied from the time fruit begins to color until harvest. Since this insect pest prefers to live within the berries or fruit, it may be difficult to suppress populations with insecticide spray applications. Application timing and coverage are critical as insecticides must be applied in time to kill adult females before they lay eggs as spraying will not kill larvae already inside the berry or fruit. The interval and frequency of insecticide spray applications may also influence the effectiveness of spotted wing drosophila suppression programs. Due to the number of generations per year, spraying once or even twice per week may be warranted. Application frequency may vary depending on the environment (e.g., temperature and rainfall) and growing conditions. The frequent applications required to treat multiple generations per year may result in intense selection pressure and thus lead to insecticide resistance. Therefore, it is important to use insecticides with different modes of action and to rotate them in order to reduce the development of insecticide resistance. However, only two insecticides are available to organic producers for use against the spotted wing drosophila; spinosad (Entrust) and pyrethrins (Pyganic). Both insecticides have short residual activity (one to three days), and spotted wing drosophila females exposed to pyrethrins (Pyganic) may recover to lay fertile eggs. In California, certain populations of spotted wing drosophila have developed resistance to pyrethrins (Pyganic).
Therefore, organic producers cannot solely rely on insecticides to manage spotted wing drosophila populations. It is important to monitor using traps, and implement cultural and sanitation practices in order to effectively reduce problems with spotted wing drosophila during the growing season.
For more information on spotted wing drosophila, consult the following extension publication from Kansas State University, Department of Entomology (Manhattan, KS):
Cloyd, R. A., and C. Copeland. 2014. Spotted wing drosophila. Kansas State University Agricultural Experiment Station and Cooperative Extension Service. MF-3158. Kansas State University, Manhattan, KS. 4 pages.
Raymond Cloyd is Professor and Extension Specialist in Horticultural Entomology/Plant Protection at Kansas State University. Contact him at [email protected]