Spotted Wing Drosophila, Part 2: Natural History

Spotted wing drosophila (SWD) is an invasive vinegar (fruit) fly that was first detected in the western United States in 2008 and in the Northeast in 2011. SWD female adults have large, serrated ovipositors that allow them to pierce and lay eggs in unripe and ripe fruit. Thus, their larvae can be found in ripe fruit (Figure 1), whereas most other drosophila can lay eggs only in over-ripe and rotten fruit. The presence of SWD has resulted in some growers discontinuing production of certain berry crops, while others have found that they need to apply insecticides much more frequently to protect the fruit.

Environmental Preferences

SWD prefers moderate temperatures and high humidity. Adults are most active at temperatures around 70°F; their activity is greatly decreased when temperatures are only 15 degrees colder or warmer than this. Adults are the primary overwintering life stage, though in the Pacific Northwest, adults have emerged from pupal cases in early winter. Adults need shelter when temperatures drop. Female adults exposed to cold temperatures lay few eggs, and eggs and larvae can be killed by several days of constant exposure to temperatures barely above freezing. In the Northeast, populations start out very low in the spring and increase as temperatures warm.

SWD has been found in both field and high tunnel environments, though SWD infestations are often lower in tunnels, possibly due to better cultural management in tunnels (e.g., more frequent harvest intervals, no missed harvests) warmer temperatures in midsummer, or drier conditions.

Life Cycle

SWD progresses through four life stages: egg, larva, pupa, and adult. The time required to complete each stage depends on temperature, with warmer temperatures accelerating development to a point. In one study, an increase in temperature from 59 to 77°F decreased the time required to complete all four stages from 23 days to 10 days.

SWD primarily overwinters as adults in protected locations, and females may become active a few days earlier than males. Adult SWD have been found flying during the winter on abnormally warm days, but they generally are expected to become active in the Northeast in May.  Trap captures in low numbers start occurring in mid-June with numbers ramping up during July.

Adults may live for 2 to 9 weeks during the growing season. Females begin laying eggs in fruit as it starts to turn color. An individual female may lay between 100 and 600 eggs during her lifetime depending on the host crop and the temperature. Larvae (Figure 2) hatch in 2 hours to 3 days and then feed in the fruit for 3 to 13 days.  Pupation (Figure 3) lasts for 3 to 15 days either inside the fruit or on the ground. Females emerging from pupation begin laying eggs an average of 2 days after emergence.

Figure 2.  Larva of spotted wing drosophila greatly magnified (Photo: K. Demchak, Penn State)

Figure 3.  Spotted wing drosophila pupa partially visible in fruit (Photo: K. Demchak, Penn State)

Multiple generations per year occur, with eight or nine generations per year plausible for the Mid-Atlantic. Thus, the proportion of fruit affected can increase rapidly during the season.

Host Plants and Feeding Preferences

SWD was originally called the "cherry vinegar fruit fly" in its native region; SWD adults have been found in cherry orchards in central Pennsylvania whether fruit was present or not. In the Northeast, SWD has had the most impact on raspberries and blackberries, especially fall-harvested cultivars. With fall raspberries, SWD may be found feeding on juice on raspberry receptacles even after the fruit has been harvested (Figure 4).

Day-neutral strawberries harvested in the fall have been severely affected on occasion. Other crops in the Northeast suffering significant losses include summer raspberries and mid- to late season blueberry cultivars. Peaches, nectarines, and hardy kiwi have also been affected. To date, June-bearing strawberries and cherries in Pennsylvania have escaped injury. Though not a preferred crop, SWD oviposition wounds in grapes have served as entry points for fungi that cause fruit rots. Wild host plants include raspberry, blackberry, wineberry, blueberry, elderberry, pokeberry, bush honeysuckle, Autumn olive, and even dogwood. SWD has been found in large numbers on cull muskmelons and cantaloupes discarded in the field.

In its native range, SWD has been observed feeding on oak sap and ovipositing in certain species of flowers, and adults can feed on honeydew excreted from piercing/sucking insects such as spotted lanternfly. This ability to use a wide range of food sources may explain why SWD has been found in unexpected locations and how it survives when fruit is not available.

Site-Specific Effects

The extent to which a particular crop is affected by SWD varies widely. Fruit farms that grow a succession of attractive crops are especially at risk. Other host plants in the area, SWD host preferences, planting size, pesticide applications, and the timing of fruit ripening among hosts will affect SWD damage. For example, fall berry crops near fields of discarded muskmelons were heavily infested with SWD. Conversely, small, isolated plantings of berry crops surrounded by agronomic crops were unaffected or only minimally affected by SWD, suggesting that rearranging farm crop layout may be helpful.

Fruit variety also affects SWD preferences. Generally, SWD prefer darker colored fruit, and a thicker/tougher skin may dissuade SWD from choosing certain varieties. Sugar or volatile levels of individual varieties could play a role, though little data currently exist.

Monitoring each susceptible crop on a farm is strongly recommended since the sites most at risk are probably farm specific. See "Part 3: Monitoring" for more information on methods.

References

Coop, L. "Online phenology and degree day model for agricultural and decision-making in the US." Corvallis: Integrated Plant Protection Center, Department of Botany and Plant Pathology, Oregon State University, 2010.

Dalton, D. T., et al. "Laboratory survival of Drosophila suzukii under simulated winter conditions of the Pacific Northwest and seasonal field trapping in five primary regions of small and stone fruit production in the United States." Pest Mgt. Sci. 67 (2011): 1368-74.

Kanzawa, T. "Research into the fruit fly Drosophila suzukii Matsumura (preliminary report)." Abstract. Kofu, Japan: Yamanashi Prefecture Agricultural Experiment Station, 1935.

------. "Studies on Drosophila suzukii Mats." Abstract. J. Plant Protection 23, nos. 1-3 (1936): 66-70, 127-32, 183-91.

------. "Studies on Drosophila suzukii Mats." Abstract. Kofu, Japan: Yamanashi Prefecture Agricultural Experimental Station, 1939.

Kimura, M. T. "Cold and heat tolerance of drosophilid flies with reference to their latitudinal distributions." Oecologia 140, no. 3 (2004): 442-49.

Lee, J. C., et. al. "In focus: Spotted wing drosophila, Drosophila suzukii, across perspectives." Pest Mgt. Sci. 67 (2011): 1349-51.

Thistlewood, H., et al. "Drosophila suzukii (Matsumura) (Diptera: Drosophilidae), a new pest of stone fruits in western North America." IOBC/WPRS Bulletin 74 (2012): 133-37.

Walsh, D. B, et al. Drosophila suzukii (Diptera: Drosophilidae): Invasive pest of ripening soft fruit expanding its geographic range and damage potential. J. Integrated Pest Mgt. 2, no. 1 (2011): G1-G7.

Prepared by K. Demchak, D. Biddinger, and B. Butler. Photos by D. Biddinger and K. Demchak. Reviewed by H. Burrack and G. Loeb.

Parts 1, 3 and 4:

Spotted Wing Drosophila Part 1: Overview and Identification

Spotted Wing Drosophila, Part 3: Monitoring

Spotted Wing Drosophila Part 4: Management