Drosophila suzukii (Diptera: Drosophilidae): Invasive Pest of Ripening Soft Fruit Expanding its Geographic Range and Damage Potential

. Spotted wing drosophila, Drosophila suzukii Matsumura, a native of eastern and southeastern Asia, is a pest of small and stone fruits. First detected in California in 2008, the insect is now found across the Pacific Coast states. Its penchant for attacking healthy, ripening fruit (as opposed to overripe and rotting fruit favored by other so-called “vinegar flies”) makes it a potential economic threat to a host of soft- and thin-skinned fruit crops including cherry, raspberry, blackberry, blueberry, strawberry, peach, plums, pluots, nectar-ines,juicegrape,tablegrape,andwinegrape.Coordinatedresearchprojectstodeterminehostpreference,seasonalphenology,biology,andmanagementoptionsaretakingplaceamongentomologistsinWashington,Oregon,andCalifornia.Adescriptionofthepestandinitialfindingsonitsbiology,lifehistory,knownandexpectedgeographicrange,managementandmonitoringtechniques,andeconomicconsiderationsarepresentedanddiscussed.

Of the Ϸ1,500 species of Drosophila (Markow and O'Grady 2006) commonly known as vinegar flies, Drosophila suzukii Matsumura (Diptera: Drosophilidae), known as cherry drosophila in Japan and now commonly called spotted wing drosophila in the United States, is one of only two species (the other being Drosophila pulchrella Tan, found in tree fruit in Japan) known to oviposit in healthy (whole) fruit as opposed to fruit that is damaged or overripe Sato 1995, 1996). Two unique characteristics of D. suzukii make it a particularly onerous pest: its propensity for favoring ripening (as opposed to overripe) fruit (Mitsui et al. 2006) and the prominent serrated ovipositor of the female, which upon insertion can cause physical damage to the host fruit. Very often oviposition wounds, including those caused by Drosophila flies, provide access to secondary infection by both insects and pathogens including fungi, yeasts, and bacteria causing additional losses (De Camargo and Phaff 1957, Molina et al. 1974, Louise et al. 1996. Eggs develop into larvae within the fruit, causing it to become soft and rot rapidly, resulting in reduced crop yields and significant financial losses. Drosophila suzukii was first observed in mainland Japan in 1916Japan in and later observed in 1930Japan in -1931 in other parts of what was then the Japanese empire, including parts of today's Korea andChina (Kanzawa 1936, 1939). There it was found on wild strawberry and cultivated cherry. While initial infestations were confused with those by cherry fruit fly, it was soon apparent that D. suzukii had the potential to severely impact the Japanese cherry industry. Kanzawa's observations on the general biology and habits of D. suzukii, as well as his initial experiments with trapping and control of the fly, formed the basis for subsequent studies and are cited extensively throughout this paper. Drosophila suzukii continues to be a pest in Japanese soft and stone fruits Sato 1995, 1996).
This paper seeks to define the pest and present historic and recent findings on its biology, life history, known and expected geographic range, management and monitoring techniques, and economic considerations.

Range Expansion and Introduction to North America
In the decades since the initial Asian observations, D. suzukii has been recorded in China, Myanmar, India, Italy, Thailand, Spain, and Russia (Toda 1987, Oku 2003, Hauser et al. 2009). It has been established in Hawaii since at least 1980 (Kaneshiro 1983). The first detected North American mainland invasion of D. suzukii was in Santa Cruz County, CA, in August 2008, on strawberries and caneberries (Bolda et al. 2010). In May 2009, economically damaging infestations were detected in cherry orchards in the Santa Clara Valley, and from Yolo to Stanislaus counties in California (Bolda et al. 2010). Further trapping and identification efforts in the fall of 2009 confirmed D. suzukii presence over a wide geographic range, including the entire length of California's coastal counties, western Oregon, the Columbia River Gorge, counties throughout western Washington, and north into British Columbia (Bolda et al. 2010, D.B.W. unpublished Kanzawa (1939, also identified as "Kamizawa" in some journals and reports) compiled an extensive early database on D. suzukii, cataloguing egg, larva, pupa, and adult sizes (Tables 1-4). He described the eggs as half transparent, milky white, and glossy, with transparency increasing and the developing larva becoming visible toward emergence. Larvae emerge milky white with visible internal organs and black mouthparts; visible respiratory ducts evolve from silvery white, inconspicuous preducts at the first instar to branched, open, and emerging to the larva's exterior by the third instar. The pupae were described as grayish yellow with soft pupal cases initially, later hardening and turning brown. A characteristic dark spot on the leading edge of the males' wings began developing 10 hours after adult emergence but took 2 days to become obvious. Kanzawa (1939) observed the life cycle of the fly in captivity (Table 5). He kept oviposition records and found the most active period of the 10 generations studied to be the span between the fifth and sixth generations, during the peaks of the cherry and grape seasons and at the most highly preferred temperature for D. suzukii, 20 -25°C. In his initial overwintering studies, only the adult life stage successfully survived winter. More recently, it has been observed that adults emerging from pupal cases during the early part of winter survived as adults (V.M.W., unpublished data). Only females survived extended cold periods in this study. Mitsui et al. (2010) found that D. suzukii adults collected in autumn were reproductively immature, suggesting winter reproductive diapause. Our findings on the development and life history of D. suzukii are summarized as follows.

Life Cycle
Adult. Adult D. suzukii are small (2-3 mm) flies with red eyes and a pale brown or yellowish-brown thorax and abdomen (Figs. 1 and 2).      Kanzawa (1939). These data were taken from experiments conducted in 1934. Ten pairs from each generation were observed, with earliest emergence reported in the table. Materials and methods were 8.7 cm diam glass enclosures 18.5 cm tall with gauze at the bottom and the mouth of the enclosure and bait hung from the top.
They have black stripes on the abdomen and males have a distinguishing dark spot on the leading edge near the tip of each wing. The spot may play a role in courtship but results are ambiguous for flies under diurnal light cycles (Fuyuma 1979). The female's serrated ovipositor (Fig. 3) is able to penetrate the skin of most thin-skinned fruit, leaving a small depression, scar, or "sting" on the fruit's surface (Fig. 4). A female lays Ϸ1-3 eggs per oviposition site, averaging 380 eggs in her lifetime. The number of eggs laid may depend on fruit maturity and eggs tend to be randomly distributed on cherry fruit (Mitsui et al. 2006). Multiple clutches of larvae are quite possible on the same fruit because many females may visit the same piece of fruit. Anecdotal observations have indicated that oviposition is most likely when fruits are entering peak ripeness and marketability. Initial research supports this: where red raspberries were observed and oviposition measured, no green fruit contained eggs, Ϸ5% of green/pink and pink fruit contained eggs, and Ϸ80% of ripe (not over-ripe) fruit contained eggs (Tanigoshi et al. 2010).
Egg. The egg develops and subsequently hatches inside the fruit in which it has been laid. Eggs are translucent, milky-white, and glossy; the milky quality diminishes toward emergence, and larvae become more visible.
Larva. Larval development occurs inside the fruit. Larvae are milkywhite and cylindrical with black mouthparts; they are Ϸ0.67 mm in length at emergence and may grow to 3.5 mm. Internal organs of the larvae are visible, particularly after it has consumed some of the fruit in which it resides. The larva develops through three instars, feeding upon the host fruit until it exits to pupate. This feeding creates brown, soft, sunken areas on the fruit, compromising its marketability and creating opportunity for other pests such as other vinegar flies and for infection by fungal and bacterial disease pathogens (Fig. 5).
Pupa. Pupae are initially grayish yellow with soft cases, gradually becoming brownish, then yellowing and hardening. Pupation occurs both inside and outside of the fruit, however pupation within the fruit is more common.

Host Range
Kanzawa observed that the fly oviposited most often on cherries, peaches, plums, persimmons, strawberries, and grapes in Japan, but was also opportunistic and would feed on other fruits dropped on the ground that spoiled or fermented. In the absence of preferred fruit, he found D. suzukii would feed upon oak tree sap. Drosophila suzukii has now been documented in apricots, blackberries, blueberries, cherries, figs, grapes, hardy kiwis, nectarines, peaches, pears, persimmons, plums, pluots, raspberries, and strawberries; it has additionally been observed feeding upon injured or culled fruit including apples and oranges. Primary concerns in the Pacific Coastal states of Washington, Oregon, and California are the small fruits blueberry, blackberry, raspberry, marionberry, boysenberry, and strawberry, and the stone fruits cherry and peach; wine and juice grapes are also of concern.
The wide variety of documented hosts, plus the presence throughout the Pacific Coastal states of ornamental and native fruiting plants (e.g., wild rose, dogwood, snowberry, crab apple, flowering cherry) that may serve as alternate hosts and that ripen at different times throughout the year, exacerbates cause for concern about the spread and potential pest status of D. suzukii. In addition, adult D. suzukii have been observed in the upper reaches of trees and ovipositing in some species of flowers in the absence of preferred hosts. Mitsui et al. (2010) found that D. suzukii bred on Stylax flowers at the end of May and the start of June, when the fruiting season of wild Prunus starts. Nishiharu (1980) observed this fly bred (rarely) on Camellia japonica L. flowers in early spring in southern Japan. Toda (1987) reported finding D. suzukii most often at a height between 9 and 11 m in the forest trees, but does not suggest why the flies might dwell so high in the canopy. In the Seattle, WA, region, high numbers of D. suzukii were reported in the upper one-fourths of a fruit tree where aphids and honeydew were prevalent. We speculate that the adults are staying alive there until preferred fruit hosts are available. The pest's mobility and its utilization of a range of agricultural, home landscape, and native plants make educational outreach and eventual control challenging.

Potential Geographic Range in North America
Drosophila suzukii prefers a moderate climate such as that along the Pacific Coast, but Kanzawa (1939) observed it surviving and thriving in relatively cold areas of northern Japan. Mitsui et al. (2010) suggested that the fly may migrate to avoid resource poor conditions. Based on the fly's expansion in 2009 and a preliminary Maxent climate matching model by Martin Damus of the Canadian Food Inspection Agency (Damus 2009), range expansion might be anticipated throughout the Pacific Coast from Northern Baja California to Central British Columbia (Fig. 6). Inland, preliminary Climex modeling using the pest's native range and extrapolated to North America indicates that much of the United States from roughly east of the Mississippi River might be suitable for D. suzukii to complete its life cycle (Damus 2009).
Climatic Limitations. D. suzukii are most active at 20°C (68°F) and activity becomes reduced at temperatures above 30°C (86°F). David et al. (2005) found thermal heat sterility thresholds variable from 23°C in heat-sensitive Drosophila spp. to 31°C in heat-tolerant species; D. suzukii seem to be toward the heat-tolerant end of the spectrum, with frequent anecdotal reports of sterility in males at temperatures above 30°C. Eggs, larvae and adults can die at temperatures below freezing, but this does not necessarily mean a whole population will be eradicated at low temperatures because D. suzukii is firmly established on the island of Hokkaido in Japan where winters average Ϫ12 to Ϫ4°C. Kimura (2004) found the cold LT 50 for female D. suzukii originating from Sapporo to be Ϫ1.6°C and the hot LT 50 to be 32.6°C. For males, the temperatures were Ϫ0.1°C and 32.6°C, respectively. Temperatures were similar for D. suzukii originating from Tokyo. Kimura (1988) reported that the adult stage is more tolerant to cold than other drosophilid developmental stages except in those species that overwinter in the larval or pupal stage.
Preliminary findings from research conducted in Oregon in the winter of 2009 -2010 suggest that some percentage of D. suzukii larvae, pupae, and adults have the potential to survive fluctuating overwintering conditions for periods up to 60 days. Adults were able to withstand longer periods of cold than larvae or pupae (A.J.D., unpublished data). In these preliminary, replicated studies, laboratoryreared subject pests were placed in 15 cm vials with moisture and a cornmeal-yeast-based media and exposed to fluctuating outdoor temperatures from late January through late March. The temperatures ranged from 0.72°C to 17.0°C and averaged 8.6°C with an average relative humidity of 82% in protected pockets of leaf fall for exposure periods of 2, 7, 14, 30, and 60 days. Six percent of the larvae and 13% of pupae survived to adulthood, while 39% of adults survived over the 60-day exposure period. Surviving adult females were able to oviposit. When flies were acclimated before direct exposure to outdoors and the amount of moisture accumulating in the vials was regulated to prevent condensation and subsequent drowning of D. suzukii, survival increased. However, when exposing pupae and adults to constant cold temperatures in lab studies, higher mortality rates occurred at colder temperatures (V.M.W., unpublished data). Individuals were acclimated to respective constant temperatures during a 2-week period, after which they were subjected to five temperatures of which 10°C was the highest for 6 weeks (42 days). Surviving individuals were then subjected to 10°C for an additional 42 days followed by another 22 days at 20°C. Less than 2% of adult females survived the total trial period of 120 days.
In addition to temperature sensitivities, vinegar flies in general are very sensitive to desiccation, and will die within 24 hours in the absence of water. Other Drosophila spp., most notably D. melanogaster Meigen, have been shown to exhibit differentiation in tolerance to cold as well as desiccation depending upon climatic conditions (Davidson 1990). Bradley et al. (1999) demonstrated that desiccation resistance could evolve over time among successive generations of D. melanogaster. While similar studies have not been conducted on D. suzukii, the relationship of the species is known to be close: Harr et al. (2000) identified the two as belonging to a monophyletic clade and Schmidt et al. (1993) confirmed the two as having a homologous sex-peptide. Degree-day models for D. suzukii are under development for the Pacific Coast states and Canada (Damus 2009, Coop 2010. Kanzawa (1939) observed D. suzukii lifespans (from egg through oviposition) ranging from 10 to 24 days, with seven of the eight generations for which lifespan was recorded being 16 days or fewer (Table 5). Recent laboratory observations of the D. suzukii life cycle document development from egg to egg-laying female as 12-15 days at 18.3°C (65°F). Another study at 21.1°C (70°F) found it took a little more than a week. Kanzawa's early observations across a wide geographic range in Asia indicated that the number of generations could range from 3 to 13 annually, depending upon a variety of climatic factors. Drosophila suzukii is believed to complete up to 10 generations per year in California. Based on degree-day models, it is estimated that 3-9 generations may develop annually within its current range in the Western United States and Canada. Coop (2010) developed a preliminary degree-day model as a framework for asking questions about D. suzukii biology and for guiding research projects. The model is based on a combination of local data, Sasaki and Sato (1995) and Kanzawa's (1939) research findings. Coop estimates a lower threshold of 10°C and an upper threshold of 30°C, beginning January 1 using a single sine curve calculation method. Initial spring activity is predicted to begin at 250DD, 50% egg laying on 490DD, and an accumulation of 744DD for development of egg to adult in the first generation.

Monitoring and Management
Kanzawa experimented with a variety of baits (molasses, grape wine, rice wine, cherry wine, acetic acid, citronella oil, geranium oil, sugar, and cherry juice, in various combinations) and trap densities, and also tested the efficacy of several chemical control regimens. He found that camphor oil was the most efficacious of the treatments he employed, followed by nicotine sulfate, pyrethroid soap, kerosene emulsion, and neoton. No treatment totally prevented oviposition, however. Kanzawa also explored the efficacy of natural enemies on D. suzukii, but his laboratory experiments with parasitoid wasps (Proctotrypoiclea Diapriidae Phaenopria sp.), had minimal impact on pest mortality. Mitsui et al. 2007 evaluated dominant parasitoids attacking frugivorous Drosophilidae species in Japan. They determined that Asobara (Braconidae, Alysiinae), Leptopilina, and Ganaspis species (Figitidae, Eucoilinae) were distributed throughout Japan. Other parasitoids showed rather restricted distributions; Asobara tabida Nees, A. rossica Belokobylskij, A. rufescens Foerster, and Leptopilina heterotoma Thomson occurred mainly in northern to central parts of the main islands, Ganaspis xanthopoda Ashmead from central to southern parts of the main islands, A. leveri Nixon in a southern part of the main islands, and A. pleuralis Ashmead, L. victoriae Nordlander, and Ganaspis sp. mainly in the subtropical islands. They concluded that host use considerably varied among parasitoid species, especially in the subtropical islands.
Trapping/Monitoring. Growers of known D. suzukii host fruits are encouraged to monitor for the presence of the pest before initiating control measures. No species-specific commercial traps are currently available, but traps used for other vinegar flies should attract D. suzukii as well. Emerging recommendations in the Pacific Coast states involve use of bucket-style traps or quart containers with Ϸ5-to-10 mm-diameter mesh or holes baited with an attractant. Baits currently in use include mixtures of yeast, sugar, and water; fruit purees; various distillates from apple cider vinegar to wine; ethanol, acetic acid, and phenylethanol 1:22:5; and Yellow Muse from AgBio. Other baits that may potentially attract D. suzukii and require testing include the commercial lure for D. melanogaster; a yellowjacket lure known as Invite; ammonium acetate crystals; and methyl salicylate. The yeastsugar-water mixture creates CO 2 and is highly attractive. Use of apple cider vinegar is also widely suggested because it has the advantage of longevity in the field as well as being transparent enough for easy fly identification. A small drop of dish soap added to the apple cider vinegar as a surfactant or placement of a sticky card within the trap results in more fly captures (not because the soap or the card attract more flies, but these elements increase the traps' success at holding flies that enter the traps). Traps for D. suzukii seem to perform best when deployed in the cooler, shadier areas of the field or orchard. Scouts are sometimes encouraged to count male D. suzukii only, as they are easier to positively identify. One example of a trap is shown in Fig. 7.
Cultural Management. While no set management program has yet been determined for D. suzukii, a successful one will be similar to that used for other Drosophila species except that there is a need to keep the flies at low densities when ripening fruit, not just overripe fruit, are present. Grower focus should include proactive control of adult vinegar flies before they mate and lay eggs, and sanitation targeted toward eliminating breeding sites in overripe fruit where D. suzukii can complete its life cycle. There are no registered insecticides that will control maggots within fruit, so chemicals must target adult control. Essential components of a management program will likely include basic sanitation and attention to timely harvest intervals before fruit reaches peak susceptibility to D. suzukii.
Sanitation measures include removal and destruction of both infested fruit and any ripe, overripe and rotten fruit at the crop site that can serve as a host. Methods for fruit removal will vary based upon the host crop, size of operation, the number of harvests taken by crop, and the economics of the situation, but growers in general are encouraged to remove cull piles and to deal with fallen fruit in some fashion. Research is underway to evaluate solarizing, burying, bagging, crushing, and spraying infested fruit to discourage D. suzukii colonization. Removal/destruction of overripe/rotting fruit will be especially important if a nearby susceptible crop will soon be ripening. Regarding harvest intervals, it is important to remember that extending harvest intervals, as may occur for a processing crop, may result in larger populations, more fruit damage, and a greater risk for future infestations of fruit crops that will be harvested later in the area.
Backyard gardens are considered an important part of an areawide approach to D. suzukii management. Coordinated efforts by Cooperative Extension personnel in California, Washington, Oregon, Idaho, and British Columbia are underway to educate homeowners and regional pest control boards about monitoring (how to and when), pest identification, sanitation, mass trapping, pest exclusion (i.e., covering fruiting plants with netting before oviposition), methods for checking fruit for infestation (e.g., fruit dunk flotation), and appropriate use of insecticides. A mapping and reporting program of fly numbers for noncommercial gardeners is also underway.
Biological Control. Parasitoid wasps (Braconidae and Cynipidae) are thought to be the primary Hymenopteran families targeting Drosophila spp. and have potential as biocontrol agents against D. suzukii  (Kanzawa 1939, Janssen 1989, Driessen and Hemerik 1991. For instance, some cynipid parasitoids are known as exclusively primary parasitoids of Drosophila spp. (Dubuffett et al. 2009), A search of literature on Drosophila parasitoids revealed more than 10 genera belonging to three Families of Hymenoptera. Timing of emergence and other factors have yet to be evaluated and little is known about the potential contribution these parasitoids can make to an integrated management system. The role of other beneficial organisms including predators, parasitoids, and pathogens has yet to be assessed. Orius, a generalist predator in wildlands and agroecosytems, was observed feeding on D. suzukii larvae in backyard raspberries in the fall 2009 (A.J.D., unpublished data). Preliminary studies in the laboratory indicate that Orius insidiosus Say can feed on D. suzukii larvae infesting blueberries (J.L. and D.J.B., unpublished data).
Chemical Management. Several insecticide classes have proven effective against D. suzukii including spinosyns, organophosphates, pyrethroids, and neonicotinoids. Longest-duration control was achieved with malathion and diazinon, with reports of good duration from zeta-cypermethrin, cyfluthrin, beta-cyfluthrin, spinetoram, and beta-cyfluthrin plus imidacloprid as well. Permethrin, imidacloprid, thiomethoxam, esfenvalerate, lambda-cyhalothrin, and some formulations of spinosad have proven effective but with a shorter duration of control (D.J.B. and J.G., unpublished data). Several studies have shown that there are no organic insecticides as effective and persistent against D. suzukii as the conventional products. The spinosad formulation GF-120 has no knock-down activity, but will suppress populations over time. However, efficacy of GF-120 was variable when populations increased and attractiveness of the product decreased as the fruiting season progressed. Pyganic and Entrust have a degree of efficacy, but residual impact is limited to 1-3 days. Horticultural oils, azadirachtin, Organocide, and Ecotrol offered no significant control in California field trials. Recent laboratory trials in Oregon supported early reports that the organophosphates diazinon and malathion had the longest residual activity against D. suzukii, and indicated that zeta-cypermethrin had the longest residual efficacy of the pyrethroids tested (D.J.B., unpublished data). In these same trials, spinetoram had residual activity comparable to zeta-cypermethrin; this was supported by results from field trials conducted in California.
Drosophila suzukii's preference for ripening fruit presents timing difficulties with respect to preharvest intervals and pollinator protection. The most effective time for applying controls is when the fruit is ripe or very nearly ripe, necessitating chemicals with a shorter preharvest interval. Growers of bee-pollinated crops may find that they need to remove their bees slightly earlier than optimum to spray late-flowering fruit before D. suzukii infestation if bee kills are to be minimized. For short-residual insecticides, evening applications may be recommended. Research is underway both in the laboratory and in the field to determine the time of oviposition, taking factors including fruit coloration, sugar levels, pH, and firmness into consideration.

Economics
While it is difficult to estimate economic damage because of significant gaps in scientific knowledge about D. suzukii's biology and control, yield loss estimates from 2009 observations range from negligible to 80%.
The majority of United States small fruit production occurs in the Pacific Coast states. In 2008, California, Oregon, and Washington accounted for virtually all commercial United States raspberry and blackberry production, 84% of the value of commercial cherry production, 83% of the value of strawberry production, and 26% of the value of blueberry production. For these five crops as a group, 76% of the total value of United States commercial production is grown in these three states, or $2.6 billion of value. Table 6 reports the value of production by state for each crop, and the share of each state's production in the national total (USDA NASS 2009).
Very little is known about the aggregate potential for crop damage because of D. suzukii. Bolda et al. (2010) offered a benchmark estimate that specifies identical yield losses across crops and regions. Assuming a yield loss of 20% for all crops, production of these five crops in these three states could sustain $511 million in damages annually because of D. suzukii. Closure of markets because of quarantine and postharvest fumigation of fruit shipped from D. suzukii infested regions will also impact grower returns.

Future and Next Steps
As we enter into the 2010 season, the fact is that we do not know what to expect of D. suzukii in the Pacific Coast states or other areas of North America. Area-wide monitoring is taking place throughout California, Washington, and Oregon as well as in southern British Columbia. Upper Midwestern states including Michigan, Atlantic Coast states such as North Carolina and Virginia, and southern sections of Canadian provinces including Ontario are taking an active interest in tracking the pest in case it should appear in their growing regions. The full range of D. suzukii crop hosts is unknown, as is the fly's ultimate geographic/climatic range. Cooperative projects involving Western states and Canada are underway to address knowledge gaps and prepare recommendations for dealing with this invasive new pest.