The spotted wing drosophila (SWD) is an invasive pest from Asia, although it has been established in Hawaii since about 1980. It was first discovered in the continental US in California in 2008. Detections had been made throughout the Pacific Northwest by 2010, including eastern Washington. Detections have been made in much of the US, Europe, and South America at this point in time, making it a very cosmopolitan pest. Spotted wing drosophila differs from the common drosophila (Drosophila melanogaster) in that it can attack undamaged fruit still on the tree; the common species will only attack damaged, overripe, or dropped fruit. Because of the relatively recent nature of this introduction, we are still learning about its biology and control.
Sweet cherries are currently the tree fruit host of greatest concern in eastern Washington. Pome fruits, non-cherry stone fruits, and grapes are generally not at risk in our area. Berries are high-risk hosts, including blueberries, blackberries, marionberries, raspberries, and strawberries. Other recorded hosts are figs and kiwis. It should be noted that while SWD, like most Drosophila, will attack damaged or dropped fruit of many types (which are often included in “host lists”); however, this is not the same as the definition of a crop pest.
Eggs are translucent white, elongate, and about 0.6 x 0.18 mm. They are laid in the flesh of the fruit. There are a pair of “breathing tubes” attached to one end of the egg; these protrude from the puncture made by the female’s ovipositor and are visible as white threads on the surface of the fruit. These are frequently fused at the tips, giving the appearance of a single thread.
Fully grown larvae are typical Drosophila larvae, with black mouth hooks at the front end, and a pair of distinctive tan caudal spiracles at the rear end. The caudal spiracles, along with the prothoracic spiracles, can be used to differentiate the larvae of Drosophila from the larvae of cherry fruit flies (tephritids). There are three larval instars, with the last instar about 4 mm long.
Pupae are medium brown with two distinctive structures at the anterior end, resembling the prothoracic spiracles. Pupae are about 3 mm in length, with females slightly larger than males.
The adults resemble Drosophila melanogaster except for a few distinguishing characteristics. They are slightly larger (about 3 mm in length), with a tan body and red eyes. There are 1 pair of wings, plus the halteres (vestigial hindwings) common to the Diptera. The abdominal segments have dark brown bands on the posterior edge. The key character of female D. suzukii is their heavily sclerotized and serrated ovipositor, which is lacking in D. melanogaster. The key character for the males is the single spot on each wing, from which the species gets its common name. The spots are on the leading edge of the wing near the tip. However, it should be noted that the spots are not visible when the adult male first emerges; it takes 8-10 hours for them to be fully formed. An additional characteristic of the males are the dark bands encircling the forelegs.
Development of SWD from egg to adult requires about 8-11 days at optimal temperatures under laboratory conditions (mid- to high 70s). The egg stage lasts from 1-3 days, the three larval stages 3-13 days, and the pupae 4.5 days. Development is proportionately slower at lower temperatures. The life cycle in the field during the cherry season is likely about 2-3 weeks during the growing season. Summer adults can live 3-9 weeks, although the adults that overwinter live for many months. The females can lay several hundred eggs in their lifetime.
The life history is typical of an insect that can reproduce quickly, and produce succeeding generations in a short amount of time as long as a suitable host and temperatures are present. SWD overwinter in the adult stage, with higher survival in mild-winter climates, or milder winters in eastern Washington. While the cold winters and hot, arid summers are suboptimal for this species, it is permanently established here. It thrives best in mild-winter climates with a temperate summer, although it is also established in hot summer climates such as California’s central valley. Activity is suppressed by extremely cold or hot temperatures but resumes when temperatures are more favorable.
The pattern of activity for SWD during the growing season in eastern Washington has been reasonably predictable. Adults are first detected in May or June and continue to increase throughout the summer. There is a peak of adult capture in the fall, usually September-October, but activity can continue high through early December in very mild years. The high numbers of adults caught in the mid-August onwards is generally after the harvest of sweet cherry, thus there can be a certain amount of ‘temporal escape’ for early cherry cultivars. In very mild winters, activity starts earlier, and risk of damage may be higher as fruit approach maturity.
Damage is caused by oviposition by the females, and larval feeding in the fruit. Fruit becomes soft, and subject to decay. The initial oviposition site takes on a sunken appearance. After larvae become larger, they cut breathing holes in the side of the fruit, further enhancing the opportunity for decay.
Cherries are generally not considered susceptible to damage when the fruit is green, and only marginally so when they reach straw color. They are fully susceptible starting several weeks before harvest, and growers should be most vigilant at this point. The period of susceptibility is similar to cherry fruit fly.
Because is it already widely distributed in the US, there is no internal quarantine on this insect (unlike cherry fruit fly). Sixteen countries list SWD as a ‘Harmful Organism’, but this pest is considered a food quality issue regardless of quarantine status.
The adult flies can be monitored using a variety of traps. A trap system is composed of 1) an attractant and 2) some means of capturing and retaining the adults so they can be identified. The attractants have been chemically characterized as volatiles from fermenting wine and vinegar (which involves yeasts) and can be presented as a mixture of liquid products, or as synthesized components placed in a pouch (or pouches). While originally only home-made traps and baits were available, there are currently a number of commercial baits, lures and traps on the market.
The characterization of the volatiles from fermenting vinegar and wine mixtures was the impetus for creating synthetic lures; several are available from commercial sources and offer a long-lasting alternative to liquid bait mixtures. Synthetic lures also offer consistency. Liquid baits may evaporate or volatilize rapidly, or if based on yeast and sugar, be contaminated by the insects that fall in the liquid. The degree of attractiveness has varied in different crops or regions of the country, thus data from other areas should be interpreted with caution. One of the original liquid baits used (apple cider vinegar) was more effective at cooler temperatures, which is past the critical decision-making period for cherries.
Traps fall into two basic categories: liquid reservoirs (cups or jars) and sticky panels. A liquid trap can use either a pouch lure or a liquid bait as an attractant, while the sticky panels must necessarily use a lure pouch as an attractant. Yellow, white, and red sticky panels have all been tested for SWD capture, but yellow panels have worked consistently well in Washington. Most of the liquid trap designs are similar, whether commercial or home-made. They have about a ½ to 1 quart capacity with about 200-300 ml (7-10 fl oz) of either the bait mixture, or if a lure pouch is used, drowning fluid. The cup traps have a series of small holes or larger mesh-covered holes to permit diffusion of the odor of the bait or lure, permit the entry of SWD, and screen out larger insects. This fluid (water with a few drops of surfactant and preferably, some form of preservative) serves one of the functions of the bait mixture, which is to keep specimens pliable for identification. The sticky traps are easier to examine in the field, and the spotted wings of the males can be readily identified; but examining the ovipositor on a desiccated female can be difficult. For this reason, most sticky trap users count only male SWD, but this ignores the more important (egg laying) form, the females.
Traps should be checked at least weekly to examine them for SWD. For liquid traps with a lure pouch, the lure may last up to 6 weeks (see manufacturer recommendations) before it needs to be changed. Regardless, the drowning fluid should be removed and replaced, and the flies counted weekly. The same is true if the attractant is a liquid bait. Sticky panels should also be checked weekly, but flies can be removed and counted, and the trap left in place as long as the adhesive is still viable. When it becomes dirty or unusable, the sticky trap should be replaced, and the lure replaced as per the manufacturer recommendation.
If your goal for trapping is spray decisions only, you may elect to discontinue trapping once that decision is made (see Management). However, it is useful to have trap records in the fall to get an idea of the overwintering population, and to compare population density over years.
Trapping is a compromise between maximum catch and ease of use. Maximum catch of SWD is often linked to high levels of by-catch (other drosophila species). These other species greatly complicate sorting out the target species, SWD, because of their similar appearance. Thus, trap selectivity is a potentially important aspect of a trap system.
Predators and parasitoids of SWD have been studied since the discovery of this pest in the US. A number of predators have been identified, mostly generalists. While the broader impact of predators is still unknown, specific studies have shown good predation rates of pupae in the soil by ground-dwelling predators, including spiders, ants, rove beetles, and earwigs. The egg and larval stages are hidden in the fruit, but some predators, including anthocorid bugs (Orius sp.), may forage there.
The current focus for biological control, however, is on parasitoids. Early surveys in the western US indicated a cosmopolitan species, Pachycrepoideus vindemmiae (a pteromalid pupal parasitoid) was present, but overall impact was low. There are several species of figitid wasps (larval parasitoids) that appear to be more host specific, including Leptopilina spp. and Ganaspis brasiliensis. The latter is an Asian species that has been collected from the native range of SWD, and is currently being tested in US quarantine facilities as a potential classical biological control candidate. The goal for parasitoid releases is primarily suppression in non-crop areas, because parasitoid wasps tend to be susceptible to the insecticides used against spotted wing drosophila.
To date, the primary method of preventing damage by this pest is full-coverage sprays. The bait sprays that have worked so well for western cherry fruit fly are currently NOT a stand-alone tactic against spotted wing drosophila. The bait system works on western cherry fruit fly (or other Rhagoletis species) because of the biology of the females; they have a ca. 10 day pre-oviposition period after they emerge where they feed in order to mature their eggs. Spotted wing drosophila has a relatively short pre-oviposition period (as short as 1 day). Even if the adults are attracted to (and killed by) the bait, it may not provide protection against this pest.
Place traps in the orchard about straw color of cherries, and check them at least weekly. The provisional threshold to trigger a spray is the capture of a single adult; this may change in the future as we gain a better understanding of active area of the traps and economic thresholds. If the crop has not been harvested, then a protective schedule of full coverage sprays is needed to prevent fruit infestation. If the crop has been harvested, or left unharvested due to condition or markets, post-harvest sprays targeting both adults and larvae in the fruit should be applied.
Pesticides, for either pre- or post-harvest use, have not been rigorously tested in field tests eastern Washington; however, field-lab bioassays provide a good profile of efficacy and longevity of residues. See the WSU Crop Protection Guide recommendations below for specific products.
Spotted wing drosophila may build up in dropped fruit of crops not really at risk from preharvest damage, and these populations may serve as a reservoir for nearby at-risk crop hosts. Similarly, SWD may also use native fruit species, such as Himalayan blackberry, but is only common in about five native species. Be aware of your unmanaged surrounding habitat if you are experiencing persistent SWD problems.
Updated June 2021
Original article: Spotted Wing Drosophila | WSU Tree Fruit | Washington State University