Spotted wing drosophila (Drosophila suzukii), or SWD, is a vinegar fly originating from Asia, but unlike most vinegar flies, D. suzukii is able to attack fully intact susceptible ripening and ripe fruit. Currently, susceptible fruit include blueberries, caneberries and cherries. In some cases, wine and table grapes may be affected dependent on compromised berries due to cracking, disease and bird damage. Wine grapes are however not a comparatively good host fruit. Larvae develop within fruit rendering it unmarketable. Damage is therefore similar to that of true fruit flies, as the larvae feed in the developing fruit, which contribute to secondary infections by bacteria and fungi. D. suzukii has not yet been documented from South Africa, or indeed Africa, but is able to survive within all important susceptible fruit producing regions. It is currently present in North and South America, Europe, the UK and Asia.

Biology

Up to 15 generations have been recorded per year, although generations overlap early in the season, while the life cycle (from egg to adult) can be completed in 10 days at 25°C. Due to its high reproductive capacity and ability to disperse easily, it is particularly important to be aware of the presence of this pest as it may cause significant economic losses within a short time period. Growers have been known to harvest and ship perceived uninfected fruit only to face rejections at their destination.

Economic impact and threat

Crop losses of 20 – 40% have been reported in Washington and Oregon on berry crops, amounting to yearly management costs ranging between US$12 and US$16. This does not take into account the loss of export markets, which is associated with the threat status of this pest. The risk of introduction is via movement of fresh fruit between countries, as the larvae are difficult to detect in fruit. Suitable habitats for establishment include natural areas, cultivated agricultural land and glasshouses, as well as urban areas in agricultural regions. Other pathway causes for introduction include cut flowers, food, as hitchhikers and on ornamentals.

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Male with spots on wings (photo: Vaughn Walton, OSU).

SWD damage on blueberries (photo: Vaughn Walton, OSU). Female with serrated ovipositor (photos: Vaughn Walton, OSU). Difference between SWD and Drosophila melanogaster pupae (photo: Vaughn Walton, OSU). Trap design for SWD (photo: Kate Mitchell, SU).

Damage symptoms to look out for include:

  • Small holes in fruit with sap leaking out.
  • Splitting of grape berries.
  • Development of early mould, wrinkling and softening.
  • Dark spots on white grapes and vice versa on black grapes.
  • Presence of either larvae or pupae at infestation site.

Identification

Molecular tools are available for identification, through DNA barcoding techniques. No reliable diagnostic features have been identified for eggs, larvae and pupae. Adults of most Drosophila species have the same basic features; they are no larger than 3 mm in length with prominent red eyes, light brown body with some black striping. Males also tend to have a solid black abdomen and be slightly smaller than the females. Female D. suzukii are the only drosophilid with a prominent ovipositor that allows this particular species to pierce the skin and lay eggs in undamaged susceptible fruit. Males exhibit black spots towards the outside edge of the wing that give the species its common name.

Monitoring and surveillance

Traps can be made from any 250 – 750 ml clear plastic containers, with a tight fitting lid. Small holes (±3 mm) in the sides will allow flies to enter, and mostly exclude larger non-target insects. The trap should be baited with a combination of apple cider vinegar and wine, with a small drop of liquid soap.

Management

Control currently is reliant on chemical pesticides and targets egg-laying adults. This method of control however needs to be employed weekly as adults only make up 5 – 10% of the total population and newly emerging adults again attack fruit as soon as the residual of the sprayed pesticides wear off. This phenomenon necessitates many applications to manage SWD. Post-harvest mitigation treatments are available for exporting fruit into pest free areas.

References

Invasive Species Compendium, www.CABI.org. Accessed 16 October 2014, modified 7 October 2014.

Walton, V.M., Lee, J., Bruck, D. & Dreves, A.J., 2010. Recognizing fruit damaged by spotted wing drosophila (SWD), Drosophila suzukii. OSU Extension Service, EM 9021.

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