Spotted Wing Drosophila (SWD): FAQs And Links

What is spotted wing drosophila?

Spotted wing drosophila (Drosophila suzukii) or SWD is a species of fruit fly, about 3mm long. Unlike the more common fruit fly (Drosophila melanogaster), the male SWD has a distinctive spot on each wing (hence the common name). The female has black marks on the forelegs and a distinctive 'saw-like' ovipositor discernible by trained entomologists.

Where does it come from?

SWD is not native to the UK, but originated in Asia and has long been a species that people have had to contend with in its native range.

Where does it occur?

SWD now occurs in most food producing continents. It arrived in the UK in 2012 and is now established in woodland and cultivated host plants.

Is it a notifiable pest?

No - in July 2012 the EU Standing Committee on Plant Health agreed that because of its biology and life-cycle official phytosanitary measures were unlikely to prevent further spread and introduction. As a result SWD would not be regulated at the EU level.

How does it spread?

It is spread by flying and through the movement of contaminated fruits. However the movement of contaminated fruit allows it to move between countries and across continents.

Which cultivated fruit crops does it affect?

Experience from other countries, where SWD occurs, has reported the following fruit crops as suitable hosts: blackberry, blackcurrant, blueberry, cherry, fig, kiwifruit, nectarine, peach, plum, raspberry, redcurrant, strawberry and table grapes.

Other species potentially at risk include: apple (damaged), apricot, elderberry, gooseberry, pear (damaged), tomato (damaged) and wine grapes (damaged).

Does it have other hosts?

Apart from fruit crops, other hosts include: wild blackberry, wild cherry, dogwood, elderberry, hawthorn, honeysuckle, mahonia, mountain ash (rowan), mulberry, nightshade, wild raspberry, rose and snowberry. Early and late sugar sources include; holly, insect honeydew and ivy.

SWD occurs in a wide range of habitats including; suburban areas, gardens, hedgerows, forests, and woodlands.

What do we know about its life-cycle?

Overwintering adults are the parents of the summer generation. UK research has demonstrated that the adults overwinter in a different form (winter morph) which withstands lower temperatures, enabling populations to survive cold Winter weather. Like most fruit flies and other insects, the female uses its ovipositor to lays eggs in the host plant or fruit. The adults will fly during mild periods in Winter and Spring, but they do not lay eggs between November and April.  The duration and temperatures of the UK winter will affect winter survival rates and the time of detection in Spring and Summer. Therefore it is important to monitor early.

Populations will build up as the temperature gets warmer, with numbers if not managed reaching a peak in autumn/winter.

How does it affect fruit crops?

The pest can cause characteristic sunken blemishes on ripening fruit. These blemishes are identified by trained workers and the fruit removed during the packing process.

I grow one of the host plant species, how do I know if SWD is present?

The best way is to monitor for the presence of adults. This can be achieved by amateurs using home-made bottle traps containing a solution of apple cider vinegar. Growers and gardeners can purchase purpose-built monitoring traps or sticky cards.

Whichever method is used, the trap should be checked each week for the presence or absence of SWD. The liquid bait and/or sticky card should be replaced weekly.

Further information is included in the section on ‘Spotting, monitoring and trapping spotted wing drosophila’.

Where should I put my trap?

Locate the trap as near the potential host plant as possible, for instance, near the edges of plantations, near hedges, wild hosts and vulnerable crops.

• In stone fruit, cane fruit and bush fruit crops, hang the trap at one third of the canopy height on the shaded side of the row.
• In outdoor strawberry crops, place the trap within the leaf canopy and secure well.

Further information is included in the section on ‘Spotting, monitoring and trapping spotted wing drosophila’.

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What can I do to control it?

Good plant hygiene is essential for both amateurs and commercial growers. All waste and overripe fruit should always be removed from the plant and disposed of responsibly. This is good practice for all gardeners as it will also reduce the incidence of other pest and disease species occurring in fruiting plants.

Commercial growers have other options available but should always seek advice on control measures by consulting a BASIS qualified agronomist.

Further information is included in the section on ‘Spotting, monitoring and trapping spotted wing drosophila’.

How is the horticulture industry managing SWD in the UK?

Following industry consultation, it was agreed that the UK Plant Health Service would work in collaboration with the industry to monitor for SWD in the UK, track its movement and provide guidance on its management and control.  

An SWD Working Group has supported the industry through the following activities:

• Production of a code of practice for fruit growers and fruit packhouses to follow when monitoring for SWD and to reduce the spread
• Production of identification sheets and posters for growers and packhouses including our identification poster and our flotation test poster
• Working with the Chemical Regulation Directorate to identify and approve potential new crop protection products which may contain and control the pest
• Monitoring current and new research projects. The results are continually shared with the industry.

The working group meets on a regular basis to maintain the momentum of its work and reports back (including growers, all fruit marketing groups and bodies from other horticultural sectors) to ensure the industry is fully aware of continuing developments.

Is there any UK specific SWD research happening?

There is a significant volume of research being funded on the biology and control of SWD in USA, Canada and mainland Europe. This information is being collated and communicated to the industry by UK scientists. In addition to this there are a number of UK specific research projects under way to improve our understanding of how SWD behaves in the UK climate and UK cropping systems whilst seeking to find new control measures which are suited to UK conditions.

Is there further support available to help identify SWD?

If you need help to confirm if you have caught SWD, an adult identification service is being offered by specialist entomology teams at NIAB EMR and The James Hutton Institute. Contact the following for further information:

Michelle Fountain, East Malling Research, New Road, East Malling, Kent, ME19 6BJ, Tel: 01732 843833

Gaynor Malloch, The James Hutton Institute, Mylnefield, Invergowrie, Dundee, DD2 5DA, Tel: 01382 562731