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Foot-and-mouth disease
The fly called spotted wing drosophila (SWD, Drosophila suzukii) is emerging as a global plant pest of significance. It attacks a range of soft skinned fruit and reduces crop yield and quality through direct feeding damage and secondary infection of the fruit.
Adult male flies (right) show distinct spots on the ends of their wings, which give the species their name. Adult females (left) do not have spots on their wings. Photo courtesy of John Davis
The adult is similar in size and appearance to the common vinegar fly (Drosophila melanogaster) to which it is related. SWD adult flies are 2-3 mm long, yellow-brown in colour, with dark abdominal bands and red eyes. The males have distinct spots on the ends of their wings and are smaller than the females.
Unlike other vinegar flies that you may be familiar with, the larvae of SWD feed on maturing fruit, not just over-ripe or decaying fruit.
The preferred hosts include berries, cherries, grapes, nectarines and plums. Apples, pears and other fruit with thicker skins are also hosts when fruit is damaged or begins to rot.
Although it has never been found in Australia, recent incursions in the United States and Europe have raised concerns about the impact if this fly were to breach Australian borders. It’s number three on Australia’s list of National Priority Plant Pests.
Left: Spotted wing drosophila in ablueberry. Image: Matteo Maspero and Andrea Tantardini – Centro MiRT Fondazione Minoprio. Right: Spotted wing drosophila larva. Image: Frank A Hale, University of Tennessee. One generation, from egg to adult, may occur in as few as 7-10 days, depending on temperature.
The circumstances surrounding first detections of SWD in over 30 countries – spanning North America, South America, and Europe – has been investigated. Researchers at cesar and Plant and Food Research Australia looked at how SWD travels long distances, how quickly it spread after the initial incursion, and the best way to detect it, to learn what we can do here in Australia.
This fly is adaptable and its ability to survive in warm and cool climates has helped it to rapidly spread between countries, and between regions within countries. Only one year after a confirmed 2008 detection of SWD in California, the fly had been found in 20 counties across the state, as well as the states of Oregon, Washington and Florida.
How does it travel so quickly? Its rapid spread is partly due to the ability of SWD larvae to make a home out of developing or ripe fruit until pupation. With the larvae hidden away inside fruit, it can be dispersed long distances by the movement of produce. Its apparently rapid spread is also a result of increased surveillance. Once people knew what to look for, detections occurred across a wide area.
Recent modelling work predicts that spotted wing drosophila would spread down the eastern seaboard of Australia, as well as Tasmania, and the south west of Western Australia within about six years in the absence of control measures.
Detecting SWD early is one of the best defences we have against an incursion of this pest. Most early detections overseas have occurred in horticultural regions, which may be because of proactive trapping and grower vigilance.
Overseas, SWD has also first been found at fruit transit hubs. Examples include near a grocery store in Sweden, by a selling point for imported fruit in the Netherlands, and in a tourist area in Croatia. The first detection in Hungary was at a highway rest stop. Detections near major seaports have also been common, showing humans can help the spread of this tiny fly by moving and disposing of fruit poorly.
The movement of imported fruit is the main risk for bringing SWD into Australia. If SWD were to arrive in Australia via this route, and subsequently establish, it would need to:
When it comes to the spread of the fly within a country or region, the rate seen overseas suggests that vehicles play a major role, rather than the insects’ ability to fly (this is a very small fly and not a strong flyer).
Being on the list of 2019 National High Priority Plant Pests, surveillance for this pest is included in the Australian Government’s National Plant Health Surveillance Program.
Where proactive traps are set, or where pest awareness is high, is likely to play a big role in where this fly is first detected, and the possibility of successfully eradicating it, if it does make it to Australia. Here’s a few things that you can do.
Spotted wing drosophila identification, Pest Bites (cesar)
Shows what can happen to blueberries and raspberries in 4-7 days
Spotted wing drosophila fact sheet (Plant Health Australia)
Drosophila suzukii image gallery (Fruit Fly ID Australia)
Pest reporting and responses fact sheet (Plant Health Australia)
Tips for taking photos with a macro lens (Plant Health Australia)
2019 National Priority Plant Pest list (Australian Government)
Free training course: Growers – Pest Reporting and Responses (Plant Health Australia)
Acknowledgement: based on an article by Jessica Lye from cesar in December 2019-January 2020, Australian Tree Crop magazine.
The research project is a collaboration between Plant Health Australia, Plant & Food Research Australia, and cesar. This project has been funded by Hort Innovation, using the strawberry, raspberry and blackberry, cherry and summerfruit research and development levies and contributions from the Australian Government. Hort Innovation is the grower-owned, not-forprofit research and development corporation for Australian horticulture.
