Most drosophila fruit flies hang around overripe or rotting fruit. But the spotted-wing drosophila prefers to raise its offspring on fresh fruit. By Anna Mouton.
Almost everyone is familiar with fruit flies in the genus Drosophila — they are those annoying little vinegar or wine flies that haunt fruit bowls. The lesser fruit fly — Drosophila melanogaster — is popular with scientists studying genetics and other aspects of biology. Five Nobel prizes have resulted from research based on the lesser fruit fly.
In contrast, the spotted-wing drosophila — Drosophila suzukii — has gained notoriety as an invasive pest of fruit. Drosophila suzukii is the only member of its entire family that has a significant economic impact on humans by destroying fresh fruit.
Why does spotted-wing drosophila matter?
Unlike other drosophila, Drosophila suzukii is attracted to healthy fruit as it changes colour — which is also when the fruit develops a softer skin and increased sugars. The softer skin allows the female fly to deposit her eggs into the fruit where her larvae can be nourished by the extra sugars.
Developing larvae damage fruit directly through feeding. Infested fruit are also more susceptible to decay-causing organisms.
When healthy fruit is not available, spotted-wing drosophila will resign itself to overripe, damaged, culled, or fallen fruit. Larvae have even been reported to complete their development in flowers of a few species of plants.
Besides not being too fussy about fruit condition, spotted-wing drosophila is also not choosy about fruit type — dozens of plant species have been recorded as hosts, including cherries, peaches, plums, apricots, and grapes. Spotted-wing drosophila seems to be especially fond of — and devastating to — berries, notably blueberries, strawberries, and cane berries.
In 2008, estimated economic losses for cherries and various berries in California, Oregon, and Washington were 20–50 %, depending on crop type. Similar losses were reported for various berries from Oregon and Washington in 2009. According to a 2011 publication, annual control costs of Drosophila suzukii in small-fruit industries in Oregon were USD 12–16 million per year.
Drosophila suzukii does not occur in South Africa. If it were to establish here, it could become a serious pest of berries, stone fruit, and grapes. Any phytosanitary measures required by our trading partners would also affect pome fruit and citrus, as these can act as hosts — albeit not preferred hosts — for Drosophila suzukii.
How do I recognise this fly?
The spotted-wing drosophila looks pretty much like any other vinegar fly — adults are 2–3 millimetres long and have red eyes. Only males have spotted wings and there is only one spot per wing. Wing spots take about two days to appear after flies emerge.
How do these flies make more flies?
Female spotted-wing drosophila deposit their eggs one at a time under the skin of ripening fruit. A female lays eggs for 10–65 days and can lay roughly 100–300 eggs in her lifetime.
Eggs hatch after 1–3 days. Larvae feed on the fruit and render them unmarketable. There are three larval stages. Larvae mature in 3–13 days and then pupate for another 4–43 days before adults emerge. Pupae mostly develop inside fruit.
Spotted-wing drosophila reproduces best at 21°C. They struggle to reproduce at temperatures below 10 °C and above 30°C. Depending on climatic conditions, spotted-wing drosophila can produce 3–13 generations per year.
Adults can become inactive during cold weather and survive harsh winters. They feed on damaged or spoiled fruit, or even tree sap.
Where did it come from?
Drosophila suzukii was first described from sweet cherries in Japan in 1931 by Dr Shounen Matsumura — he called it cherry drosophila. It is probably native to eastern and south-eastern Asia, including Japan, China, and Korea, although it may have been introduced to Japan. It occurs throughout much of Asia, including India, Israel, and Taiwan.
Global trade in fresh fruit most likely led to the spread of Drosophila suzukii. The first records of spotted-wing drosophila in California and Europe were in 2008. It has subsequently been reported from most European countries and expanded its range, including South America. In Africa, it is present in Kenya, Morocco, and Réunion.
Spotted-wing drosophila has not been found in South Africa.
Control of spotted-wing drosophila
Control relies mostly on insecticides that target adult flies — life stages inside fruit are generally well-protected from chemicals. Spotted-wing drosophila presents a challenge in that infestation occurs relatively close to harvest when spray applications carry an increased risk of leaving residues.
Baits containing spinosyns have not proved consistently effective against Drosophila suzukii. This may be because sufficiently attractive baits have not yet been formulated — a problem that also hampers trapping and monitoring.
Researchers have identified many biological agents with the potential to control Drosophila suzukii, including parasitoids, predators, and microbes, but to date, these have not been widely adopted in practice.
Fruit that may be infested with Drosophila suzukii can be fumigated or subjected to cold treatment. Preliminary data from the United States suggested that methyl bromide fumigation kills all life stages in strawberries, cherries, peaches, and nectarines, whereas a combination of sulphur dioxide and carbon dioxide fumigation followed by cold disinfestation was effective for table grapes.
A Japanese study reported that exposure to temperatures of 1.7–2.2°C for 96 hours could kill all eggs and larvae.
Early-warning systems are highly recommended for countries or regions that are currently free of spotted-wing drosophila. Traps containing sugar-yeast baits or apple cider vinegar, with added wine and sugar, can be deployed for monitoring.
Drosophila suzukii disperses and reproduces rapidly. In the case of the United States and Europe — neither practised monitoring for drosophila — it was only detected after it was already entrenched and impossible to eradicate.