Spotted wing drosophila arrives in South Africa

The survey that led to the detection of spotted wing drosophila in South Africa. By Anna Mouton.

The first records of damage by spotted wing drosophila are from Japan in 1916. By the 1930s, it was a serious pest of cherries in Japan and parts of Korea and China — it still plagues berries and stone fruit in Japan today.

The flies were first seen outside Asia in 1980 in Hawaii. The initial detections in Europe and North America occurred in 2008. By 2013, spotted wing drosophila had reached southern Brazil. Within a year, they were recorded at other Brazilian locations 1 700 km from where they were initially discovered.

International trade has now spread the intrepid flies to every continent except Australia and Antarctica. Their indiscriminate feeding habits and rapid reproduction have enabled their astonishing success in colonising new territories.

The global economic impact of spotted wing drosophila is difficult to quantify. One 2008 study estimated the losses due to direct damage of cherries and berries in California, Oregon, and Washington states at USD 551 million per year. However, the flies also cost money to manage, and chemical control can limit market options.

In Africa, spotted wing drosophila was trapped in Morocco and on the island of Réunion in 2013. Subsequently, it was found on neighbouring islands, Algeria, Kenya, and, most recently, South Africa.

Proactive surveillance

Many countries only detected spotted wing drosophila once it started causing significant damage. For example, Californian entomologist Dr Martin Hauser and co-authors describe how the first flies submitted for identification by a Santa Cruz raspberry grower in 2009 were dismissed as harmless drosophilids.

Within a few months, Californian cherry growers also began reporting damage, initially ascribed to the western cherry fruit fly (Rhagoletis indifferens). As losses mounted, so did the evidence for a drosophilid culprit. Entomologists identified spotted wing drosophila when they eventually received adult flies for examination.

In South Africa, spotted wing drosophila was confirmed before any fruit damage had occurred, thanks to a survey initiated in 2019.

“I was asked who took the decision to monitor for Drosophila suzukii,” recalls Matthew Addison, previous crop-protection programme manager at Hortgro Science. “It was three people in a room.”

Addison, along with fellow entomologists Prof. Pia Addison and Dr Leigh Steyn, both at the Department of Conservation Ecology and Entomology at Stellenbosch University, had met to discuss a Hortgro and SATI-funded project to develop specialised pest-control services. They decided to include surveillance for spotted wing drosophila, as they recognised its potential to invade South Africa.

Applied entomologist Dr Leigh Steyn was the postdoctoral researcher on the project. In 2021, she placed the first funnel traps in Western Cape fruit-growing areas. These traps tended to collect rain rather than drosophilids. They were later replaced with modified McPhail traps and a lure formulated by Insect Science.

“Leigh did a lot of research on different traps and lures,” remembers Dr Gulu Bekker, applied entomology researcher at Stellenbosch University and technical consultant at BerriesZA. “I even had to get her a specific variety of wine that is supposed to be better for attracting suzukii.”

A mountain of flies

The surveillance network eventually encompassed about 70 traps throughout the Western Cape. “They ended up catching a mountain of flies,” says Matthew Addison. These were mostly vinegar flies in the genus Zaprionus, but also many Drosophila species. The project ran for two years, and the highest trap catches — nearly 15 000 insects — were from the Koo.

“Monitoring this fly is incredibly difficult,” says Prof. Pia Addison. “Of all our insect pests, it’s the worst because you have to sift through thousands of specimens before you find the one with a spot on it.”

At the time, Bekker shared an office with Steyn. “She was sitting day after day, examining every little fly, working through the backlog,” he says. But none of the flies were spotted wing drosophila.

In April 2022, Matthew Addison travelled to the Langkloof, and Steyn took advantage of the trip to place some traps in that region. One of these went into a hedge near a farm office in the Eastern Cape town of Misgund. The trap was collected on 25 April and became another peak in the mountain range of fly samples.

Meanwhile, the Hortgro- and SATI-funded project ended, and Steyn left. “We had these traps in the lab, and we were just getting students to help process them,” says Prof. Pia Addison.

And then, in October 2023, the sample from Misgund came under the microscope, and there it was — a single male spotted wing drosophila. DNA analysis confirmed the identification.

“Suddenly, it’s everywhere”

Thinking back, Prof. Pia Addison admits she was surprised at the discovery. “We had been monitoring for two years and catching nothing. But once we caught the first one, suddenly, we were trapping spotted wing drosophila everywhere. It was like an explosion.”

“That catch in Misgund in 2022 must have been one of the first Drosophila suzukii that invaded the Cape,” says Bekker. “I think we all agree that, when Leigh did the survey, we didn’t have it in the Western Cape because we now have traps in the same areas she did, and we’re catching it a lot.”

As soon as spotted wing drosophila was confirmed, the Addisons and Bekker assembled a multidisciplinary team of scientists to discuss research options. They then met with Hortgro, SATI, SAWine and BerriesZA research managers.

All four industries have since joined forces to fund work on modelling and managing spotted wing drosophila. Read more about these here.

“Spotted wing drosophila is still spreading and establishing in the Western Cape. We’re almost in front of the wave, and that’s why we’re not seeing massive numbers and significant damage yet,” says Bekker.

He is optimistic that this improves South African fruit growers’ chances of successfully managing spotted wing drosophila. “We’re in a good position in terms of timing, and I think the proactive research being funded puts us in a good spot.”