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202409 Fresh Quarterly Issue 26 20 Vaughn Walton
Issue 26September 2024

Spotted wing drosophila

What lessons can South African fruit growers learn from the United States? By Anna Mouton.

Spotted wing drosophila is an invasive fruit fly that attacks many wild and cultivated fruits. Female flies lay eggs under the skin of ripening fruit. The developing larvae damage fruit directly while creating favourable conditions for decay organisms.

“The whole life cycle can be as fast as seven days under ideal conditions,” said Oregon State University entomologist Prof. Vaughn Walton. He has studied spotted wing drosophila since it appeared in Oregon in 2009.

The flies prefer brightly coloured, soft-skinned fruits with at least 10 °Brix. Fruits with sugar levels below 10 °Brix tend to be too firm for egg deposition.

Spotted wing drosophila’s favourite Oregon crops are raspberries, strawberries, blackberries, cherries, blueberries, and table grapes. Wine grapes and peaches are usually only infested if damaged.

Walton described how Oregon growers harvested and packed apparently perfect berries, which subsequently collapsed at retailers.

“We have spent tens of millions of dollars on this issue,” he said. “Oregon has seen initial revenue losses of around 10% because of this insect — spotted wing drosophila causes annual crop losses of about USD 720 million in the United States.”

The new reality

Walton showed a video clip of blueberries being sprayed. “In 2010, this was our new reality,” he commented. “As the crop ripened, Oregon growers would spray their fields 2–3 times per week — 20 times per season. They were spraying 24 hours a day to get through the acreage.”

The challenge is that spray applications only affect adult flies. Other life stages are sheltered inside fruit or (in the case of some pupae) in soil. For most of the year, less than 10% of the fly population consists of adults.

“So you’re spraying, the pesticide breaks down, and the next flies emerge from pupae,” said Walton. “You have waves and waves of flies emerging after spraying.”

Spotted wing drosophila can multiply when temperatures rise above 10 °C for more than five consecutive days. The optimum temperature range for reproduction is 18–32 °C. As Walton pointed out, the South African climate is highly suitable for this invasive fly.

Originally from Western Asia, the flies were first observed in North America and Europe in 2008. The insects now occur throughout Europe.

In the United States, spotted wing drosophila was reported from California in 2008 and then spread over the Rocky Mountains, the Cascade and Sierra Nevada Ranges, the Mississippi River, and the prairies to reach the East Coast. It was first detected in Brazil in 2013.

When looking at a map of the United States, the areas where spotted wing drosophila occurs coincide with about 80% of fruit production. Walton cautioned that the flies will likely find most of South Africa’s significant fruit-growing areas equally good habitats.

Dealing with drosophila

“This problem needs to be attacked systematically,” said Walton. Control measures include chemical, cultural, and biological practices, as well as developmental disruption.

Oregon growers are pruning berry plants to reduce the shaded, humid canopy conditions where flies like to lay eggs. Other interventions that suppress spotted wing drosophila include weed fabric and drip irrigation — the pests prefer sprinklers. Some growers have even turned to exclusion netting.

However, pesticides remain the primary control measure. Walton presented a ranking of the insecticides most commonly used for spotted wing drosophila across multiple states. Most of the pesticides that achieved good control were pyrethroids. Carbamates, diamides, organophosphates, and spinosyns were also effective.

“Generally, our growers are using the harsher compounds earlier in the season,” said Walton. “When spraying those persistent compounds early, they know there will be no residues later.”

Previously, growers achieved control with pesticides but then had perfect fruit rejected when residues exceeded the maximum limit. Now, growers use spinosyns for later applications, as these can be sprayed within hours of harvest and provide adequate protection during that window.

“We spent hundreds of thousands of dollars trying to figure out how to deal with residues,” said Walton. “South African growers must make sure they’re not caught out with those.”

Tapping into nature

“Plants have been attacked by insects for millions of years. They’ve evolved to defend themselves,” said Walton. “Let’s use those basic principles to find solutions to the drosophila problem.”

He described how some plants produce volatile compounds that influence insect behaviour. “We’re using this for spotted wing drosophila. We developed a technology where we signal to the flies that there’s something worthwhile for them to feed on.”

Research has demonstrated that spotted wing drosophila find food-grade gum irresistible. They would rather lay eggs in the gum than in developing fruit, but their larvae can’t survive in the gum. Even better, adding a spinosyn to the gum turns it into a deathtrap for adult flies.

Field trials of the gum dispensers in cherries, wine grapes, blueberries, raspberries, and blackberries in California and Oregon showed significant reductions in egg deposition in fruit.

Bait sprays are another control option. “South African growers already use this for Mediterranean fruit flies,” said Walton. “Bait sprays have very low labour costs, low volumes of the active ingredient, and quick coverage with low quantities of the active ingredient on the fruit, compared with full-cover spray applications.”

He shared data on laboratory trials comparing bait sprays with gum-based traps. Both significantly reduced the number of eggs laid per berry relative to an untreated control. Adding an insecticide to the gum-based trap resulted in a significant increase in fly mortalities.

In conclusion, Walton stressed that full-cover pesticide application, although a keystone of crop protection, is not sustainable as a standalone measure. He emphasised the importance of integrated control, including areawide management, using technologies such as attract-and-kill traps.

“It looks as if South Africans are ready for spotted wing drosophila,” said Walton. “I believe you will be able to continue producing high-quality fruit.”

Watch Walton’s presentation on the Hortgro YouTube channel.

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