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A widely-used agricultural chemical sprayed on fruits and vegetables to prevent fungal disease is also killing beneficial insects that play a critical role in pollination and wider ecosystems.

New Macquarie University-led research in Royal Society Open Science, shows chlorothalonil, one of the world's most widely used agricultural fungicides, deeply impacts the reproduction and survival of insects, even at the lowest levels routinely found on food from cranberries to wine grapes.

"Even the very lowest concentration has a huge impact on the reproduction of the flies that we tested," says lead author, Ph.D. candidate Darshika Dissawa, from Macquarie's School of Natural Sciences.

"This can have a big knock-on population impact over time because it affects both male and female fertility."

The insect species Drosophila melanogaster, commonly called fruit fly or vinegar fly, was used as a laboratory model representing countless non-target insects found in agricultural environments.

"D. melanogaster is also at the bottom of the food chain, becoming food for a whole lot of other species," says Dissawa.

Unlike major horticultural pests in Australia, such as the Queensland fruit fly (Bactrocera tryoni) and the Mediterranean fruit fly (Ceratitis capitata), D. melanogaster feed on rotting fruit and play an important role in nutrient recycling in agriculture.

Testing the fungicide

Scientists exposed D. melanogaster larvae to chlorothalonil amounts matching levels typically found in fruits and vegetables.

Even at the lowest dose tested, the flies showed a 37% drop in egg production at their maturity, compared with unexposed individuals.

Supervising author Associate Professor Fleur Ponton, from Macquarie's School of Natural Sciences, says the dramatic decline was surprising.

"We expected the effect to increase far more gradually with higher amounts. But we found that even a very small amount can have a strong negative effect," Associate Professor Ponton says.

The findings add to mounting evidence of what researchers call the "insect apocalypse"—a global phenomenon that has seen insect populations plummet by more than 75% in some regions in recent decades.

Where the fungicide is used

Although banned in the European Union, chlorothalonil is extensively applied to Australian crops to control fungal diseases such as mildews and leaf blights.

The chemical has been detected in soil and water bodies near agricultural areas, with residue levels in fruits and vegetables ranging from trace amounts to 460 milligrams per kilogram.

"We need bees and flies and other beneficial insects for pollination, and we think this is an important problem for pollinator populations."

"Chlorothalonil is particularly common in orchards and vineyards and is often used preventatively when no disease is present," Associate Professor Ponton explains.

"People assume fungicides like chlorothalonil only impact fungal diseases, but they can have devastating, unintended consequences for other species," says Associate Professor Ponton.

Knock-on effect

The study found that chlorothalonil exposure during larval development caused severe reproductive damage in adult flies.

Females showed significantly reduced body weight, fewer egg-producing structures called ovarioles and drastically reduced egg production. Males had reduced iron levels, suggesting disruption to metabolic processes essential for sperm production.

The scientists also found the larvae consumed the contaminated food normally, ruling out taste aversion as an explanation.

"We didn't find a significant aversion for food contaminated with chlorothalonil, except when there was a very high concentration of the chemical," says Associate Professor Ponton. "This means the impacts are due to chlorothalonil ingestion."

Knowledge gap has broad implications

In agricultural landscapes where entire orchards and vineyards are treated with fungicides, insects cannot escape chemically-contaminated food sources.

"We need bees and flies and other beneficial insects for pollination, and we think this is an important problem for pollinator populations," Associate Professor Ponton says. "There is a strong commercial incentive to understand the impact in the field and address the use of this chemical."

The research highlights a critical knowledge gap in pesticide regulation. Chlorothalonil is one of the most extensively used fungicides globally, but fewer than 25 scientific papers examine its effects on insects, despite mounting evidence of widespread insect population decline.

"People assume fungicide only affects fungal diseases, but it has an effect on other non-target organisms," Associate Professor Ponton says.

The researchers have called for more sustainable agricultural practices, such as reduced frequency of applications to allow insect populations to recover between treatments.

"We need field trials to explore options and develop evidence-based guidelines to consider the knock-on effects of fungicides on beneficial insects," says Associate Professor Ponton.

Future research will examine whether the reproductive damage carries over to subsequent generations and investigate the combined effects of multiple agricultural chemicals typically used together in farming operations.

This content was originally published on The Macquarie University .