How crop protection is fueling the global decline of insects

Insects are being harmed by a fungicide sprayed on fruits and vegetables – even at levels that are considered safe.

A new study from Macquarie University reveals that the chemical chlorothalonil, which is used globally to prevent fungal disease, is damaging insect fertility and survival.

Testing the fungicide on fruit flies

Researchers at Macquarie’s School of Natural Sciences tested the effects of chlorothalonil on fruit flies, a common insect model used in labs. Fruit flies are representative of a large number of similar insect species in the wild.

The team used Drosophila melanogaster, known as the common fruit fly or vinegar fly. Unlike pests that target crops, these flies feed on rotting fruit. They play an important role in breaking down waste and recycling nutrients on farms and in orchards.

“D. melanogaster is also at the bottom of the food chain, becoming food for a whole lot of other species,” said Ph.D. candidate Darshika Dissawa, lead author of the research paper.

A surprising decline in insect fertility

The experiment exposed fruit fly larvae to chlorothalonil at levels that match what’s found on real agricultural produce, such as cranberries and wine grapes. What they found shocked the team.

“Even the very lowest concentration has a huge impact on the reproduction of the flies that we tested,” said Dissawa. “This can have a big knock-on population impact over time because it affects both male and female fertility.”

Flies exposed to the chemical showed a 37% drop in egg production at maturity, compared to those that were not exposed.

“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,” noted Professor Fleur Ponton.

Fungicide still in use, despite risks

Chlorothalonil is banned in the European Union. But in Australia, it’s still sprayed widely to stop fungal diseases like mildew and leaf blight.

Residue from this fungicide has been found in soils and water near farms. On food, the levels can range from almost zero to 460 milligrams per kilogram.

“Chlorothalonil is particularly common in orchards and vineyards, and is often used preventatively when no disease is present,” explained Professor Ponton.

“People assume fungicides like chlorothalonil only impact fungal diseases, but they can have devastating, unintended consequences for other species,” she stated.

Fungicide fuels insect decline

The study revealed that chlorothalonil exposure during the larval stage caused major reproductive issues in the adult flies.

Female flies were smaller, had fewer egg-producing structures, and laid far fewer eggs. Male flies had lower iron levels, which can interfere with sperm production and other metabolic functions.

The researchers also checked whether the flies were avoiding the contaminated food. They weren’t.

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

Effects on insect pollinators

In agricultural settings where whole crops are treated with fungicides, insects like flies and bees have no way to avoid the chemicals. This creates a larger problem for pollination, ecosystem stability, and food production.

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

Despite the widespread use of chlorothalonil, there’s a surprising lack of research into how it affects insects. Fewer than 25 scientific studies have examined this – far too few, considering the global decline in insect numbers.

A call for smarter farming

The team behind the study says it’s time to rethink how fungicides are used in agriculture. Reducing the frequency of spraying could give insect populations a chance to recover. But that requires more research.

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

Next, the researchers will study whether these reproductive problems are passed on to future generations.

The team also plans to investigate what happens when insects are exposed to combinations of chemicals – something that’s far more common in real-world farming.

The full study was published in the journal Royal Society Open Science.

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