Common Farm Fungicide Devastates Insect Reproduction

A fungicide widely sprayed on fruits and vegetables to prevent crop disease is quietly devastating insect populations critical for pollination and ecosystem health.

Macquarie University research reveals that chlorothalonil—one of the world’s most extensively used agricultural chemicals—severely impairs insect reproduction and survival even at the lowest concentrations routinely detected on food. The findings add urgency to concerns about global insect decline, with some regions already reporting population drops exceeding 75% in recent decades.

Published in Royal Society Open Science, the study exposes a troubling gap in pesticide regulation: despite chlorothalonil’s ubiquitous use, fewer than 25 scientific papers have examined its effects on insects, leaving regulators largely blind to its ecological impact.

Surprising Toxicity at Trace Levels

“Even the very lowest concentration has a huge impact on the reproduction of the flies that we tested,” explains lead author Darshika Dissawa, a PhD candidate 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 researchers exposed fruit flies to chlorothalonil concentrations matching those typically found in produce ranging from cranberries to wine grapes. Even at the lowest tested dose—just 5 milligrams per kilogram—flies showed a dramatic 37% drop in egg production compared to unexposed individuals. At higher concentrations matching levels found in some fruits and vegetables, reproductive output plummeted by 58%.

The team also discovered that chlorothalonil exposure significantly reduced the number of ovarioles—egg-producing structures in female insects—while disrupting iron metabolism in males, a process crucial for sperm development and fertility.

More Than Just Reproductive Damage

The chemical’s impact extended beyond reproduction to fundamental survival processes:

• Larval mortality increased dramatically with exposure, establishing a lethal concentration affecting 50% of larvae at just 13.7 mg per kg
• Surviving larvae took significantly longer to develop, suggesting metabolic disruption
• Female body weight decreased substantially, indicating overall fitness impairment
• Male iron levels dropped by half, potentially compromising fertility through disrupted sperm production

Widespread Environmental Presence

Chlorothalonil’s extensive use creates unavoidable exposure for insects across agricultural landscapes. The fungicide is frequently applied preventatively—even when no disease is present—and often coincides with flowering periods when pollinators are most active. Contaminated pollen and nectar get carried back to hives, exposing entire colonies.

Supervising author Associate Professor Fleur Ponton describes the team’s surprise at the chemical’s potency: “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.”

The chemical works by targeting sulfhydryl groups in proteins and glutathione—essential components of cellular defense systems. This mechanism, while effective against fungi, proves equally destructive to beneficial insects, disrupting their ability to neutralize oxidative stress and maintain cellular health.

Regulatory Blind Spot

While chlorothalonil has been banned in the European Union due to health and environmental concerns, it remains extensively used on Australian crops including orchards and vineyards. The research reveals a critical knowledge gap: despite being one of the world’s most widely applied fungicides, its effects on non-target insects have received minimal scientific attention.

This oversight is particularly concerning given that insects like fruit flies play crucial ecological roles beyond pollination. They contribute to nutrient cycling, serve as food for birds and other animals, and help maintain ecosystem stability. Population declines can cascade through food webs, affecting entire agricultural and natural systems.

“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 emphasizes.

The researchers recommend more sustainable practices, including reduced application frequency to allow insect populations time to recover between treatments. They also advocate for better integration of botanical fungicides derived from plant compounds, which offer effective disease control with lower toxicity to beneficial species.

As global insect populations face mounting pressures from habitat loss and climate change, the study underscores how overlooked agricultural chemicals may be accelerating declines in species essential for food security and ecosystem health.