Frogs exposed to a mix of pesticides at extremely low concentrations like those widely found around farms suffer deadly infections, suggesting that the chemicals could be a major culprit in the global disappearance of amphibians, UC Berkeley scientists reported Tuesday.
When tadpoles were exposed in laboratory experiments to each pesticide individually, 4% died before they turned into frogs. But when atrazine and eight other pesticides were mixed to replicate a Nebraska cornfield, 35% died.
The frogs developed an array of health problems, including meningitis, because the chemicals suppressed their immune systems. They also took longer to complete the transformation from tadpole to frog, which reduces their chances of survival.
At least one-third of amphibians worldwide, or 1,856 of the known species of frogs, toads, salamanders and caecilians, are in danger of extinction, according to an international group of conservation biologists.
A variety of factors are thought to be involved, including climate change, ultraviolet radiation, disease, parasites and habitat destruction.
"We demonstrated that a realistic pesticides mixture [based on a mixture applied to an actual field] at low, ecologically relevant concentrations can have dramatic effects on amphibian development and growth and ultimately, we predict, survivorship," Tyrone Hayes, a professor at the university's department of integrative biology, and his colleagues reported in the online version of the scientific journal Environmental Health Perspectives.
"Given these adverse effects and the continued increase and use of pesticides in agriculture over the past 50 years, it is likely that pesticides have played and will continue to play a role in amphibian declines," they wrote.
Three years ago, Hayes triggered a controversy after reporting that the herbicide atrazine, used in large volumes in the United States primarily on corn, feminizes frogs by mimicking estrogen.
Chemicals that disrupt hormones can have a wide range of seemingly unrelated effects on animals because hormones regulate many bodily functions, including reproduction, immune cells and brain development.
Syngenta, the manufacturer of atrazine, has criticized Hayes' previous work, saying its own research has been unable to replicate his findings of feminized frogs. Hayes, who formerly worked for a consulting firm hired by Syngenta, has said the company pressures researchers to hide results that link atrazine to ecological effects, which the company denies.
