Pesticides, Fertilizers Linked to Frog Decline

Oct. 30, 2008 -- A pesticide compound commonly used in the United States is linked to the growth of tiny parasites that sicken and kill frogs and also harms the amphibians' immune defenses against infection, according to a study published Thursday.

The impact of this chemical is boosted in the wild by phosphate fertilizers, the investigators believe.

Runoff from fertilizers into ponds encourages the proliferation of snails that are a natural host to the flatworm parasites, they say.

The flatworms, called trematodes, are notorious for causing limb malformations, kidney damage and sometimes death in several species of frog.

The new study points the finger at atrazine, an active ingredient in several herbicide products manufactured by a Swiss-based company, Syngenta.

Atrazine was banned in the European Union in 2004 after the chemical showed up in drinking water, but has over the last 15 years become a leading farm chemical in the United States, especially in corn-growing regions.

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In a field survey led by Jason Rohr of the University of South Florida, scientists measured more than 240 variables in 18 Minnesota wetlands that could account for the rate at which frogs are infected by trematodes.

The strongest link by far was with atrazine concentrations, which accounted for more than 50 percent of the likelihood that the amphibians would become diseased.

When the presence of atrazine was combined with traces of phosphate fertilizer -- runoff from nearby agricultural plots -- the rate of diseased frogs went up to 75 percent.

Seeking to find out more, the researchers raised tadpoles for four weeks in several 290-gallon tanks containing snails, leaves and insect larvae, to approximate a natural environment.

In tanks where atrazine was added in concentrations found in wetlands, four times as many snails grew compared with the population that was in water free of the herbicide. The population of the parasitic flatworms exploded too.