Ecologists firm up link between agricultural chemicals and potentially devastating flatworm infections

Atrazine, the second-most widely used agricultural pesticide in America, can pose a toxic double whammy to tadpoles. The weed killer not only increases the likelihood that massive concentrations of flatworms will thrive in the amphibians’ ponds, a new study reports, but also diminishes the ability of larval frogs to fight infection with these parasites.

Moreover, the new data show, runoff of phosphate fertilizer into pond water can amplify atrazine’s toxicity. The fertilizer does this by boosting the production of algae on which snails feed. Those snails serve as a primary, if temporary, host for the parasitic flatworms, which can sicken frogs.

Amphibian populations around the world have been declining in recent decades, with many species on the brink of extinction. Infection with any of several species of tiny flatworms, known as trematodes, can trigger debilitating limb deformities in frogs. Severe infections can kill the amphibians. The question was why high rates of those deformities – and, presumably, trematode infections – began showing up across the nation in the mid-1990s (SN: 7/12/1997, p. 31). The new study suggests that one answer lies in atrazine’s quick rise to dominance in U.S. agriculture; for many years the compound has been a – if not the – leading herbicide used by farmers, especially in corn-growing regions.

Val R. Beasley of the University of Illinois at Urbana-Champaign and his colleagues quantified more than 240 separate factors in 18 Minnesota wetlands that might affect amphibian trematode infection rates, including water quality, habitat and resident species. In the Oct. 30 Nature, the team now reports that atrazine concentrations stood out as the prime correlate with trematode infection rates in Minnesota’s declining northern leopard frog.

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