Corn Pesticides in Combination Kill Frogs & Threaten Public Health

Nature (online news section)
February 7, 2006

http://www.nature.com/news/2006/060206/full/060206-4.html

Pesticide cocktail kills US frogs
Weedkillers combine in fields to form a deadly brew.

By Michael Hopkin

Pesticides that seem harmless on their own can have severe effects on frogs
when in combination.

Pesticides used by US corn growers are combining to kill off the country's
native frogs. Research shows that commonly used pesticides, fairly harmless
by themselves, are mixing to create a toxic soup in runoff water. This
stunts the frogs' sexual development and leaves them susceptible to fatal
infections.

Environmental toxicologists led by Tyrone Hayes of the University of
California, Berkeley, have spent the past four years surveying the effects
of pest-control chemicals on frogs that live in the US corn belt in the
Midwest.

The team raised northern leopard frogs (Rana pipiens) in water containing a
combination of four weedkillers, two fungicides and three insecticides, and
examined the effect on their growth.

Global amphibian declines are the result of multiple assaults on their
environment.

"The nine-compound mixture represents that applied to the field at the time
that eggs are laid in water that runs off the field," Hayes says.

Sum of all fears

The cumulative effect of the nine chemicals was greater than the sum of
their individual properties, the researchers found.

When tested at concentrations of 0.1 parts per billion (ppb), six of the
compounds had no effect at all, two had small but noticeable effects on
development, and one - atrazine - caused serious developmental problems, but
not death.

Yet all together, the combination killed 35% of frogs, and lengthened the
time taken to develop from tadpole to adult by 15 days, or 25%.

The researchers suspect that the mixture stimulates production of stress
hormones, which retard a frog's development. They publish their results on
the website of the journal Environmental Health Perspectives (1).

Small doses

Concentrations of 0.1 ppb are the lowest levels found in runoff water from
cornfields during the summer, when frogs are growing and breeding.
Concentrations can reach as much as 10 ppb, says Hayes.

For atrazine, concentrations of 0.1 ppb can be quite harmful, causing male
African clawed frogs (Xenopus laevis) to grow 'feminine' gonads and show
stunted growth of the larynx. Hayes has previously demonstrated this in both
wild and lab frogs (see 'Atrazine gives frogs male and female gonads, says
field study'); some of his new work shows that just a week of atrazine
exposure is enough to cause the effect (2).

But only the combination of all the pesticides seems to cause crippling
developmental problems and death.

A delay in reaching adulthood may mean that ponds and puddles may dry out
before frogs get the chance to reproduce, Hayes fears. And immune
suppression may mean that they fall prey to otherwise benign infections.

Multiple assaults

Pesticides are just one of several reasons why amphibian populations are
declining worldwide, Hayes says. "Global amphibian declines are the result
of multiple assaults on their environment."

Other potential threats to frogs and toads include increases in ultraviolet
radiation from the Sun, climate change, draining of wetlands, and emerging
diseases. Pesticide exposure may increase the damage done by these factors,
Hayes suspects.

"Many 'emerging diseases' and deformities associated with infections may
have bigger impacts in pesticide-exposed populations," he says. "Other
researchers have shown similar effects, but not at concentrations as low as
we used here."

A ban on atrazine was this year extended to encompass the entire European
Union, but the chemical is still licensed in the United States. "I cannot
predict whether it will be banned," says Hayes. "The industry is fighting
hard to keep it on the market."

References

1. Hayes T. B., et al. Env. Health Perspect., doi:10.1289/ehp8051 (2006).
2. Hayes T. B., et al. Env. Health Perspect., doi:10.1289/ehp8067(2006).
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