September 29, 2008
Contact: Morgan Kelly
[412-624-4356 (office); 412-897-1400 (cell); [EMAIL PROTECTED]
Pitt Research Shows That Low Concentrations of Common Insecticide Can
Decimate Tadpole Populations Through the Food Chain
Insecticide malathion initiates chain reaction that deprives tadpoles of
food source, indirectly killing them at doses too small to kill them directly
PITTSBURGHThe latest findings of a University of Pittsburgh-based project
to determine the environmental impact of routine pesticide use suggests that
malathionthe most popular insecticide in the United Statescan decimate
tadpole populations by altering their food chain, according to research
published in the Oct. 1 edition of Ecological Applications.
Gradual amounts of malathion that were too small to directly kill developing
leopard frog tadpoles instead sparked a biological chain of events that
deprived them of their primary food source. As a result, nearly half the
tadpoles in the experiment did not reach maturity and would have died in
nature. The research was funded by a National Science Foundation grant.
The results build on a nine-year effort by study author Rick Relyea, an
associate professor of biological sciences in Pitts School of Arts and
Sciences, to investigate whether there is a link between pesticides and the
global decline in amphibians, which are considered an environmental
indicator species because of their sensitivity to pollutants. Their deaths
may foreshadow the poisoning of other, less environmentally sensitive
speciesincluding humans. Relyea published papers in 2005 in Ecological
Applications suggesting that the popular weed-killer Roundup® is extremely
lethal to amphibians in concentrations found in the environment.
For his current research, Relyea and the studys coauthor, Pitt alumnus
Nicole Diecks (CGS 05), created simulated ponds from 300-gallon outdoor
tanks containing wood frog and leopard frog tadpoles. They exposed the ponds
to no malathion, moderate concentrations in a single dose, or low
concentrations in weekly doses that mirror the levels tadpoles experience in
nature. Malathion is commonly used worldwide to thwart crop pests and
control mosquitoes that carry malaria and West Nile virus. It has been
detected in the wetlands where frogs and other amphibians live.
The doses of malathion in the simulated ponds were too low to directly kill
the amphibians, but instead wiped out tiny animals known as zooplankton that
eat algae that float in the water. With few zooplankton remaining, the
algae, known as phytoplankton, grew rapidly and prevented sunlight from
reaching the bottom-dwelling algae, or periphyton, which tadpoles eat. This
chain of events occurred over a period of several weeks. The wood frog
tadpoles, which mature quickly, were largely unaffected.
Leopard frog tadpoles, on the other hand, require more time to develop into
frogs and experienced slower growth as a result of the reduced amount of
periphyton. Ultimately, 43 percent of the leopard frog tadpoles did not
mature as a result of the repeated application of malathion at very low
concentrations. Relyea reported that the multiple low doses were a greater
detriment than the single dose, which had a concentration 25-times higher
than the multiple applications combined. The single doses also wiped out the
zooplankton, but they eventually recovered and the pond reverted back to its
original state. The repeated doses prevented the zooplankton from recovering.
The chain of events caused by malathion deprived a large fraction of the
leopard frog tadpoles of the nutrients they needed to metamorphose into
adult frogs, Relyea said. Repeated applications sustained that disruption
of the tadpoles food supply. So, even concentrations that cannot directly
kill tadpoles can indirectly kill them in large numbers.
The research results should apply to several other insecticides that are
highly lethal to zooplankton, including carbaryl, diazinon, endosulfan,
esfenvalerate, and pyridaben, Relyea said. All of these chemicals are toxic
to humans as well and are commonly used in the United States, although some
are banned in other countries. The effect of insecticides and other
pesticides on amphibians are not widely known because current regulations
from the U.S. Environmental Protection Agency do not require amphibian
testing. The EPA also relies on single-species tests to assess a pesticides
risk and does not account for potential indirect repercussions.
The indirect impacts on the amphibians observed in this study could not be
observed in traditional, single-species tests, Relyea said. These results
demonstrate that we need to take a much broader view of the consequences
pesticides might have in our world.
Leopard and wood frogs naturally range across North America, including
Pennsylvania and the Northeastern United States. Once plentiful, leopard
frogs have declined in recent years.
The journal Ecological Applications is available online at
www.esajournals.org/loi/ecap.
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