(Beyond Pesticides, October 6, 2008)
University of Pittsburgh researchers have found that the commonly used insecticide malathion can decimate tadpole populations by altering their food chain. The study, published in the October 1 edition of Ecological Applications, finds that 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 results build on a nine-year effort 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. According to the researchers, their deaths may foreshadow the poisoning of other less environmentally-sensitive species, including humans.
According to the U.S. Environmental Protection Agency (EPA), malathion is the most commonly used insecticide in U.S. agriculture and the third most commonly used insecticide in the U.S. home and garden sector. It has been detected in the wetlands where frogs and other amphibians live.
The researchers 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. 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. Study author Rick Relyea, Ph.D., an associate professor of biological sciences at the University of Pittsburgh School of Arts and Sciences, reported that the multiple low doses are a greater detriment than the single dose, with a concentration 25-times higher than the multiple applications combined. The single doses also wipe out the zooplankton, but they eventually recover and the pond reverts back to its original state. The repeated doses prevents 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,” Dr. 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, Dr. 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 EPA do not require amphibian testing. The EPA also relies on single-species tests to assess a pesticide’s 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,” Dr. Relyea said. “These results demonstrate that we need to take a much broader view of the consequences pesticides might have in our world.”
A U.S. Geological Survey study in 2007 found that the breakdown products of chlorpyrifos, malathion and diazinon are ten to 100 times more toxic to amphibians than their parent compounds, which are already highly toxic to amphibians.
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.
Dr. Relyea has published a number of papers on the effects of pesticides on amphibians and aquatic communities, including a 2005 study suggesting that the popular weed-killer Roundup is “extremely lethal” to amphibians in concentrations found in the environment.