Just a tad is too much: less is worse for tadpoles exposed to chemicals.

The herbicide atrazine is more likely to kill developing amphibians when it is highly diluted than when it's much more concentrated in aquatic environments, a new study suggests. Although counterintuitive, the finding is consistent with some controversial past research on atrazine and studies showing that other hormonally active compounds are most damaging at trace concentrations.

Atrazine, the most commonly used weed killer in the United States, shows up across the environment. Typically applied to crops in spring, it lingers in aquatic habitats for months.

Scientists who have looked for biological effects of the contaminant have reported inconsistent results. Some scientists have observed no problems in animals exposed to atrazine, even at high concentrations. Other research has linked atrazine to testicular deformities in frogs (SN: 11/2/02, p. 275) and feminizing hormonal changes in mammals. Either of these effects could occur because the chemical keeps cells from recognizing male sex hormones and accelerates the conversion of male hormones into estrogen.

To investigate whether atrazine affects amphibians living in the wild, researchers at Pennsylvania State University in State College collected more than 800 frog and toad embryos and tadpoles. In the lab, Sara Storrs, now of the University of Missouri in Columbia, and Joseph Kiesecker, who has also left Penn State, let the animals grow for about a month in water that contained either no atrazine or the chemical in concentrations of 3, 25, or 65 parts per billion (ppb). All these concentrations have been measured in the environment, and up to 3 ppb atrazine is permitted in U.S. drinking water.

The researchers analyzed data on each of four species at one or two stages of maturation. In six of seven data sets, death before full maturity occurred more frequently among tadpoles exposed to 3 ppb atrazine than among those not exposed to the chemical. The number of animals dying in water containing the higher concentrations of atrazine was between the numbers for the low concentration and for pure water, Storrs and Kiesecker report in the July Environmental Health Perspectives. Other herbicides, such as mecoprop and dicamba, also have more damaging effects at low concentrations than they do at higher concentrations (SN: 10/12/02, p. 228).

Alan Hosmer of atrazine-maker Syngenta's Greensboro, N.C., offices objects that the new study offers no hint of how a dilute solution of atrazine could kill animals. No compelling study has ever demonstrated greater effects of the chemical at low concentrations than at high concentrations, he adds.

Some researchers have criticized past findings of inconsistent toxic effects from various atrazine concentrations, says her petologist James Hanken of Harvard University. "This [study] helps to disarm that criticism," he says.