Controversial Weed Killer Gives These Toxic Toads Extra-Deadly Venom

Cane toads develop more venom when raised in water containing a glyphosate-based herbicide. Tim Wimborne / REUTERS

Herbicides containing glyphosate, the main ingredient in Monsanto's Roundup weed killer and other similar products, have been implicated in harming aquatic wildlife. New research shows that exposure to a glyphosate-based herbicide causes cane toads to produce elevated levels of venom.

Cane toads, brownish amphibians that can grow to more than five inches in length, are native to South and Central America but have been introduced accidentally or purposefully to many islands throughout the world. In Australia, they have wreaked havoc on native wildlife because, among other reasons, they often kill animals that try to eat them; their skin, even when they are tadpoles, contains glands that pump out potent toxins. Humans can also be poisoned if they touch their mouths or eyes after handling cane toads, and people have died from the amphibian's venom, though rarely.

In a study published in the journal Proceedings of the Royal Society B, researchers from the Centre for Agricultural Research at the Hungarian Academy of Sciences exposed cane toad tadpoles to varying levels of herbicide in water. Several weeks later when the tadpoles were about to metamorphose into adults, the team measured how much of their normal deadly chemicals— bufadienolides, which attack the heart—the venom contained. They found that cane toads that swam in herbicide-infused water as tadpoles generally grew up to produce more dangerous venom than those raised without exposure to the chemical. The results held true for experiments conducted in the lab and in a more naturalistic pond-like enclosure. In the enclosure, toads raised in the presence of the herbicide developed more than 150 percent more venom than those raised in pristine water.

The scientists were surprised by the findings. They had expected that herbicide would reduce the quantity of toxins found. "We had expected a negative effect, because herbicide exposure usually makes the animals weak and sick," says Veronika Bokony, lead author of the study. "So we hypothesized that they would be less able to produce their defensive chemicals too."

The concentrations of glyphosate-based herbicide used in the studies, between 2 and 4 milligrams per liter, are many times higher than the levels at which it is usually found in large bodies of water and streams. However, these amounts are "in the range of maximum concentrations observed in runoff" and could be reached in very small, shallow ponds, the types of places that many amphibians, including cane toads, like to breed, Bokony says. "We were interested in worst-case scenarios in which high doses of the herbicide accidentally pollute a small water body," she says.

Monsanto, which manufactures glyphosate and Roundup, took issue with these claims. "The researchers exposed the toads to environmentally unrealistic levels of the glyphosate-based formulation," says spokesperson Charla Lord. The concentration of glyphosate-based herbicide that wildlife would likely be exposed to from a single spraying or application would be 50 to 100 times lower than the level used in this study, Lord adds. Monsanto also noted that it doesn't manufacture the product used in the study, Glyophogan Classic, which is produced by a company called Adama, though its label says it is identical in formulation to Roundup Classic, a Monsanto product. (Adama didn't immediately reply to a request for comment.)

Bokony says the 150 percent increase in toxins is "ecological significant," meaning that such an effect in the wild would have important ramifications for cane toad predators (or would-be predators). One 2009 study, for example, found that a two-fold increase in bufadienolides can drastically increase the lethality of cane toad tadpoles. But Lord again disagreed, saying the "ecological relevance of this study to natural toad populations is highly questionable" because of the high concentrations used.

Cane toads, by virtue of the toxic venom produced in glands along their back, can be deadly to native wildlife that try to eat them. David Gray / REUTERS

Rick Shine, a cane toad expert at Australia's University of Sydney who wasn't involved in the study, says that glyphosate-based herbicides may already be altering wildlife. "Tadpoles can be sensitive to extremely low concentrations of such chemicals, so yes—it's probably happening in nature," Shine says. However, nobody has measured this effect in the wild, and it would be difficult to tease apart the potential effect of the herbicide from other environmental stressors, he adds.

Exactly why the weed killer may increase levels of natural toad toxins is unknown, though Shine suspects that the chemicals stress the toad, which triggers toxin production. "Basically, a stressed tadpole reacts by investing more in defenses," he says. The study authors also suggest that herbicide could interfere with the molecular "brakes" the toads use to limit the production of toxins; research in a related species shows the herbicide decreased the activity of several enzymes involved in breaking down toxins.

Bokony notes that the herbicide formulation used in the study contains additives other than glyphosate. This weed killer, like many similar products, contains chemicals called polyethoxylated tallow amine, which enhance the effectiveness of glyphosate. But this controversial substance has also been shown to harm many animals, Bokony says, and human cells in lab studies. In Europe, Monsanto is already making the transition away from using this additive.