Our Stolen Futurea book by Theo Colborn, Dianne Dumanoski, and John Peterson Myers


  Relyea, RA and N Mills. 2001. Predator-induced stress makes the pesticide carbaryl more deadly to gray treefrog tadpoles (Hyla versicolor). Proceedings of the National Academy of Sciences, 98: 2491-2496.

[How did they conduct the experiments?]
[What did they find?]

[More about frogs...]

Gray Treefrog
photo by James Harding

This study shows that frogs are likely to be far more sensitive to pesticides in the real world than traditional laboratory tests used to establish regulatory standards would indicate.

Relyea and Mills reach this conclusion for two reasons. First, the standard amphibian tests are far too short. Extend their time and low contamination levels of carbaryl that appeared to cause no deaths wind up causing significant mortality. [More...]

More important, however, is what Relyea and Mills discovered when they made the experiments resemble the real world by adding a predator to the mix. They set the experiment up in a way that the tadpoles' natural predator was present and the tadpoles could detect it, but that the predator, a type of salamander, could not actually reach the tadpoles. Not surprisingly, previous work has shown that the presence of a predator induces stress in prey, even in amphibians. The combination of stress and pesticide made the pesticide 2-4 times more lethal than the pesticide alone. [More...]

Thus this experiment documents a hitherto completely unexplored variable affecting the lethality of a pesticide, i.e., the interaction with predator-induced stress. By itself, that is highly significant and should be repeated in appropriate experimental designs for a wide array of organisms.

But predation is only one of the many natural factors that stress organisms living in the real world, including people. Virtually all toxicity testing for regulatory purposes ignores these realities. It is highly likely, therefore, that current regulatory science even in its simplest form (does a pesticide kill) has dramatically underestimated the impacts of many pesticides.

Another important dimension to this study is that the levels of carbaryl used in the experiments was only a small percentage of carbaryl concentrations measured in ponds in the real-world after application on crops. Because carbaryl can be persistent under some settings for years, and because its use is so widespread, there is every reason to believe that amphibians in many places are encountering carbaryl levels within the concentration range found in these experiments to be deadly.

What did they do? Relyea and Mills performed a series of experiments examining the effect of carbaryl, a commonly used pesticide, and predator-induced stress on survival of tadpoles of a common frog in North America, the gray treefrog, Hyla versicolor. These experiments were carried out using contamination levels of carbaryl that were low compared to the doses normally required to kill tadpoles.

In their experiments they varied the dose of carbaryl and also whether or not the predator, a caged larval Spotted Salamander Ambystoma maculatum,was present in the tadpoles' container.

adult Spotted Salamander
Photo by James Harding

According to Reylea and Mills, "caged predators emit chemical cues that induce antipredator responses in their prey without allowing the predators to kill the target animals." By placing the caged predator in the vicinity of the tadpoles, Reylea and Mills increased the stress level of the tadpoles.

The dose levels they employed were a small percentage of the concentration of carbaryl normally required to kill tadpoles.

What did they find? The first revelation was that simply by extending the time of exposure of the tadpoles to relatively low levels of carbaryl, they were able to cause significant tadpole mortality.

Prior testing of carbaryl had lasted 2 to 4 days at much higher levels of exposure. Relyea and Mills showed that by extending the experiments out only a few days longer, carbaryl at much lower concentrations caused tadpole mortality.
Adapted from Relyea and Mills 2001

Prior amphibian toxicity testing of carbaryl had been too short to reveal this effect. "Very low concentrations of carbaryl can still cause widespread amphibian death, it just takes a few more days to observe the effect."

Relyea and Mills found that the effect of stressing the tadpoles by adding the predator to their tanks dramatically increased mortality. "Predator cues made the pesticide 4 times more lethal."

Adapted from Relyea and Mills 2001
The graph on the left shows the impact of combining predator stress with carbaryl on tadpole survival. All tadpoles were exposed to a low level of carbaryl and died at a higher rate than normal (compare to "water" in graph above). Tadpole survival drops even lower when predator-induced stress interacts with the contaminant.


Relyea and Mills also observed other impacts of carbaryl and predatoress the tadpoles

  • The presence of the caged predator "did not affect tadpole activity when carbaryl was absent but significantly reducted activity when carbaryl was present."
  • Predators did not affect growth in the absence of carbaryl but reduced tadpole growth by 50% in the presence of carbaryl.







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