Pesticides may affect how frogs handle other threats

While increasing evidence suggests that wood frogs can tolerate pesticides, these adaptations may change the way the frogs deal with others.

In a study published in Evolutionary Applications, researchers found that wood frogs (Lithobates sylvaticus) that live closer to agriculture and have evolved a tolerance to pesticides are more susceptible to ranavirus, an often-fatal pathogen, but less susceptible to trematodes, a parasitic flatworm.

“As we continue to rely on pesticides, we need to consider how wildlife populations respond to these contaminants and also how their responses influence reactions to other threats, such as parasites,” said Jessica Hua, an assistant professor at Binghamton University and lead author of the study. “Wood frogs are a really great model species for addressing these issues. They’re widespread and live in ponds where they encounter both pesticides and parasites.”

As part of the study, Hua and her colleagues tested the susceptibility of wood frogs to ranavirus and trematodes using 15 populations at varying distances from agriculture and with varying tolerances of pesticides.

First, they tested for susceptibility to trematodes, which frogs can pick up from infected snails in wetlands. Hua and her colleagues placed wood frog tadpoles from each of the 15 populations into individual cups. Then, they collected infected snails and extracted the trematode by placing the snail under a heat lamp. They added trematodes to the cup with the tadpoles and counted how many of the parasites made it into the tadpoles’ kidneys.

To test for ranavirus, which causes hemorrhaging and bloating in the frogs, the researchers exposed the tadpoles to the viral particles and after measuring their time of death, dissected the kidney and liver and measured the viral loads.

The results showed that wood frogs close to agriculture have a higher tolerance to pesticides, are more susceptible to ranavirus and are less susceptible to trematodes than those that live farther away.

Hua said this may have something to do with the multiple life stages of trematodes that, in areas of more agriculture, have more nutrient input that can increase resources for things like snails. As a result, the parasite may spend more time in other stages, impacting fewer tadpoles.

“While I think it’s important to replicate this in other amphibians, this is an important first step,” she said.

Hua said it’s important to balance the need for pesticides in agriculture with the health of the ecosystem. “This study shows that it is important to understand that there are costs,” she said.

Header Image: An adult wood frogs stands in Pennsylvania’s Elk State Forest. ©Devin G. DiGiacopo