I’ve been working with a pesticide called carbaryl over the last 16 years. There’s a lot of research looking at how different chemicals result in mortality and what levels may be environmentally safe, which is good and valuable research. One of my goals, however, has been to understand the ecological ramifications of pesticide exposure; to do this, we have used rather few pesticides, but asked a diversity of ecological questions…well, at least 16 years worth. There’s nothing particularly special about this chemical, although it’s considered relatively benign and wasn’t even registered as a toxic substance when I started using it. Carbaryl is a neurotoxin that is found in Sevin dust or liquid Sevin, which many people use in the garden, and it’s also found in many other products including some flea powders for pets. This chemical has lower toxicity to mammals than to many aquatic species, like the frogs and salamanders that I study. However, one of the surprising things my collaborators and I have found is that sometimes pesticide exposure can have what looks like positive effects on tadpoles. The most likely explanation for apparent positive effects on tadpoles is due to a reduction in algal grazers, like water fleas (Daphnia spp.) that are very sensitive to insecticides, which can result in algal blooms. When the algae that tadpoles eat increases, then they can reach larger sizes or transform sooner. For instance, in some studies we did with Green Frog tadpoles, which frequently take over a year to transform into frogs, we found that pesticide exposure lead to early metamorphosis under some conditions. So, theoretically, that doesn’t seem so bad for the frogs in this case. Unless…it’s setting them up for failure in some other way.
We have been doing some studies over the last couple of years to understand if this precocious metamorphosis is solely the results of changes in food resources, or if timing of exposure may influence the developmental pathway in some way. One of our recent lab study suggested that carbaryl exposure for just three days can lead to changes in thyroid responsive genes in the brain, suggesting that exposure at key times during development could have long term impacts on amphibians. We are attempting to look at this in the field in experimental mesocosm ponds, which you can see above.
The last two summers we have tried field experiments with green frogs, but we weren’t getting a large enough number of juvenile metamorphs to look at changes in thyroid receptors in the brain, so this year we’re using a different species, northern leopard frogs, which hatch from eggs in the early spring and live as tadpoles for a couple of months before transforming into juvenile frogs. I collected some egg masses of northern leopard frogs in Somerville, OH just last week (see below), and I could hear the male’s chuckling call in the background during the day. One of the neighbors said they could even go outside at night without the deafening frog calls.
Although we expect tadpoles to turn into frogs, it’s a major developmental change—it would hardly be more impressive if you started your life out as fish and crawled out onto land as a cute little human baby. Tadpoles go from swimming, herbivorous animals with tails and no limbs to four-legged, hopping, tailless carnivores that spend the majority of life on land (for most species). This massive reorganization and development is controlled by thyroid hormones which start out very low early in development and increase through metamorphic transformation and then drop back down—a similar trajectory to our own thyroid hormones during gestation and birth, suggesting that amphibian metamorphosis can tell us something about the human animal as well. But, if a chemical affects thyroid hormones, the time of exposure could be critical and in some cases you might expect to see early metamorphosis (or maybe birth in a human).
So, if a chemical causes earlier metamorphosis, could it result in some tradeoff like increased mortality or reproductive problems? Well, that is we hope to find out, so stay tuned. Experiments can be well planned and well-executed, but there’s always the elements of surprise. (See that wind turbine in the picture above? I hope it doesn't surprise us.) I am definitely hoping for good surprises this year.