Neonicotinoid Insecticide Exposure Harms Amphibians Across Multiple Life Stages

(Beyond Pesticides, September 27, 2022) Exposure to widely used neonicotinoid insecticides harms amphibians at multiple life stages, adversely affecting their ability to survive in the wild, according to research published in the Journal of Zoology. As long-lived, systemic insecticides, neonicotinoids are consistently found in US waterways, often above federal safety limits, making these findings particularly dangerous for frogs and other amphibians throughout the country. As troubling data piles up on this class of dangerous insecticides, which are damaging pollinators, birds, deer, aquatic wildlife, and human health, it is left to the public to place pressure on federal regulators and members of Congress to act.

To understand the impact of neonicotinoids on amphibian life stages, researchers conducted a range of experiments. These were designed to investigate how exposure to the neonicotinoid imidacloprid affected larval survival, sexual development, locomotor skills, and avoidance behavior of the wood frog (sylvatic frog).

Larval survival was examined by exposing tadpoles to 10 parts per billion (ppb) of imidacloprid, at a rate lower than the lethal concentration expected to kill half of other frogs species in acute toxicity tests. Four treatment protocols were established, adding the variable of natural pond drying to half of the tested frogs to see if there was interplay between chemical exposure and natural stressors in the environment. Of the four groups, two were exposed to imidacloprid, one in a non-drying tank and another in a slowly drying tank, while the other two acted as controls, including one non-drying and one drying tank. Researchers observed larval survival, size at metamorphosis, and sex ratio.

To see how pesticide exposure affected frogs at the terrestrial stage of their life, they were also subject to endurance trials, assessing their ability to jump on a circular track. Frogs exposed to imidacloprid as tadpoles were evaluated, and then exposed to imidacloprid again and tested to see how subsequent environmental exposures impacted overall fitness. Lastly, neonicotinoid exposed frogs were placed in a tank where half of it was sprayed with imidacloprid to see if the frogs attempted to avoid exposure to the chemical.

Moreover, over 15% of neonicotinoid exResults showed that frogs exposed to imidacloprid in drying ponds experienced the highest mortality rate. Chemical exposure also resulted in an altered sex ratio. Unexposed frogs had 10% more males survive metamorphosis. Exposed frogs could not be sexed due to “unclear morphology of the reproductive organs,” while one exposed frog was “clearly hermaphroditic,” implicating imidacloprid as a likely endocrine disruptor.

Yet interestingly, exposed frogs developed into larger and heavier frogs than those in the unexposed control. However, this story is quite complicated and ultimately does not indicate a benefit from this phenomenon, but instead a likely detriment.

Study coauthor Cassandra Thompson, PhD, explained the situation in an Ohio University release: “Unexpectedly, we found that frogs from Imidacloprid treated tanks were significantly larger than control frogs and prior to exposing them to pesticides in the terrestrial environment, they outperformed control frogs in endurance trials. They may be able to travel further distances and are overall better marathon runners! Unfortunately, they also seem to crash harder,” Dr. Thompson said. “We wanted to know what would happen to a frog if [it] travel[s] across and temporarily settle in areas that have been recently sprayed with Imidacloprid. After 12-hour exposure to imidacloprid, we found reduced endurance capabilities of frogs from all treatments. Additionally, pre-exposure to imidacloprid as a tadpole caused greater declines in locomotor capability when exposed to imidacloprid again as a recently metamorphosed frog. So if you were exposed as a tadpole and were exposed again in the terrestrial stage, you could have reduced endurance capabilities or how far you can move in the environment.””

In the study, researchers indicate that neonicotinoid exposed frogs may have grown larger for a number of reasons, including greater access to food, and/or lack of competition due to the higher mortality rate among exposed tadpoles in their experimental group.

Neither exposed nor unexposed tadpoles appear to recognize or attempt to avoid imidacloprid in the environment. “If these frogs come across one of these drenched soil sites, they not only won’t be able to behaviorally assess and avoid the pesticide, but also risk mortality if they settle there for a short period as their locomotor abilities will be hampered,” said Dr Thompson.

Frogs and amphibians are not specifically tested by the US Environmental Protection Agency (EPA) prior to the registration of a pesticide. At best, they are provided “aquatic life benchmarks” which are unenforceable, arguably arbitrary ranges that EPA indicates can be helpful “in identifying and prioritizing sites and pesticides that may require further investigation.”

While EPA effectively ignores the impacts of pesticides on sensitive amphibians with complex life cycles, independent science has shown a range of harm. Earlier research, published in 2017, found that imidacloprid-exposed wood frogs experience delayed metamorphosis, placing populations at risk of increased mortality. Frogs exposed to neonicotinoids have also been shown to display a weakened response to a predator attack. While unexposed frogs hopped away to avoid researchers mimicking a heron attack, those in contact with imidacloprid did effectively nothing to avoid becoming lunch.

Data indicating a disrupted endocrine system response with imidacloprid exposure is concerning in light of research on how other pesticides affect frog sexual reproduction. Renowned scientist Tyrone Hayes, PhD, of the University of California Berkeley, has published extensive research documenting a range of impacts to various amphibian species from exposure to the herbicide atrazine. Results have shown the ability for atrazine not only to skew sex ratios, but also cause hermaphroditism in male frogs, and in some cases make male frogs completely female with the ability to lay eggs.

Despite a range of concerning independent data on chronic effects that, while not outright killing an animal, increases its likelihood of death in the wild, EPA has done little to rein in the use of the insidious pesticides harming the health of both wildlife and humans. Skewed sex ratios in frogs, birth defects and increased mortality in fawns, immune system damage and lower nutritional value in shrimp and oysters, inability for songbirds to orient during migration, reduced fruit productivity resulting from disoriented and uncoordinated pollinators, and evidence of hormone-dependent breast cancer in humans are all findings in independent, peer-reviewed studies. Yet, EPA discounts or dismisses this data and relies on non-peer reviewed studies produced by the manufacturer of the pesticide in accordance with EPA protocol, but not subject to regular unannounced audits.

If this information makes you queasy, take a breath, catch your breath, and then take action. Educate friends, family, neighbors, and your broader community about the dangers posed by neonicotinoids and other EPA registered pesticides. Protecting wildlife doesn’t mean reverting to a different pesticide or different way of applying a toxic pesticide, but by changing practices that embrace the pest management abilities of natural systems. Help move your community toward that approach by asking your local leaders to embrace organic land management, and push for changes at the top by urging your federally elected representatives to cosponsor the Saving America’s Pollinators Act (SAPA). By eliminating neonicotinoids, SAPA would not only protect pollinators, but frogs and entire ecosystems currently contaminated with these long-lived insecticides.

All unattributed positions and opinions in this piece are those of Beyond Pesticides.

Source: Ohio University press release, Journal of Zoology

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