Neonicotinoids are systemic insecticides that act as agonists of nicotinic acetylcholine receptors, the molecular switches that nerve cells use to communicate. They were engineered to bind insect receptors far more strongly than vertebrate ones, which long underpinned claims of their relative safety for wildlife with backbones. Yet their chemical stability, rapid absorption, and resistance to degradation mean they persist in soils and waterways, and thiamethoxam in particular is frequently detected in surface waters and sediments of farming regions at concentrations ranging from 0.001 to 225 micrograms per liter. Adding to the concern, thiamethoxam converts in insects, plants, and the environment into clothianidin, a metabolite that behaves as an even more potent receptor agonist, meaning organisms may face a shifting and potentially more toxic chemical landscape than exposure measurements alone suggest.

Amphibians are uniquely positioned to absorb this chemical burden. Their skin is highly permeable, their eggs and larvae develop directly in water, and their biphasic life cycle ties them to both aquatic and terrestrial habitats, multiplying the routes by which pesticides can enter their bodies. These traits make anurans classic sentinel organisms, early-warning sensors whose biological responses flag environmental degradation before it becomes catastrophic. With amphibian populations collapsing worldwide and emerging pollutants increasingly implicated in the decline, the research team set out to close a conspicuous gap: while neonicotinoid effects on bees and aquatic invertebrates are well documented, the genetic and lethal toxicity of thiamethoxam in frogs had been barely explored.

The experiment followed internationally recognized testing guidelines and was anchored in environmentally realistic concentrations. Tadpoles at Gosner stage 25, obtained from a Brazilian breeding facility, were acclimated in dechlorinated water and then exposed for 96 hours to seven concentrations of a commercial thiamethoxam formulation, Cruiser 350 FS, ranging from 14.40 to 3516 micrograms of active ingredient per liter. The intermediate dose of 225 micrograms per liter was chosen to mirror concentrations reported in the field. Actual exposure levels were verified by liquid chromatography coupled to tandem mass spectrometry at the Federal University of Goiás, a rigorous analytical step that confirmed the insecticide was the sole pesticide present and that the test concentrations were accurate. Water quality parameters, including pH, dissolved oxygen, conductivity, and ammonia, remained within ranges compatible with valid acute toxicity testing throughout the study.

The lethal results were striking. Probit analysis estimated the concentration killing 10 percent of tadpoles within 96 hours at 624.44 micrograms of active ingredient per liter, the LC50 at 1070.13 micrograms per liter, and the LC90 at 1833.03 micrograms per liter. Those figures place the American bullfrog among the most sensitive amphibian species ever tested against this class of chemicals. For comparison, earlier studies reported 96-hour LC50 values for other South American anurans exposed to thiamethoxam ranging from 11.28 to more than 71.2 milligrams per liter, roughly ten to seventy times higher than the bullfrog’s threshold. Against the related neonicotinoid imidacloprid, the contrast is even sharper: values of 82 milligrams per liter for Rana limnocharis, 74.18 for Xenopus laevis, and 173.55 for Rana nigromaculata dwarf the bullfrog’s 1.07 milligrams per liter, which is approximately 98 percent lower than the 52.62 milligrams per liter recorded for Hypsiboas pulchellus exposed to a commercial imidacloprid product.

More alarming still were the sublethal findings. The team exposed a second cohort of tadpoles to 9.4, 31, and 59 micrograms of active ingredient per liter, corresponding to 1.25, 2.5, and 5 percent of the LC50, doses squarely within the range measured in contaminated waterways. Using the Comet assay, a technique that detects DNA strand breaks in individual cells by measuring how fragmented genetic material migrates during electrophoresis, the researchers analyzed 1,600 nuclei per treatment group across three damage metrics: tail length, percentage of DNA in the tail, and the Olive Tail Moment. All three parameters responded significantly, with damage peaking at the intermediate concentration of 31 micrograms per liter. The Micronucleus test, which scores permanent chromosomal damage in dividing cells, revealed increased frequencies of lobed nuclei, nuclear budding, and anucleated cells, with mutagenic effects most pronounced at the highest dose of 59 micrograms per liter.

The non-monotonic pattern, in which an intermediate dose produced more DNA damage than a higher one, is a signature increasingly recognized in neuroactive compounds. The authors suggest several non-exclusive explanations. At lower concentrations, neonicotinoids may destabilize cellular processes through partial receptor activation, while at higher doses the damage may become so severe that cells die outright, removing the most injured cells from the assay and producing an apparent dip in measured genotoxicity. The significant rise in anucleated cells at 59 micrograms per liter, indicating loss of nuclear integrity, is consistent with advanced cytotoxic processes of exactly this kind. Guidelines for the Comet assay from the Organisation for Economic Co-operation and Development explicitly warn that high cytotoxicity can mask genotoxic responses, a caveat the researchers took seriously in interpreting their dose-response curve.

Mechanistically, the study situates its results within a growing literature showing that thiamethoxam genotoxicity is driven largely by oxidative stress. Exposure to the compound has been linked to overproduction of reactive oxygen species, lipid peroxidation, and disruption of antioxidant defenses including superoxide dismutase, catalase, glutathione, and glutathione peroxidase. This redox imbalance promotes oxidative DNA lesions such as 8-hydroxy-2′-deoxyguanosine, a hallmark of oxidized nucleotides, which manifests as the strand breaks the Comet assay detects. Beyond direct chemical attack on DNA, thiamethoxam has been shown in vertebrate models to alter the expression of genes governing xenobiotic metabolism, cell cycle control, and apoptosis, while promoting inflammatory signaling and, at high doses, mitochondrial dysfunction and ferroptosis. The positive correlations the Brazilian team found between Comet parameters and lobed nuclei frequencies suggest a mechanistic continuum: primary, potentially repairable DNA lesions that escape repair before cell division can progress into chromosome fragmentation, spindle errors, and the fixed nuclear abnormalities scored in the Micronucleus test.

The ecological implications extend beyond individual tadpoles. DNA damage in amphibians has been experimentally linked to malformations, reduced growth, behavioral impairment, and diminished fitness, and genomic instability is a recognized precursor of tumor transformation in the framework of cancer biology. Even subclinical cellular injury can erode body condition, reproductive capacity, and survival, with cumulative effects that ripple through population dynamics. The authors also note a regulatory blind spot: commercial formulations like Cruiser 350 FS contain 83 percent unspecified co-formulants, solvents and surfactants legally classified as inert but capable of altering the bioavailability and toxicity of the active ingredient, and whose identities remain hidden from researchers. Prior work has shown that stressors such as ranavirus infection can lower thiamethoxam’s LC50 in wood frogs to near environmental concentrations, hinting that real-world toxicity under multiple stressors may exceed laboratory estimates.

The study concludes that thiamethoxam disrupts essential cellular processes in amphibians at concentrations found in the environment, and it positions the American bullfrog as a sensitive bioindicator for aquatic contamination. The Comet assay proved the more sensitive instrument for catching early, transient DNA lesions during short exposures, while the Micronucleus test captured the establishment of persistent chromosomal alterations, underscoring the value of deploying both biomarkers in tandem. The researchers call for future work across additional species, developmental stages, and biomarkers tied to cholinergic signaling, DNA repair, and cell death, to clarify the non-monotonic responses and to build more realistic ecotoxicological risk assessments for neonicotinoids. For now, the message for freshwater ecosystems is sobering: an insecticide designed to spare vertebrates is leaving its mark on their genomes at doses they actually encounter.

Subject of Research: Genotoxic and cytotoxic effects of the neonicotinoid insecticide thiamethoxam on bullfrog tadpoles

Article Title: Environmentally relevant concentrations of thiamethoxam cause cytotoxic and genotoxic damage in bullfrog (Lithobates catesbeianus) tadpoles

Article References: De Oliveira, A. A., Barradas, M. C., Tomaz, A. A., da Silva Lima, D., Assis, J. V. B., Chaves, A. R., Vieira, T. B., Monteiro, M. S., Sarmento, R. A., de Souza Saraiva, A., & De Melo e Silva, D. (2026). Environmentally relevant concentrations of thiamethoxam cause cytotoxic and genotoxic damage in bullfrog (Lithobates catesbeianus) tadpoles. Ecotoxicology, 35(8), Article 182. https://doi.org/10.1007/s10646-026-03154-0

Image Credits: AI Generated

DOI: 10.1007/s10646-026-03154-0

Keywords: thiamethoxam, neonicotinoid, bullfrog, Lithobates catesbeianus, tadpoles, genotoxicity, Comet assay, micronucleus test, DNA damage, ecotoxicology, amphibian decline, water contamination