Neonicotinoids are systemic insecticides applied as coatings on crop seeds before planting. As the seed germinates and grows, the chemical is taken up into plant tissues, protecting the young seedling from chewing and sucking insects. But only a fraction of the active ingredient ends up inside the plant. The rest can persist in the soil, where it encounters an enormous community of microorganisms that drive nutrient cycling, decompose organic matter, and form partnerships with plant roots. While the impacts of neonicotinoids on insects and aquatic invertebrates have been studied extensively, their effects on soil fungal communities have remained surprisingly underexplored, a gap the new research set out to fill.

The team designed a controlled microcosm experiment in which soil was treated with three commonly applied neonicotinoids: imidacloprid, thiamethoxam, and clothianidin. These compounds represent the backbone of modern seed-treatment agriculture, coating millions of tonnes of seed annually across the world’s major cropping systems. By working in microcosms, the researchers could isolate the effect of each compound on the fungal community without the confounding noise of weather, cropping history, or management variation that complicates field studies. Soil samples were then collected at multiple time points after exposure, allowing the team to track not just whether the fungi responded, but when.

To profile the fungal communities, the researchers turned to amplicon sequencing of the internal transcribed spacer, or ITS, region of fungal DNA. The ITS region is the standard molecular barcode for fungi, allowing scientists to identify which fungal taxa are present in a soil sample even when those organisms cannot be cultured in the laboratory. This technique revealed hundreds of fungal taxa across the samples, spanning dominant phyla such as Ascomycota and Basidiomycota as well as rarer, more enigmatic lineages. Sequencing-based approaches like this have transformed microbial ecology in recent years, making it possible to detect subtle community shifts that would be invisible under a microscope.

One of the study’s most striking findings concerns timing. Overall diversity indices remained largely stable across the treatments when averaged over the whole experiment, which might initially suggest the fungicide exposure had little effect. But post-hoc comparisons told a different story: on day 10, fungal diversity dropped significantly under both imidacloprid and thiamethoxam exposure. This kind of delayed, transient response is exactly the sort of signal that single end-point sampling would miss. The lesson, the authors suggest, is that the ecological footprint of a pesticide cannot be judged from a single snapshot in time.

Beta diversity analysis, which measures how community composition differs between samples, reinforced this temporal picture. The researchers found significant effects of sampling day and, critically, a treatment-by-time interaction, meaning the fungal communities under different insecticide treatments did not simply follow the same trajectory. Instead, each compound appeared to push the community along its own path as the experiment unfolded. Such temporally variable responses complicate risk assessment, because a pesticide that looks benign in one week of a field season may produce measurable disruption in another.

Not all fungi responded equally. The dominant phyla, Ascomycota and Basidiomycota, which include many decomposers and plant-associated species, remained relatively stable throughout the experiment. But several less abundant phyla declined over time, including Mortierellomycota, Rozellomycota, and Olpidiomycota. These obscure-sounding groups are far from ecologically trivial. Mortierellomycota species are important decomposers and plant growth promoters, Rozellomycota comprises widespread intracellular parasites of other microorganisms, and Olpidiomycota includes fungi that can vector plant viruses. A decline in these rarer lineages may signal subtle erosion of functions that only become apparent when soil health degrades.

Perhaps the most consequential result came from functional guild analysis, which classifies fungi by their ecological roles rather than their taxonomy. Here the researchers found that clothianidin exposure drove an increase in saprotroph abundance, the fungi that break down dead organic matter, alongside a decrease in symbiotroph abundance, the fungi that live in mutually beneficial partnerships with plants. The most important symbiotrophs in agricultural soil are the arbuscular mycorrhizal fungi, which colonize crop roots and exchange soil nutrients for plant sugars. A shift away from symbiotrophs and toward saprotrophs could alter how nutrients flow through the soil food web, potentially affecting crop nutrition in ways that standard pesticide evaluations never measure.

Differential abundance analysis, a statistical technique for identifying which taxa increase or decrease under specific conditions, revealed that each compound left its own fingerprint on the community. Imidacloprid was associated exclusively with suppressed fungal biomarkers, meaning every taxon linked to this compound was depleted rather than enriched. Thiamethoxam induced both enriched and depleted taxa, suggesting a more mixed restructuring of the community. Clothianidin stood out as the most disruptive of the three, associated with the greatest number of discriminatory fungal biomarkers and accompanied by the increase in saprotrophs and reduction in symbiotrophs. The compound-specific nature of these responses suggests that treating all neonicotinoids as a single ecological hazard may obscure important differences among them.

The study, which was funded by the Australian National University and conducted with colleagues Julia F. Jasonsmith, Nilantha R. Hulugalle, and Craig L. Strong, carries implications well beyond the laboratory. As regulators and farmers weigh the costs and benefits of neonicotinoid seed treatments, fungal communities have rarely featured in the calculus, even though these organisms underpin soil fertility, carbon storage, and crop resilience. The authors argue that fungal community responses deserve a place in evaluations of the broader ecological effects of neonicotinoids, and they call for long-term, functionally oriented studies in real agroecosystem contexts. A ten-day microcosm can reveal the shape of a disturbance, but only sustained field research can determine whether these shifts persist, compound across seasons, or ultimately translate into measurable consequences for the crops that depend on the hidden life below ground.

Subject of Research: Effects of neonicotinoid seed treatments on soil fungal community diversity, composition, and function over time

Article Title: Fungal Responses to Neonicotinoid Seed Treatments in Soil: Temporal Shifts in Community Diversity and Composition

Article References: Akter, S., Jasonsmith, J. F., Hulugalle, N. R., & Strong, C. L. (2026). Fungal Responses to Neonicotinoid Seed Treatments in Soil: Temporal Shifts in Community Diversity and Composition. Microbial Ecology. https://doi.org/10.1007/s00248-026-02892-2

Image Credits: AI Generated

DOI: 10.1007/s00248-026-02892-2

Keywords: neonicotinoids, soil fungi, fungal diversity, imidacloprid, thiamethoxam, clothianidin, ITS amplicon sequencing, soil microbiome, temporal dynamics, saprotrophs, symbiotrophs, agroecosystems

The research team, led by scientists at the University of São Paulo and the University of Campinas in Brazil, set out to answer a question that has lingered in ecotoxicology for years. Plastic debris floating in rivers and lakes is not a static pollutant. Sunlight, heat, and oxygen steadily break polymer chains apart, and this weathering alters everything about the particle, from its surface chemistry to its roughness and its appetite for other contaminants. Yet most laboratory toxicity studies still rely on pristine, factory-fresh plastic, potentially painting a misleading picture of what actually drifts through natural waters.

To simulate years of environmental exposure in the laboratory, the researchers bombarded commercial polyethylene microparticles with UV-C radiation for 94 days in an accelerated aging chamber, a dose they calculated to be equivalent to roughly 643 days of natural sunlight. The transformation was striking. Scanning electron microscopy showed that smooth, homogeneous particles emerged covered in cracks, cavities, and pitted surfaces, evidence of a progressively weakening polymer matrix. Fourier-transform infrared spectroscopy confirmed the chemical counterpart of this physical decay: new absorption bands at 1710 and 1181 inverse centimeters revealed the formation of carbonyl and carbon-oxygen groups, the classic fingerprints of polyethylene photo-oxidation.

These chemical changes matter far beyond materials science. Oxygenated functional groups make the plastic surface more polar and more reactive, increasing its capacity to adsorb organic pollutants such as pesticides. The aging process also nudged the particle size distribution upward slightly, with degraded particles averaging 30 micrometers compared with 27 for pristine ones, and produced a more porous structure that could offer additional binding sites for dissolved chemicals. In other words, every day a plastic particle spends in the sun converts it into a more chemically active platform for carrying other contaminants into the food web.

The ecotoxicological centerpiece of the study was the water flea Daphnia similis, a tiny filter-feeding crustacean and a standard sentinel of freshwater health. In 21-day chronic exposure tests conducted under OECD guidelines, control animals thrived, with 90 percent survival and healthy production of around 66 offspring per female. But exposure to pristine polyethylene microplastics proved unexpectedly lethal. At 80 milligrams per liter, mortality reached 90 percent, and at 140 milligrams per liter it reached 70 percent, concentrations the authors note correspond to just a few times the highest levels of microplastics reported in natural freshwaters.

Curiously, the dose-response was not neatly linear, with the strongest lethal effect appearing at the intermediate concentration rather than the highest one. The researchers attribute the mortality to physical mechanisms: daphnids readily ingest particles in this size range, and accumulated microplastics can create a false sense of satiety, starve the animals of real nutrition, abrade the gut lining, and even impair oxygen diffusion through the filtering apparatus. The observed particles were also smaller on average than the manufacturer’s specification, and prior work suggests finer particles are substantially more harmful to daphnids than larger ones.

Photodegraded particles told a different and arguably more insidious story. While they killed fewer animals outright, they delivered sharper sublethal blows, particularly to reproduction. At 140 milligrams per liter, aged microplastics cut neonate production by nearly 70 percent. Reproduction is the demographic engine of any population, and a generation of daphnids that fails to replace itself cannot sustain the base of freshwater food webs that fish and other predators depend upon. The study demonstrates that the toxicity profile of a microplastic particle is not fixed; it is rewritten by every hour of sunlight it absorbs.

The most provocative results emerged when imidacloprid entered the picture. This neonicotinoid insecticide, which acts on the nervous systems of insects by overstimulating nicotinic acetylcholine receptors, is highly water soluble, persistent to hydrolysis, and routinely detected in surface waters worldwide at concentrations ranging from nanograms to hundreds of micrograms per liter. Because polyethylene is known to adsorb imidacloprid efficiently, the researchers expected the plastic to act as a vector, concentrating the pesticide and amplifying its harm.

Instead, the mixture data revealed an antagonistic interaction. In combination with 1.5 milligrams per liter of imidacloprid, pristine microplastics caused no lethal effects at any concentration, and a model deviation ratio analysis confirmed that the observed toxicity at 80 and 140 milligrams per liter fell significantly below what additive behavior would predict. The likely explanation lies in feeding behavior: daphnid feeding depends on the rhythmic beating of filtering appendages under nervous control, and imidacloprid’s neurotoxic action disrupts this neuromotor coordination. Animals exposed to the insecticide simply ingest fewer plastic particles, indirectly shielding them from the plastic’s physical harms, even as reproduction still dropped by up to 75.7 percent in some mixture treatments.

The authors caution that this apparent mercy is temporary. Previous research shows that prolonged imidacloprid exposure starves daphnids by suppressing feeding until energy reserves are exhausted, and the long-term ecological cost of a contaminated, underfed zooplankton community could be severe. Meanwhile, mixtures involving photodegraded particles followed a largely additive pattern, with progressive, dose-dependent reproductive losses up to fourfold below controls, likely reflecting hydrogen bonding between the polymer’s new oxygenated groups and the insecticide’s amine moieties. Taken together, the study delivers a clear message for regulators and risk assessors: the ecological risk of microplastics cannot be evaluated in isolation from their weathering state or their chemical traveling companions, and realistic assessments must embrace chronic exposures, aged materials, and contaminant mixtures if they are to protect the integrity of freshwater ecosystems.

Subject of Research: Photodegradation of polyethylene microplastics and its combined chronic toxicity with the insecticide imidacloprid to the freshwater cladoceran Daphnia similis

Article Title: Integrated Study of Polyethylene Microplastic Degradation and Its Interaction with the Insecticide Imidacloprid: Chronic Toxicity to Daphnia similis

Article References: Kiihl, M. E., da Silva Pinto, T. J., Dias, M. A., Montagner, C. C., Espíndola, E. L. G., & Alexandre, D. S. (2026). Integrated Study of Polyethylene Microplastic Degradation and Its Interaction with the Insecticide Imidacloprid: Chronic Toxicity to Daphnia similis. Archives of Environmental Contamination and Toxicology, 91(2), Article 16. https://doi.org/10.1007/s00244-026-01213-5

Image Credits: AI Generated

DOI: 10.1007/s00244-026-01213-5

Keywords: microplastics, polyethylene, photodegradation, imidacloprid, Daphnia similis, freshwater ecotoxicology, chronic toxicity, contaminant mixtures, UV aging, reproduction, neonicotinoids, water pollution