The diligent carabid ground beetles, members of the Carabidae family, are ubiquitous nocturnal predators found in nearly every terrestrial habitat imaginable. While some view them as mere garden nuisances, notorious for the pungent odor they release when threatened, farmers often appreciate their insatiable appetite for common pests like snails, caterpillars, slugs, and other small invertebrates.
Lately, however, an alarming addition has crept into their diet: microplastics.
These tiny fragments, films, and fibers of plastic pollution are pervasive, contaminating our oceans and steadily accumulating in our lands. In soil, they can alter its structure, impact water retention, and interact unpredictably with the complex microbial communities surrounding them. The smaller these plastic pieces are, the more readily organisms ingest them, leading to documented adverse effects like tissue damage and metabolic stress.
A Clever Solution to a Tricky Problem
Despite extensive research on microplastic distribution and consequences in aquatic environments, tracking them in terrestrial ecosystems has remained a significant hurdle. The primary challenge lies in the very nature of soil itself: unlike water, which can be filtered and analyzed relatively easily, soil is a complex, diverse matrix. Microplastics become intricately entangled with organic matter, clay, and mineral grains, making extraction and study technically demanding. Even when recovered, distinguishing synthetic microplastic fragments from natural fibers requires specialized laboratory techniques.
Furthermore, burrowing creatures like earthworms and other insects inadvertently spread microplastics through the soil, and plant roots can carry them deeper underground. Consequently, scientists have often resorted to laborious soil sampling, followed by chemical treatments and expensive spectroscopic analyses.
To bridge this knowledge gap, a truly innovative approach was sorely needed.
Recently, a team of Italian scientists embarked on a novel quest, seeking out insects capable of detecting microplastics in soil, thus reducing the need for direct human sampling. These ideal insect candidates, they reasoned, would need to be easy to collect, widely distributed, and well-documented in scientific literature for further analysis.
The common carabid beetle perfectly fit the criteria.
Both Promising and Disturbing Discoveries
In a groundbreaking study published in Ecological Indicators, the scientists deployed pitfall traps along Italy’s Conero coast between July and October 2020. These traps, essentially plastic containers buried flush with the ground, were used to collect carabid beetles from various sites with differing levels of human activity, including meadows, woodlands, and a stony beach.
After collecting a sample of 50 carabid beetles, researchers meticulously dissected each specimen, extracted the contents of their guts, digested them with oxidizing solutions, filtered the remaining materials, and then examined them under microscopes and using infrared spectroscopy.
The findings were a potent mix of encouraging and unsettling news. Nearly a third of the beetles examined were carrying microplastic fragments, predominantly polyester and silicone, ranging from 0.1 to 1 mm in size—often no larger than a grain of sand. The highest rates of microplastic ingestion were observed on a stony beach popular with summer tourists, where an astonishing 87% of beetles captured in August contained plastic.
The encouraging aspect confirmed that carabid beetles are indeed legitimate bioindicators for microplastics. While the authors acknowledge this study as a foundational step, they suggest that more standardized techniques can be developed from this work. For instance, carabid beetles could be integrated into routine biomonitoring programs, offering a cost-effective method to assess the extent of microplastic contamination in specific locations.
Why Insects Make Perfect Environmental Detectives
The concept of using bioindicators is far from new. In India, for example, farmers have long relied on natural indicators to predict rainfall, manage pests, gauge soil fertility, and evaluate the efficacy of various organic farming practices. A significant body of research is also underway across the country, exploring the use of insects to detect contaminants in rivers and agricultural lands.
But why insects? The Italian study itself also revealed earthworms (which are annelids) that had ingested microplastics.
In a recent review published in Plant Archives, a team of scientists from Navsari Agricultural University (NAU) in Surat highlighted the distinct advantages of using insects as bioindicators.
The fundamental purpose of a bioindicator is to provide a simple and sensitive marker of an ecosystem’s health.
According to the Plant Archives paper, insects excel as bioindicators because they exhibit rapid responses to environmental stress, are abundant, and possess short life cycles. They can offer early warnings of ecological changes, are cost-effective to monitor, and accurately reflect the repercussions of pollutants on ecosystems, plants, and even humans.
“Their prevalence in most terrestrial habitats, coupled with their ability to migrate, makes them an excellent choice,” noted NAU entomologist Malireddi Prasanna, the lead author of the review, specifically referring to carabid beetles.
Humanity’s Call to Action
Prasanna further added that even if their populations are temporarily affected by scientific sampling, the knowledge gained could ultimately contribute to protecting these vital insects.
The review paper also acknowledged important challenges associated with using insects as bioindicators. These include natural fluctuations in their populations, temporary declines due to natural calamities, predation pressure, and their seasonal absence. Their complex life cycles and varied activity patterns can also complicate data interpretation, necessitating careful, long-term monitoring for truly reliable assessments.
Research globally has unequivocally shown that microplastics have infiltrated almost every known ecosystem and biosphere. Carabid beetles and other insects could prove to be indispensable sentinels, guiding humanity’s crucial response to this escalating environmental crisis.