The carabid ground beetles of the family Carabidae are as common as they come. These little nocturnal hunters are found in almost any terrestrial habitat on the planet. To some, they’re simply pests — infamous for their pungent smell, which they emit when they’re threatened. Others, but farmers in particular, are fond of them because they sate their voracious appetite by snacking on a variety of garden pests, including snails, caterpillars, slugs, and other small invertebrates.
Unfortunately, their diet of late has also begun to include microplastics.
Small fragments, films, and fibres of this nuisance material are dispersed in our oceans and our lands. In soil, they have a tendency to accumulate, altering the soil’s structure and the way it retains water, and reacting almost unpredictably with the microbes in their surroundings.
The smaller a piece of microplastic is, the more easily it’s digested by organisms. Researchers have found that their subsequent effects include deleterious effects on the surrounding tissue and metabolic stress.
An innovative approach
However, while the distribution and consequences of microplastics are well documented in aquatic environments, tracking them in terrestrial ecosystems has proven quite difficult. The primary challenge is the soil itself: unlike water, which can be filtered and analysed, soil is a complex, heterogeneous matrix. Here, microplastics become entangled with organic matter, clay particles, and mineral grains, rendering scientists’ methods to extract them for study technically demanding. Even when scientists do recover them, they have difficulty distinguishing synthetic microplastic fragments from natural fibers, and need specialised labs to do so.
Burrowing earthworms and insects also help spread microplastics through the soil, and they can be carried further underground by the roots of affected plants. As a result, scientists often have to engage in laborious soil-sampling endeavours, followed by chemical treatment and expensive techniques like spectroscopy.
To fill the gap an innovative approach was necessary.
Recently, scientists from Italy took a new look at this problem. The group started looking for insects that could help detect microplastics in soil rather than scientists having to do so directly. These insects, the Italy group figured, would have to be easy to sample, be distributed widely, and be well documented in the literature for further analysis.
The carabid beetle fit the bill.
Encouraging and unsettling
In a study published in Ecological Indicators, the scientists set pitfall traps along the Conero coast of Italy from July to October 2020. According to ScienceDirect, “Pitfall traps are plastic containers dug into the substrate until flush with the surrounding surface, often covered with plastic plates to prevent rain and debris from entering, and may contain preservatives such as ethylene glycol or saline solution.”
These traps captured carabid beetles from different sites with varying levels of human activity: a meadow, woodland, and a stony beach, among others.
When the scientists had a sample of 50 carabid beetles, they dissected each specimen, extracted the contents of their guts, digested them with oxidising solutions, filtered the remains, and examined what was left under microscopes and using infrared spectroscopy.
What they found was both encouraging and unsettling. Nearly a third of the beetles carried microplastic fragments, most of them polyester and silicone, ranging from 0.1 to 1 mm in size and often no larger than a grain of sand. The highest rates of microplastic ingestion appeared on a stony beach popular with summer tourists, where up to 87% of beetles captured in August contained plastic.
The encouragement came from the fact that the carabid beetles proved to be legitimate bioindicators of microplastics. While this study represents only the first step in this direction, according to the authors, more standardised techniques can be developed on this work. For example, carabid beetles can be integrated into bio-monitoring programs as a cost-effective way to assess the extent of microplastic contamination in a location.
Simple, sensitive marker
The idea of using bioindicators is not new. In India, farmers in particular already use bioindicators to help anticipate impending rain, to manage pests, to assess soil fertility, and to evaluate the success of certain organic farming practices. Plenty of research is also ongoing in the country to detect contaminants in rivers and agricultural lands using insects.
But why insects? The new study had in fact also unearthed earthworms (which are annelids) that had ingested microplastics.
In a review published recently in Plant Archives, a team of scientists from Navsari Agricultural University (NAU) in Surat explained why insects present clear advantages.
The principle purpose of a bioindicator is to be a simple and sensitive marker of an ecosystem’s health.
According to the Plant Archives paper, insects are effective bioindicators because they respond quickly to environmental stress, are abundant, and have short life cycles. They are also able to provide early warnings of ecological changes, are cost-effective to monitor, and sensitively reflect the consequences of pollutants on ecosystems, plants, and humans.
“Their prevalence in most terrestrial habitats followed by an ability to migrate makes them the best choice,” NAU entomologist Malireddi Prasanna and lead author of the paper said of the carabid beetles in this regard.
Humans’ response
She added that even if their populations are negatively affected when scientists pick up individuals from an area for study, what they learn could help protect the insects.
The review paper also enumerated important challenges when working with insects as bioindicators. They include natural fluctuations in their populations, temporary declines from calamities, predation pressure, and their absence in particular seasons. Their complex life cycles and varying activity patterns can also complicate scientists’ ability to interpret the data and thus demand careful, long-term monitoring for reliable assessments.
Research worldwide has established that microplastics have contaminated almost all known ecosystems and biospheres. The carabid beetles and other insects could be an important part of the way humans respond to this crisis.
Sumed Shinde hasa master’s degree in animal behaviour and conservation and is a science writer.
