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Toxic pollutants in the air are taking the toll on pollinating insects

Read time: 6 mins
Toxic pollutants in the air are taking the toll on pollinating insects

A colony of Giant Asian Honey Bee (Apis dorsata) from Bengaluru [Image credits: Elephant Corridor Films]

New findings show air pollution is affecting the survival, health and genes of honeybees in Bengaluru.

Honey, where are the bees?
They seem to have gone, away from the trees!
For the air in the cities is mal,
Choking and killing 'em all!

It’s not just the bees. In reality, most insects are dying at a never-seen-before rate, and the world is heading towards an insect apocalypse. In the last three decades, the insect population has silently but surely dwindled by a quarter across the globe. In Kansas, USA, grasshoppers have declined by 30%, and in Germany, the population of insects has nosedived by a massive 75%. Closer home, farmers in Odissa noticed an 80% plunge in the number of bees—the primary pollinators of many crops—in their farms.

For years, chemicals in pesticides were thought to be the culprits, leading to the mass death of insects. Now, a new study has found that toxic pollutants in the air are equally responsible for this misery. Much like in humans, polluted air is affecting the survival, behaviour, health and genes of pollinating insects, honey bees in this case.

“During some local observations in Bengaluru, we noticed fewer pollinators in urban centres of the city. Was it pesticide use that caused this, or the lack of water or shade? We wanted to find out,” says Dr Shannon Olsson, talking about the motivation behind this study.

She is a professor at the National Center for Biological Sciences (NCBS), Bengaluru. Her lab, called the NICE lab, studies insects and works on the chemical ecology of pollinating insects.

The study, conducted by researchers from NCBS along with collaborators in the United States of America, was published in the Proceedings of the National Academy of Sciences (PNAS). It was partially funded by Tata Education Trust, the Department of Atomic Energy (DAE) and the Science and Engineering Research Board (SERB).

For four years, Geetha Thimmegowda, the lead author, travelled to different locations of Bengaluru to observe and collect Giant Asian honey bees (Apis dorsata).

“The Giant Asian honey bee is not only a common resident of Indian cities, but it is an important contributor to India’s food security and ecosystems. This bee produces over 80% of the country’s honey and pollinates over 687 plants in Karnataka alone. Thus, it is an excellent system to understand the impact of urbanisation on our ecosystem services,” says Dr Olsson, talking about why they chose to study this particular bee.

At each sampling site across the city, the researchers observed the number of Giant Asian honey bees visiting the yellow-flowered trumpet bush (Tecoma stans). They collected 1.820 bees across different locations and further analysed the havoc wreaked by air pollutants in these bees. They studied the survival, heart and respiration rates, immune system functions, blood cell count, the expression of genes for stress, metabolism, and immunity, and the amount of tiny particulate matter deposited on various body parts.

Pollutants harm bees, inside out

Air pollutants, like particulate matter (PM 10 and PM 2.5) are known to choke our lungs and cause severe respiratory, cardiovascular and neurological illnesses. In bees, the study found, they do more harm and kill about 80% of them. When they observed the collected bees under an electron microscope, they discovered that the hindlegs, antennae and wings of the bees were laden with particulate matter. Besides, toxic metals like arsenic, lead, tungsten and aluminium were also found on the bees. At sites with higher pollution levels, the amount of deposited particulate matter and metals were high too.

Non-coated scanning electron micrograph of the leg of a foraging Giant Asian honeybee collected from a highly polluted site in Bengaluru, India, with a PM10 concentration of  98.6 µg/m3. [Image Credits: Geetha Thimmegowda with a Zeiss microscope at 540X magnification]

Inside the bees, the researchers found anomalies in the insect’s heart rate which increased with pollution. The number of blood cells (hemocytes) in those exposed to pollutants was less than those collected from rural areas, indicating the effect of air pollution on their physiology.

When the genes of the bees were analysed, the study found that the genes responsible for stress response, immunity, homeostasis, and  metabolism were upregulated in bees from polluted sites. That is, the proteins involved in these mechanisms were produced more than normal levels. Some proteins, like vitellogenin—an antioxidant that protects cells from oxidative damage—were found in less-than-normal levels.

The researchers then exposed lab-grown fruit flies (Drosophila melanogaster) to the air in the polluted sites and studied them to see if the effect of pollution would be any different in these insects. They observed that, akin to the bees, the fruit flies also had particulate matter deposited in their bodies, differences in heart and respiratory rates and disparities in gene activities.

Could air pollution similarly affect other pollinating insects?

“We cannot say if the results would be the same for all of the wide variety of insects (flies, moths, butterflies, beetles, ants, etc.) that serve as pollinators. However, our study does show that our results are not specific to one species,” says Dr Olsson on the findings.

Its time to revisit air pollution guidelines

So far, many studies have explored the disastrous effects of air pollutants on humans—one of the millions of species calling this planet home. The current study is an attempt to understand how our actions affect the remaining ones.

“Our study is one of the few studies in the world to address the physiological and molecular impacts of air pollution on our wild plants and animals,” says Dr Olsson.

Bengaluru, a megacity in India, is not as polluted as other cities and has about 50μg/m3 concentration of PM10. The National Ambient Air Quality Standards prescribe an upper threshold of 100 μg/m3 in ecologically sensitive areas, which could have a greater impact on ecologically-vital systems, as study results show. It calls for a need to reevaluate these standards by taking into account the effect of air pollution on other vulnerable species. These effects also have an economic bearing in India, which is a significant producer of the world’s fruits and vegetables, resulting from the hard work of insect pollinators, like the bees.

“This study covers important new ground, by examining the impact of air pollution on pollinators, which have serious implications for agricultural output in India,” says Arunabha Ghosh. He is the founder and CEO of the Council on Energy, Environment and Water. “Similar studies for different agro-ecological zones should be initiated so that we have a better sense of how air quality is likely to impact pollinators and plants in various regions,” he adds.

The study shows the specific impacts of air pollution on wild systems, and what air quality standards would be needed to ensure their well-being.

“More research is needed on other systems to fully understand how air pollution is impacting our plants and animals. We hope that this study will inspire other researchers to tackle this crucial problem that impacts our lands, livelihoods and food security,” signs off Dr Olsson.


This article has been run past the researchers, whose work is covered, to ensure accuracy.