The adverse impact of air pollution on insect pollinators
By Priyankar Mondal
We don’t know if “Wuhan virus” is the last one to hit the human population so hard or some other menaces are waiting to wipe out the “over-smart bipedal species” from this blue planet but something we know for sure is insects are facing a massive apocalypse since the last few decades and anthropogenic deterioration of the environment is the main culprit. Insects play a crucial role in ecosystem services by pollinating crops and wild plants, recycling nutrients, and regulating the population and functioning of lower and upper trophic levels. Speaking about pollinators, more than 90% of angiosperms are pollinated by insects like bees, beetles, moths, butterflies, and flies which include all your tasty fruits and nutritious veggies. As India shares 11% and 15% of worldwide vegetable and fruit production respectively, you can imagine how much we depend on pollinators for our food and economic security. Intensive agriculture, rapid urbanization, poorly controlled emission, and concomitant air pollution have significantly contributed to the disappearance of pollinators. However, there was a lack of direct evidence about the impact of air pollutants on the biology, physiology, or immunity of the pollinators which could possibly be linked to their tragic decline.
A group of scientists led by Professor Shannon Olsson from NICE (Naturalist-inspired Chemical Ecology) lab at National Center for Biological Sciences, Bengaluru (Bengaluru) have deciphered a lion’s share of the mystery behind the “pollinator armageddon” and the study has been published this month in the Proceedings of National Academy of Sciences (PNAS) of the United States of America. The team conducted a multi-layered study in the megacity of Bengaluru (one of the nine most polluted cities in the world) to investigate the mechanistic effect of air pollution on the Giant Asian honey bee, Apis dorsata.
“During some local observations in our home city, we noticed fewer pollinators in the urban centres of the city. Was it pesticide use that caused this? Lack of water or shade? Determined to find out, Geetha (the lead author of this study and PostDoc at NCBS) travelled to different locations of this megacity to observe and collect the Giant Asian honey bee” says Prof. Shannon.
Researchers selected four different sites in the Bengaluru city and ranked them as rural, low, moderate and highly polluted sites based on the amount of respirable suspended particulate matter (RSPM: a complex mixture of particles less than 10 μM in diameter respirable by humans). A flowering perennial shrub, Tecoma stans, largely preferred by Apis dorsata was selected at each site to record observations and collect bee specimens for further investigations.
“The Giant Asian honey bee, Apis dorsata is not only a common resident of Indian cities, but also 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 urbanization on our ecosystem services” says Prof. Shannon.
The team found the number of bees foraging per day not only significantly declines with an increasing amount of RSPM but they also experience reduced survival in moderate and highly polluted sites in comparison to the rural and low polluted sites. They collected specimens from each site to investigate the deposition of pollutants on the exoskeleton and internal tissues which revealed the presence of several heavy metals like arsenic, lead, tungsten and aluminium on hindlegs, antenna, wings among external structures and crop and midgut among internal tissues with the highest amount of metallic deposition on samples collected from highly polluted sites.
SEM images of Giant Asian honey bee appendages bearing RSPM from low and high sites (Adapted from Thimmegowda et al., 2020; for details visit https://doi.org/10.1073/pnas.2009074117)
Following the thread of researches dictating the impact of RSPM on human circulatory and respiratory physiology, the scientists found similar impacts of air pollutants on the physiological and immunological systems of bees. Respiration rate of Apis dorsata collected from all four sites showed a significant difference between sites. Though the heart rate did not differ noticeably, the interbeat interval was significantly higher in bees collected from highly polluted sites which indicates the dysfunctionality of bee heart due to pollution (https://youtu.be/QST2-Y9_o90). The total hemocyte count also differed among the sites being the lowest in the highly polluted one.
The researchers went ahead one step further to correlate the pollution-induced observed differences in behaviour, survival and physiology of these bees with the expression level of genes in their antennae and heart.
“For this, we used whole antennae and heart tissue for transcriptome analysis to identify differentially expressed genes (DEGs)” says Dr. Geetha.
The qRT-PCR analysis using unexposed Apis dorsata and sample specimens of the same collected from low and highly polluted sites using nine differentially expressed genes (involved in stress metabolism, nutrition, homeostasis, immunity, and transcriptional regulation) revealed a noticeable difference in expression. Vitellogenin expression (a phosphoglycoprotein that protects the cells from oxidative damage) was found the lowest in urban bees of polluted sites probably indicating one of the causes for their reduced survival. Several other genes involved in cholesterol metabolism, detoxification, homeostasis, and innate immunity were significantly upregulated in bees collected from highly polluted sites in comparison to bees unexposed and exposed to low pollution sites. These indicate a higher amount of stress and immune response in bees exposed to severe pollution. The team found almost similar results from physiological and molecular experiments with the lab-reared and age-matched Drosophila melanogaster exposed to the same sites. The results together suggest that the RSPMs drastically alter the physiology of the pollinators leading to a steady decline in numbers with an increasing amount of pollutants and the pattern can be applied to many other insects exposed to similar situations.
“Physiological and molecular information like we gathered is essential to understand what specific impacts air pollution has on wild systems, and what air quality standards would be needed to ensure their wellbeing. That said, 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 in India and across the world to tackle this vital problem that impacts our lands, our livelihoods, and our very own food security” says Prof. Shannon.
The research has comprehensively highlighted many specific impacts of air pollutants on wild pollinators but it did not deal with the possible effects on intra and interspecific communications like host plant searching, mate finding, interaction with other trophic levels which can ultimately influence the population levels of these bees.
“As our study is field-based approach, we really cannot say only RSPM alone can influence searching behaviour of flowers in field. And Apis dorsata are wild bees we had no access to colony to study other behaviour closely” says Dr. Geetha.
This study is a breakthrough in understanding the impact of air pollution on pollinators and other insects providing important ecosystem services. This will not only serve as a valuable reference for academicians and scientists but must draw the attention of environmentalists and policymakers as well. Here’s what Mr. Arunabha Ghosh, founder and CEO of the Council on Energy, Environment and Water has to say
“So far, much of the air quality studies in India have either considered sources of pollution or impact on human health, and to an extent on economic productivity. This study covers important new ground, by examining the impact of air pollution on pollinators, which would have serious implications for agricultural output in India. From a policy perspective, we need, first, more dense air quality monitoring networks in agricultural areas (rural areas suffer from data blindspots). Secondly, impact 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. Thirdly, there is a need for clearer communication of the results of such studies to communities, so that they become more aware of the livelihood consequences of the poor air they are breathing. Finally, such findings further underscore the need to raise India's ambient air quality standards.”
For further details, please refer to "A field-based quantitative analysis of sublethal effects of air pollution on pollinators"
Priyankar Mondal is one of the Student Associate Editor of IE and working as a Researcher Scholar at Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, West Bengal, India. You can contact him @ Email: firstname.lastname@example.org