Contaminated air isn’t just harmful to humans; it poses a significant threat to bees and other pollinators. A new study reveals how air pollution interferes with bees’ ability to find flowers, which could have widespread implications for plant pollination and food production.
In the summers of 2018 and 2019, ecologist James Ryalls and his team embarked on an unusual experiment in a field near Reading, southern England. Their focus was on black mustard plants, whose small yellow flowers are a favorite of bees, hoverflies, and other insects. However, this wasn’t just a typical study of insect behavior. Some of the mustard plants were surrounded by pipes that emitted ozone and nitrogen oxides, common pollutants from power plants and cars. Other patches were exposed to clean air.
The results were alarming. In areas with polluted air, insect visits to the plants dropped by up to 70%. Even more concerning, these flowers received 90% fewer visits compared to those in unpolluted plots. The concentrations of pollutants were below levels considered safe by U.S. regulators, yet the impact on insect activity was dramatic. “We didn’t expect it to be quite as dramatic as that,” said Robbie Girling, a study co-author and entomologist at the University of Southern Queensland in Australia.
This research adds to a growing body of evidence suggesting that air pollution disrupts insects’ ability to locate flowers. This comes at a time when many insect populations are already in steep decline due to agricultural chemicals, habitat loss, and climate change. Around 75% of wild flowering plants and 35% of food crops rely on animals to move pollen, making this issue critical for the ecosystem.
Even the black mustard plants, which can self-fertilize, showed a 14% to 31% drop in successful pollination when exposed to polluted air. Scientists are still trying to understand how widespread and severe these effects are. Pollution may alter the scents that draw insects to flowers or impair their ability to smell, learn, and remember.
Insects rely heavily on their sense of smell to find food. They learn to associate certain flower scents with nectar and pollen. Honeybees, for instance, use visual landmarks and directions from their hive mates, but their sense of smell is crucial for identifying flowers from a distance. Nocturnal pollinators like moths are especially reliant on smell. “They can smell these patches of flowers from a kilometer away,” noted Jeff Riffell, a neuroscientist at the University of Washington.
One major effect of pollution is how it changes the aromas of flowers. Each fragrance is a unique blend of many compounds that are chemically reactive and degrade in the air. Pollutants like ozone or nitrogen oxides speed up this degradation, causing the scents to dissipate more quickly. “For very reactive scents, the plume can only travel a third of the distance than it should actually travel when there is no pollution,” said Jose D Fuentes, an atmospheric scientist at Penn State University.
When some scent compounds degrade faster than others, the overall bouquet changes, potentially confusing insects. Girling and his colleagues observed this effect in a wind tunnel experiment where they exposed synthetic floral scents to ozone. The scent plume shortened and narrowed as ozone reacted with the compounds, and some scents disappeared entirely.
The team trained honeybees to recognize a floral scent by pairing it with sugar water. The bees learned to associate the scent with food and would stick out their proboscises—a tongue-like organ—when they smelled it. However, when the bees were exposed to the scent after it had been altered by ozone, far fewer bees responded. “The bee is sniffing a completely different odor at that point,” explained Ben Langford, an atmospheric chemist at the UK Centre for Ecology & Hydrology.
Other research has shown that pollution affects various insects, including striped cucumber beetles and buff-tailed bumblebees. These insects struggle to recognize their host plants when ozone levels are high. The most dramatic effects have been observed at night when reactive pollutants called nitrate radicals accumulate. In one study, tobacco hornworm moths were 50% less likely to find evening primrose flowers whose scent had been altered by pollution. White-lined sphinx moths didn’t recognize the scent at all, leading to a 28% reduction in seeds and fruits produced by the plants.
Can insects adapt to these transformed scents? It’s possible, but the process is complicated. Insects need to smell the new scents while feeding on nectar to learn that the altered odors are still rewarding. However, a flower’s scent changes as it travels away from the plant, making it harder for insects to learn the new smells. Only one species, the tobacco hawkmoth, has been shown to adapt to polluted scents.
Pollution can also impair insects’ ability to learn. In a 2019 study, Girling and his colleagues found that honeybees exposed to diesel exhaust had trouble recalling a learned scent. Bees trained to recognize a particular odor lost their ability to remember it after exposure to exhaust fumes. “It looks like they’re potentially not able to form those memories as well and hold onto those memories as well,” Girling said. The reason for this is unclear, but it may involve physiological stress in the bees’ brains.
Pollution can also affect insects’ sense of smell directly. A study by chemical ecologist Magali Proffit showed that ozone exposure made bees’ and wasps’ antennae less responsive to smells. Insects exposed to moderate levels of ozone moved aimlessly rather than toward their host plants’ odors. At higher ozone levels, fig wasps even avoided the odors they usually sought out. “Something is happening in their olfactory system,” Proffit noted.
Air pollutants can also impact plants by altering their metabolism, which changes the blend of odor compounds they emit. Ozone exposure can affect the number of flowers a plant produces, the quality and quantity of nectar or pollen, and even the timing of flowering. These changes can disrupt the synchronization between plants and pollinators, potentially leading to a failure in plant reproduction.
The magnitude of these effects is still uncertain, but urban areas, which have experienced high pollution levels for centuries, might show different patterns. Adaptations may have occurred, but further research is needed. Insects failing to detect flowers miss out on nectar and pollen, forcing them to travel farther and expend more energy. Pollution also affects other aspects of insect life, such as reproduction and survival.
Overall, pollution’s impact on pollinators could be severe. A recent study estimated that air pollution causes a more than 30% reduction in pollinator performance. This research is crucial as scientists seek to understand how different species are affected and how these impacts interact with other environmental stresses.
From agricultural chemicals to climate change, many factors are making life harder for insects. “If you then make it just a little bit harder to find a flower, is that the tipping point that pushes that particular insect or hive over the edge?” Girling asks.
