Flowers Are Starting to Smell Different
7 min read
This article was originally published by Knowable Magazine.
In the summers of 2018 and 2019, the ecologist James Ryalls and his colleagues would go out to a field near Reading, in southern England, to stare at the insects buzzing around black-mustard plants. Each time a bee, a hoverfly, a moth, a butterfly, or another insect tried to get at the pollen or nectar in the small yellow flowers, the scientists would make a note.
It was part of an unusual experiment. Some patches of mustard plants were surrounded by pipes that released ozone and nitrogen oxides—polluting gases produced by many power plants and gas-powered cars. Other plots had pipes that were releasing normal air.
The results startled the scientists. The plants smothered by pollutants were visited by up to 70 percent fewer insects overall, and their flowers received up to 90 percent fewer visits compared with those in unpolluted plots. The concentrations of pollutants were well below what U.S. regulators consider safe levels. “We didn’t expect it to be quite as dramatic as that,” says study co-author Robbie Girling, an entomologist at the University of Southern Queensland, in Australia, and a visiting professor at the University of Reading.
A growing body of research suggests that pollution can disrupt insect attraction to plants—at a time when many insect populations are already suffering deep declines due to agricultural chemicals, habitat loss, and climate change. About 75 percent of flowering plants and about 35 percent of food crops rely on animals to move pollen around so that plants can fertilize one another and form seeds. Even the black-mustard plants used in the experiment, which can self-fertilize, exhibited a drop of 14 percent to 31 percent in successful pollination, as measured by three different pollination metrics.
Scientists are still working out how strong and widespread these effects of pollution are, and how they operate. They’re learning that pollution may have a surprising diversity of effects, including changing the scents that draw insects to flowers, and warping the creatures’ ability to smell, learn, and remember.
This research is still young, says Jeff Riffell, a neuroscientist at the University of Washington: “We’re only touching the tip of the iceberg, if you will, in terms of how these effects are influencing these pollinators.”
Insects often rely on smell to get around. As they buzz about in their neighborhoods, they learn to associate the flowers that are good sources of nectar and pollen with their scent. Although some species, such as honeybees, also use directions from their hive mates and visual landmarks to navigate, even they depend on their sense of smell for sniffing out favorite flowers from afar. Nocturnal pollinators such as moths are particularly talented smellers. “They can smell these patches of flowers from a kilometer away,” Riffell says.
One of the effects of pollution—and what Girling suspects was largely responsible for the pollination declines at the England site—is how it interferes with these flowery aromas. Each plant’s fragrance is a unique blend of dozens of compounds that are chemically reactive and degrade in the air. Gases such as ozone and nitrogen oxide will quickly react with these molecules and cause odors to vanish even faster than usual. “For very reactive scents, the plume can only travel a third of the distance than it should actually travel when there is no pollution,” says the atmospheric scientist Jose D. Fuentes of Penn State University, who has simulated the influence of ozone on floral-scent compounds.
And if some compounds degrade faster than others, the bouquet of scents that insects associate with particular plants transforms, potentially rendering the plants unrecognizable. Girling and his colleagues observed this in experiments in a wind tunnel into which they delivered ozone. The tunnel was also outfitted with a device that steadily released a synthetic blend of floral odors. (An actual flower would have wilted, says co-author Ben Langford, an atmospheric chemist at the UK Centre for Ecology & Hydrology.) Using chemical detectors, the team watched the flowery-scent plume shorten and narrow as ozone ate away at the edges, with some compounds dropping off entirely as others persisted.
The scientists had trained honeybees to detect the original flowery scent by exposing them to the odor, then giving them sugar water—until they automatically stuck out their tonguelike proboscis to taste it upon smelling the scent. But when bees were tested with ozonated odor representing the edges of the scent plume, either six or 12 meters away from the source, only 32 percent and 10 percent, respectively, stuck out their proboscis. The bee is “sniffing a completely different odor at that point,” Langford says.
Researchers also have observed that striped cucumber beetles and buff-tailed bumblebees struggle to recognize their host plants above certain levels of ozone. Some of the most dramatic observations are at night, when extremely reactive pollutants called nitrate radicals accumulate. Riffell and his colleagues recently found that about 50 percent fewer tobacco hornworm moths were attracted to the pale evening primrose when the plant’s aroma was altered by these pollutants, and white-lined sphinx moths didn’t recognize the modified scent at all. This reduced the numbers of seeds and fruits by 28 percent, the team found in outdoor pollination experiments. “It’s having a really big effect on the plant’s ability to produce seeds,” Riffell says.
Can insects learn to recognize these transformed scents? To recognize new scents as rewarding, they need to smell them while they’re feeding on sweet nectar—but the problem is, a flower’s scent transforms only at some distance from the flower, Girling says. Perhaps some insects can learn to follow polluted scents as they approach a flower, but so far, this has been shown only for the tobacco hawkmoth.
Pollution can also make it harder for insects to learn. In a 2019 study, Girling and his colleagues first trained honeybees to recognize an odor using the sugar water–proboscis method. Then they exposed the bees to diesel exhaust. Later on, they tested the bees to see how many of them had retained their training and still reacted to the odor by sticking out their proboscis. Remarkably, 44 percent fewer honeybees were able to recall the scent 72 hours after diesel exposure, compared with unexposed bees.
“It looks like they’re potentially not able to form those memories as well and hold on to those memories as well,” Girling says. The reason is unclear; perhaps the gas somehow causes physiological stress in the bees’ brain that leads to neurological impediments. This could hypothetically mean that in polluted air, bees may forget the original scents of flowers—or forget the polluted aromas, if they do manage to learn those.
Dirty air could even affect insects’ sense of smell. A few years ago, the chemical ecologist Magali Proffit of the French Center for Functional and Evolutionary Ecology and her team hooked up electrodes to the antennae of buff-tailed honeybees and fig wasps. They found that exposing the insects to ozone often made these odor-sensing organs much less responsive to smells. Bees and wasps exposed to moderate levels of ozone moved around aimlessly rather than toward the odor of their host plants.
At high ozone levels, fig wasps even avoided the odor. “Something is happening in their olfactory system,” Proffit says.
Air pollutants also have an influence when plants absorb them, says the ecologist Laura Duque at the University of Würzburg, in Germany: They can change plant metabolism and thus alter the blend of odor compounds that flowers emit. Depending on the species, ozone can increase or decrease flower numbers and possibly the quality and quantity of nectar or pollen. Ozone can also influence the timing of flowering, Duque says: “It’s possible that there is no plant reproduction at all if there’s a complete mismatch between flowering and insect activity.”
It’s hard to assess the magnitude of these effects on plant pollination, says the urban-insect ecologist Elsa Youngsteadt of North Carolina State University, who co-authored a 2023 paper on pollination in cities in the Annual Review of Ecology, Evolution, and Systematics. Some studies—including Girling’s study in the U.K. and Riffell’s on moths—have begun to quantify the effects.
In general, one would expect impacts to be greatest in cities, which have borne the brunt of pollution since industrial times—although this might also have led to adaptations, Youngsteadt says. Studies on nonurban plants and insects “might be giving a different picture than if we studied the plants and insect populations that have already been exposed to urban pollutants for hundreds of years,” she says.
Also unclear is the effect of pollution on insects. When insects fail to detect flowers, they miss a chance to get nectar or pollen for themselves, their young, or their hives. They must travel farther in search of flowery scents, sapping their energy, Fuentes says.
And the transformation of flower scents may be just one factor among many. Pollution particles can interfere with the ability of houseflies to smell food, for example, and ozone changes the pheromones of male flies so that they smell more like females, causing males to chase other males.
Pollution can also more generally affect insect reproduction and survival; putting all of these impacts together, a recent study estimates that air pollution causes a more than 30 percent reduction in performance of pollinators and pest regulators. Scientists need more research on different species—on their sensitivity to pollution and their interactions with plants—especially in understudied regions such as the global South.
From agricultural chemicals to climate change, “we’ve got all of these other factors that are chipping away and reducing the fitness, making it harder for these insects to go through their normal life cycles and normal processes,” Girling says. “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?”
