The ability to respond quickly to odors is important for virtually all animals. But for pollinating insects that travel at high speeds, being able to react quickly to odor molecules is crucial. A moth’s sense of smell, for instance, is comparable to that of a dog — which means it vastly outperforms humans. But as our own innovation continues to introduce novel scents into the atmosphere, some scientists have started to wonder how our machines, and their pollutants, are affecting these insects’ abilities to located nectar-rich flowers. Now, a new study, published today in Science, demonstrates that fumes from cars not only compete with flowers, but also alter the way moths decode a flower’s scent. And this effect may be powerful enough to prevent these insects from finding — and therefore pollinating — agriculturally important flowers.
The act of detecting and following an odor is extremely complex. Something as banal as turbulent air flow can quickly disperse odor molecules, which dilutes and mixes odors. So, when moths are flying quickly, they need to be able to pick up and recognize pertinent odor patches quickly.
"When the moth encounters an odor patch, it will move upwind closer to the odor source," says Jeff Riffell, a neuroecologist at the University of Washington and a co-author of the study. "But this behavior is dependent upon the moth encountering the patches of odor at a sufficient enough rate for the moth to continue moving upwind closer and closer to the flower." This rate specificity is useful, because it allows pollinators to detect and discriminate relevant, airborne signals at very small concentrations. Unfortunately, this characteristic also means that competing odors interfere with moths' ability to detect flowers.
In the study, Riffell and his team put Manduca sexta moths in a large wind tunnel that also contained a computer-controlled odor-stimulus system. "That allowed us to control the odor environment that the moths experience," he says. They tested how the moths responded to the smell of a flower called Datura wrightii at frequencies ranging from 1 to 20 Hz, and found that the moths responded most strongly to odor pulses at 1 Hz. Anything above 2 Hz was too fast, and caused the moths to act as though no odors were present. Then, the scientists introduced background odors into the tunnel while also emitting a flower-plume at 1 Hz.
they used two common vehicle exhaust pollutants
These background odors included two common vehicle-exhaust pollutants, xylene and toluene, at concentrations ranging from extremely high and unrealistic to those that would occur in suburban and urban neighborhoods. They also tested the moths to see how they fared against the odors emitted by non-flowering vegetation. As expected, these odor backgrounds — the pollutants, as well as certain plant volatiles — prevented the moths from detecting the flower’s scent.
But the researchers didn’t just test this behaviorally. They also recorded the insects’ brain activity by inserting an electrode into the area of the brain that processes odors. This allowed them to demonstrate that the background odors were actually altering the neural perception of the flower by decreasing neurons’ ability to encode pulses of odor. This means that flowers aren't just competing with other plants, but also with the gas-guzzlers that we use around them.
"Environmental volatiles, both natural and human-made, activate the same receptors that are activated by flower scents," Riffell explains. When these receptors encounter strong odors that compete with flowers, they can become adapted — meaning they get used to the plethora of odors surrounding them, and stop being able to pick them apart. In other cases, however, these odors also affect the neurons that are responsible for processing flower scents. "Both of these neuronal processes," he says, "serve to ‘mask’ the flower scent."
background odors altered the moths' neural perception of the flower
"What this study suggests is potentially alarming for pollination and agriculture in general, when we consider that their study species is perhaps one of the most well-known model organisms for insects," said Alex Smith, a biologist at the University of Guelph in Ontario, in an email to The Verge. "What if the vast majority of species involved in pollination — about which we often know much less — are also affected in the same manner?" he asked. The study also calls attention to the human-induced fracturing of a critical ecological service, Smith said. "It will be further alarming if that fracture extends into the other species of pollinators."
"alarming for pollination and agriculture."
Sylvia Anton, a neurobiologist at the University of Angers in France, agrees. "We should be concerned about volatiles emitted by anthropogenic actions," she wrote in an email, "which might modify insect behavior in general, including pollinators, but also that of pest insects."
Riffell and his team would now like to replicate the experiments in an outside environment, with other species of pollinators. But for now, "the takeaway message is that environmental chemicals like pollutants, and even chemicals from neighboring vegetation, can affect the pollinator's ability to find flowers," Riffell says. And "because pollination is important for food security," these results have implications for native bumblebees, butterflies, and moths — not to mention "agriculturally important pollinators, like honeybees."