Nocturnal pollinators such as moths have developed a highly sophisticated olfactory system that allows them to locate nectar-providing flowers. They are able to navigate correctly in complete darkness, even in the presence of background scents produced by non-target flowers. The mechanism by which moths find the correct flowers against a background of other odors was recently examined in an exciting study published in Science by Jeffrey Riffell and colleagues from the Universities of Washington and Arizona, USA. They found that scent tracking was dependent on odor frequency and background odors. However, they also made the disturbing observation that anthropogenic pollutants can perturb the moth's odor-navigation system.
To find food sources or mates, moths and other insects need to discriminate and locate different scents within a complex mixture of odours. For instance, Carolina sphinx moths (Manduca sexta) roam up to 80 miles a night to find the nectar of angel's trumpets (Datura wrightii) flowers. However, finding these flowers is challenging, as they are often isolated within dense populations of the creosote bush (Larrea tridentate), which disperses a very strong scent. Odors are not evenly emitted from flowers, the plumes are broken into filaments by air turbulence. To measure the patchy odor traces produced by Datura in the field, the scientists used a proton-transfer reaction mass spectrometer that allowed monitoring of volatile organic compounds such as oxygenated aromatics and monoterpenes in ambient air. They found that the rate at which specific Datura monoterpene odor filaments are encountered in the air (odor frequency) increases with the distance from the flower. Also, the ratio of monoterpenes to oxygenated aromatics changed in the presence of background volatiles, such as those emitted by the creosote bush.
Next, the team tested the moth's ability to locate the flower in the laboratory using a wind tunnel where they could control odour plumes. The team examined how freely flying Manduca moths responded to Datura secnt embedded in different background mixtures, including odors from the creosote bush. They found that the volatile background significantly influenced the moth's ability to discriminate and track the odor, particularly when oxygenated aromatics that occur in both Datura and Larrea scents were present. To dissect the neuronal pathways activated in the moths by antennal odor perception, the scientists went on to collect electrical recordings from the antennal lobe, the area in the insect brain that receives the input from the olfactory sensory neurons on the antennae. They also developed computational models that allowed them to compare the neural activity patterns collected in response to flower scents in isolation and flower scents embedded in various backgrounds. Combining the results of their experiments and simulations, the team found that the model agreed well with their experimental observations. They also found that odor frequencies above 1–2 Hz and certain backgrounds modified the neuronal representation of the Datura mixture in the brain and thus altered perception of the flower. Unexpectedly, traffic pollutants containing volatiles such as toluene or xylene, which are only weakly similar to odors that attract moths, elicited responses from the same olfactory neurons as oxygenated aromatics that are found in Datura and Larrea scents, fooling the insect's olfactory system.
Riffell and colleagues have demonstrated that the olfactory system is less sensitive to flower-specific volatiles than previously thought. They have also shown that, surprisingly, moths respond to volatiles present in the exhaust gases of cars and trucks. They speculate that this may affect the moth's navigation system and possibly account for the widespread disruption to other pollinators such as bees, which are economically essential.
