It wasn’t long ago that bed bugs were just an uncomfortable memory from a bygone era. However, the blood-sucking critters have recently staged a remarkable comeback, and with it new industries have emerged with the aim of tracking and eradicating the irritating pests. Vincent Harraca from Lund University, Sweden, explains that traps could be used to monitor bed bug infestations, ‘but so far the traps are not efficient because we do not know how to attract bedbugs into them’, he says. Human odour is complex, containing as many as 400 different components, and as no one had identified the key components that attract the insects to home in on juicy victims, Harraca, Camilla Ryne, Göran Birgersson and Rickard Ignell decided to find out which compounds in human odour are irresistible to bed bugs (p. 623).
But Harraca’s first challenge was to collect samples of human odours, ‘And to do that you need bags that do not release any smell’, explains Harraca. ‘The best ones are oven bags for roasting chicken or turkey, but they do not make human-sized oven bags’, he chuckles. Resourcefully heat-sealing the transparent bags together, Harraca was then faced with another dilemma: convincing his volunteers to climb inside them naked for 2.5 h while he collected their odours on filters. Having successfully collected samples from eight volunteers, Harraca teamed up with Birgersson to isolate the individual components constituting each odour. Then he tested which odours the insect could sense by recording the electrical activity in the insect’s odour receptors with a tungsten electrode.
Having isolated 100 different compounds from the human odour samples, Harraca admits that he was surprised to see that the insects only responded to five. And when he identified the compounds – four aldehydes and a ketone (sulcatone) – he was even more surprised, because these compounds are emitted by the skins of all vertebrates. He admits that the team had thought that the insects would react to scents such as geranyl acetone and lactic acid that are produced by humans in large quantities. However, instead of specifically sniffing out people, bed bugs are happy to sniff out any warm blooded creature to snack on.
The team then decided to find out how natural human odour affects the blood sucking insects’ behaviour. Constructing a double-decker arena from Petri dishes, Harraca placed a semi-circle of filter paper soaked in body odour in one side of the lower chamber of the arena and then allowed a bed bug to explore the odour in the upper chamber for 7 min while he filmed its movements.
Realising that the bed bugs were repulsed by the most concentrated odour samples, Harraca repeated the process with more dilute body odours and found that the insects were not strongly attracted to human scents alone. Explaining that they are also attracted to warmth and carbon dioxide, he suggests that bed bugs are guided by a combination of senses to locate slumbering victims. Harraca also suspects that the relative proportions of ketones and aldehydes in an individual’s scent might alter their attractiveness for bed bugs as the pests were more motivated to explore one person’s odour, which differed subtly from the others.