Sulcatone (aka 6-methyl-5-hepten-2-one) is a volatile odorant, meaning that (1) it can fairly easily end up floating about in the air around us, and (2) it has a smell. It's emitted by various plants and animals, and serves a variety of functions. For some reason, relative to other animals, humans produce a lot of it. Consequently, mosquitoes that prefer to consume human blood can use our sulcatone stink to hunt us down. Other blood-feeding insects including bed bugs and highland midges likely use the same approach.
Sulcatone is apparently found in citronella oil, which makes me wonder how the heck those citronella candles marketed to ward off mosquitoes work. Maybe they overwhelm the mosquitoes' sense of smell?
Ants of the genus Lasius will emit sulcatone if they are attacked by a predator in order to alert their neighbours and cause them to scatter. Substances used for this purpose are called alarm pheromones. In turn, it is thought that certain beetles residing in ant nests (ants form all sorts of interesting symbiotic relationships with other organisms) will emit sulcatone to avoid being attacked by their hosts.
In other ant species, sulcatone will call forth an army to attack a predator. Wolf spiders and other ant munching arthropods will thus avoid areas where the chemical has been released. Gelis agilis, a small wingless parasitoid wasp (it paralyzes a host and injects it with a single egg, which hatches into a larva that feeds on the host) also releases sulcatone when alarmed. It is thought that it uses the chemical to mimic the ant response, scaring away a predator that might also happen to eat the wasp.
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