Above: Drawing of a fruit fly (Wikimedia Commons/B. Nuhanen)
Fruit flies love to eat rotting fruit. And one of the natural byproducts of rotting fruit is ethanol, or ethyl alcohol: the same intoxicating chemical found in wine, beer, and liquor.
Scientists have recently discovered that fruit fly (Drosophila melanogaster) larvae protect themselves from parasitic wasps by increasing their alcohol consumption. During the larval stage, fruit flies are vulnerable to parasites, especially wasps. Female wasps lay their eggs inside fruit fly larvae, also called maggots. The parasitic wasp larva grows inside the fruit fly maggot, slowly eating it from the inside out.
Did you know? Fruit flies can safely consume food with an alcohol content as high as 6-11%, which is stronger than many alcoholic beverages!A parasite that leads to the death of its host is called a parasitoid and a parasite that feeds on a living host from the inside is called an endoparasitoid. Because the fruit fly maggot is still alive, the wasp larva can eat for a long time. However, if the wasp larva survives, it will eventually kill its host.
But it seems that fruit flies try to kill the parasite with alcohol before the parasite kills them. In large enough quantities, alcohol is toxic to most animals, including humans and fruit flies. There’s only so much alcohol a person or a fly can metabolize and excrete.
However, fruit flies have evolved to be able to tolerate particularly high levels of alcohol. A good thing, since alcohol is abundant in their food. By contrast, wasps just don’t have a metabolism capable of coping with large quantities of alcohol. What would be safe for a fruit fly is toxic to wasps.
A team of scientists at Emory University in Atlanta has discovered that fruit flies use their high tolerance to alcohol as a way of defending against killer parasites. Maggots infected with wasp parasites show a preference for alcohol-rich foods. This helps kill the parasites and increase the survival rate of the maggots.
Did you know? When they see female parasitic wasps, female fruit flies decrease the production of a compound called neuropeptide F in their brains. This causes them to change their egg-laying behaviour.Of course, there is a limit to how much alcohol even a fruit fly can consume. Alcohol is a toxin that needs to be metabolized and excreted, and unnecessary consumption could still harm fruit fly larvae. So female fruit flies are careful to lay their eggs in alcohol-rich food sources only when necessary.
For example, they’ll do so when female parasitic wasps are present, but not in the presence of male parasitic wasps or non-parasitic wasps. This means that adult fruit flies can identify the both the species and the sex of the wasps. Armed with this information, they are able to choose the strategy that will best protect their offspring.
So fruit flies have not only evolved with the ability to safely consume large quantities of alcohol, something that occurs naturally in their food. These insects also alter their egg-laying behaviour depending on what other insects are present. If they notice female parasitic wasps, they seek out alcohol rich foods that their offspring should be able to feed on but that will hopefully kill any wasp parasites. Otherwise, they avoid foods too high in alcohol to avoid harming their own offspring.
Fruit flies force their young to drink alcohol for their own good (ScienceDaliy/Emory Health Sciences) To Evict Parasite, Canny Fruit Flies Pick Their Poison (New York Times/Carl Zimmer)
Kacsoh, BZ, Lynch ZR, Mortimer NT, Schlenke TA. 2013. Fruit Flies Medicate Offspring After Seeing Parasites. Science. 339(6122):947-950. Milan NF, Kacsoh BZ, Schlenke TA. 2012. Alcohol consumption as self-medication against blood-borne parasites in the fruit fly. Current Biology 22:488-493.