Using various mutant fruit flies the researchers showed that it was sight and not smell which allowed the fruit fly to detect the presence of these parasitic wasps and change the location on which to lay their eggs. They also found that the female fruit fly’s choice of high-alcohol-content fruit continued for up to four days after seeing the parasitic wasps, unless they used a mutant fruit fly that lacked long-term memory. Surprisingly, male parasitic wasps were without effect on the fruit fly’s choice of food source for their offspring.
However, the fruit flies and their developing larvae are not totally immune to alcohol themselves as their success rate in raising offspring is reduced when their larvae feed on alcohol-containing fruit. In the absence of parasitic wasps, and given a free choice of food sources containing no alcohol, 3% alcohol, or 6% alcohol, the flies generally chose food with 3% alcohol. Thus the fruit fly’s behavior is equivalent to making a calculated choice in the presence of the parasitic wasp: Either risk having no living offspring by laying one’s eggs on an alcohol-free food source or accept a reduction in the number of one’s descendants knowing that those that survive on alcohol-containing fruit stand a better chance of resisting infection by the wasp.
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