Preference for ethanol in feeding and oviposition in temperate and tropical populations of Drosophila melanogaster.

The natural habitat of Drosophila melanogaster Meigen (Diptera: Drosophilidae) is fermenting fruits, which can be rich in ethanol. For unknown reasons, temperate populations of this cosmopolitan species have higher ethanol resistance than tropical populations. To determine whether this difference is accompanied by a parallel difference in preference for ethanol, we compared two European and two tropical African populations in feeding and oviposition preference for ethanol-supplemented medium. Although females of all populations laid significantly more eggs on medium with ethanol than on control medium, preference of European females for ethanol increased as ethanol concentration increased from 2 to 6%, whereas that of African females decreased. In feeding tests, African females preferred control medium over medium with 4% ethanol, whereas European females showed no preference. Males of all populations strongly preferred control medium. The combination of preference for ethanol in oviposition, and avoidance or neutrality in feeding, gives evidence that adults choose breeding sites with ethanol for the benefit of larvae, rather than for their own benefit. The stronger oviposition preference for ethanol of temperate than tropical females suggests that this benefit may be more important in temperate populations. Two possible benefits of ethanol for which there is some experimental evidence are cryoprotection and protection against natural enemies.