Host‐plant shifts and adult survival in the cactus breeding Drosophila mojavensis

Abstract. 1. In order to gain insight into how Drosophila mojavensis Patterson and Crow (which lives on the rotting tissue of cactus) successfully shifted host plants, from an Opuntia species to columnar cactus species, over evolutionary time, specific components of columnar cactus and Optunia tussue were examined for their effects on adult longevity. 2. The availability of free sugars affects the survival of adult D.mojavensis . The addition of a sugar to columnar cactus tissue significantly increased the longevity of adults that fed on those tissues. This increase in adult longevity was equal to the longevity observed on unsupplemented Opuntia tissue, which has free sugars present. 3. Live microorganisms added to Opuntia tissue had a slight detrimental effect on adult longevity (longevity was decreased by approximately 1 week). 4. Low concentrations of atmospheric ethanol (independent of diet) increased the longevity of adult D.mojavensis to levels comparable in magnitude to that provided by the addition of free sugars to the diet. 5. This information and the apparent lack of a year‐round supply of free sugars for D.mojavensis in nature suggests that the ethanol (produced by fermentation) rather than free sugars per se is the most important component of rotting cactus tissue for the survival of this species and may explain its successful host‐plant shift.