Seasonal shifts in macronutrient preferences in supercolonies of the invasive Yellow Crazy Ant Anoplolepis gracilipes (Smith, 1857) (Hymenoptera: Formicidae) on Christmas Island, Indian Ocean.
The timing and duration of sexual brood production in ants can affect their rate of spread and colony growth. Because protein is key to larval growth, queen survival and fecundity, macronutrient collection by foraging workers is expected to favour protein prior to and throughout gyne production. However, food preference driven by the production of gynes may be overridden by a preference associated with worker production, especially if investment in workers vastly outweighs that of sexual brood and workers are produced on different schedules. Food preferences alone may not indicate the availability of that food type in the environment. On Christmas Island, Indian Ocean, sexual brood of the invasive Yellow Crazy Ant, Anoplolepis gracilipes (Smith, 1857), was produced annually during a single period associated with the onset of the wet season. However, workers were produced continuously throughout the year and colony investment to worker production measured by the standing biomass of eggs, larvae and pupae, typically exceeded 98%. High, aseasonal investment in worker production, together with aseasonal worker activity, would suggest that there should be no seasonal preference shown by workers at food stations containing both protein and carbohydrate. However, workers showed a preference for one food type over another on 46 of 61 occasions at one site and on 31 of 41 occasions at another. When a preference was shown, it was predominantly for protein-rich food during the dry season and almost always for carbohydrate at the onset and during the wet season. We suggest that these preferences reflect seasonal shortages in key resources when invertebrates (protein-rich) are scarce during the dry season and honeydew from scale insects (carbohydrate) is depleted by rain during the wet season. On Christmas Island, timing and duration of the dry season preference for protein has been exploited by the control program for A. gracilipes supercolonies, which deploys a toxin in a proteinaceous bait matrix during dry periods.