NATIVE FOOD SPECTRUM, SIZE-MATCHING AND FORAGING EFFICIENCY OF THE MEDITERRANEAN HARVESTER ANT MESSOR WASMANNI (HYMENOPTERA: FORMICIDAE)

This study examined the food spectrum, size-matching and harvesting efficiency of the Mediterranean harvester ant Messor wasmanni Krausse, 1910, a major seed predator on annual grasslands, on the Croatian island of Cres. The ant workers collect available food items not only from the soil surface (post-dispersal seed predation), but also directly from the mother plant (pre-dispersal seed predation). The distributional pattern of the main food resources therefore changed constantly from May to October 2009. This investigation shows that food items were collected either from several distinct small regions, or from widely distributed zones of the foraging ground. Accordingly, the foraging strategies are a mixture of individual foraging and column foraging, adjusted to the ants’ needs. Moreover, under natural conditions, M. wasmanni workers do not show size-matching at foraging trails. Little, if any, size variation of harvested seeds and fruits can be attributed to the foragers’ body size. Small workers did not appear to be constrained by load size, which indicates that size-matching may not be an adequate measure of colony foraging success for M. wasmanni. I further observed that the harvesting efficiency of media- and major-sized workers decreased considerably from May to October 2009, whereas the harvesting efficiency of minor-sized workers constantly increased over the year. This finding suggests that the worker size and the resulting difference in individual metabolism play an important role for the harvesting efficiency of M. wasmanni.