Effect of leaf toughness on the susceptibility of the leaf-cutting ant Atta sexdens to attacks of a phorid parasitoid

Because the size of Atra spp. along foraging trails is partly determined by the characteristics of the plants harvested, and considering that parasiti c phorid flies are attracted mostly to large individuals, we hypothesized t hat plant toughness affects the susceptibility of Atta spp. to these parasi toids. To test this hypothesis, we evaluated parasitism rates of the phorid Neodohrniphora sp. and its effect on Atta sexdens (L.) foragers in a labor atory colony. We manipulated forager size by alternating tough (Anthocephal us chinensis, Rubiaceae) and tender (Rosa chinensis, Rosaceae) plants given to the colony. Ants foraging on tough leaves were larger than ants foragin g on soft leaves, and there was a significant reduction in forager size for both plants when the colony was exposed to Neodohrniphora sp. However, the re were no relative differences on forager size between the two plants afte r the introduction of the parasitoid. The lack of response of Neodohrniphor a sp. to the increase in ant size when the colony was given tough leaves ma y be attributed to the unusually large number of suitable hosts in a labora tory colony. However, large foragers are much less abundant in the field, i n which case shifts in the size of the workforce triggered by different sub strates could affect the incidence of parasitism.