Biological Control of Leaf-Cutting Ants Using Forage Grasses: Nest Characteristics and Fungus Growth

One of the major pasture pests in the neotropics is the leaf-cutting ant Acromyrmex landolti , which cuts monocots to nourish a symbiotic fungus as a food source. The objective of the present study was to characterize ant nests and to test the effect of six forage grasses on grass consumption and fungus properties of A. landolti nests. Grasses tested in a 7-week field experiment during March and April 1997 in the Paraguayan Chaco were Cynodon nlemfuensis , Panicum maximum , Cenchrus ciliaris , Brachiaria brizantha , Digitaria milanjiana and Urochloa mosambicensis . In a bioassay, grass species were evaluated with regard to fungus growth in the laboratory. Results showed that ant nests at that time of year were deeper and had more chambers than often reported in the literature. The two least and most preferred grasses were B. brizantha and C. ciliaris , and C. nlemfuensis and D. milanjiana , respectively. Consumption of the first two grasses was strongly reduced during the second half of the experiment, approaching zero in the last week. The highest fungus weights were found in nests fed with C. nlemfuensis and D. milanjiana . Fungus colony weight and colour proved to be adequate parameters with which to rate the vigour of the fungus; weight was lowest in nests fed with B. brizantha and C. ciliaris . In the bioassays, B. brizantha and D. milanjiana plant saps reduced the fungal growth most and least, respectively. In conclusion, D. milanjiana and C. nlemfuensis can be regarded as highly susceptible to A. landolti , and B. brizantha as resistant; the other grasses cannot be assigned to either group. The bioassay proved to be a quick way of testing the ability of a grass species to inhibit fungal growth.