COLONIZATION OF 2 TROPICAL FORAGE GRASSES BY ACROMYRMEX-LANDOLTI (HYMENOPTERA, FORMICIDAE) IN EASTERN COLOMBIA

Established swards of the tropical forage grass Andropogon gayanus var . bisquamulatus (Hochst.) Hack were heavily colonized by the leaf-cutt er Acromyrmex landolti Forel over a period of 33 months in the eastern plains of Colombia. Density of Ac. landolti colonies in swards of A. gayanus increased from 23 +/- 9 at planting in 1988 to 263 +/- 57 colo nies per 1800 m2 in 1991. Brachiaria humidicola (Rendle) Schweick was highly resistant to colonization by Ac. landolti. Colony density in sw ards of B. humidicola declined from 23 +/- 9 at planting in 1988 to 16 +/- 4 colonies per 1800 m2 in 1991. Mean density in adjacent savanna control plots varied over the three years from 114 +/- 23 to 225 +/- 2 4 colonies per 1800 m2. Maps of colony entrances within swards were ma de yearly during 1989, 1990, and 1991. Comparison of maps demonstrated that colonies did not persist more than a single year at a given site implying colony migration. Excavation of over 100 colonies of Ac. lan dolti revealed a single entrance per colony in all cases.