RESISTANCE TO LEAFCUTTER ANTS (HYMENOPTERA, FORMICIDAE) AND INHIBITION OF THEIR FUNGAL SYMBIONT BY TROPICAL FORAGE GRASSES

Four measures of resistance to the leafcutting ant Acromyrmex landolti (Forel) in 5 forage grasses were evaluated in a neotropical savanna: damage caused by cutting of seedlings during pasture establishment, co lonization by A. landolti of established swards, fresh weight of fungu s gardens of excavated A. landolti colonies, and ability of the leafcu tters' symbiotic fungus to grow on artificial medium containing aqueou s extracts of susceptible and resistant grass cultivars. Susceptible f orages (Andropogon gayanus 'Carimagua 1' and Brachiaria dictyoneura 'L lanero') had more seedlings cut during establishment and greater colon ization by A. landolti in established swards compared with resistant c ultivars (B. brizantha 'Marandu, B. decumbens 'Basilisk', and B. humid icola 'Pasto Humidicola'). Fresh weights of the fungus gardens of exca vated colonies that had been restricted to cutting resistant cultivars (Paste Humidicola, Marandu or Basilisk) were reduced compared with co lonies restricted to Carimagua 1, Llanero, or native savanna (control) . Growth of the symbiotic fungus isolated from fungus gardens of A. la ndolti and Atta laevigata (F. Smith) was reduced when grown on agar wi th aqueous extracts of resistant cultivars compared with growth of the fungus on agar alone. When cultured on agar with aqueous extract of C arimagua 1, fungus growth was equivalent to the control. Growth of the fungus with homogenate of Llanero was intermediate to control and res istant cultivars. Agreement between these measures of resistance to le afcutting ants and their symbiont indicates that resistance in Brachia ria is conferred by plant factors inhibitory to the symbiotic fungus.