Resistance to Leafcutter Ants (Hymenoptera: Formicidae) and Inhibition of Their Fungal Symbiont by Tropical Forage Grasses

Abstract

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, colonization by A. landolti of established swards, fresh weight of fungus gardens of excavated A. landolti colonies, and ability of the leafcutters’ symbiotic fungus to grow on artificial medium containing aqueous extracts of susceptible and resistant grass cultivars. Susceptible forages ( Andropogon gayanus ‘Carimagua 1’ and Brachiaria dictyoneura ‘Llanero’) had more seedlings cut during establishment and greater colonization by A. landolti in established swards compared with resistant cultivars ( B. brizantha ‘Marandu’, B. decumbens ‘Basilisk’, and B. humidicola ‘Pasto Humidicola’). Fresh weights of the fungus gardens of excavated colonies that had been restricted to cutting resistant cultivars (Pasto Humidicola, Marandu, or Basilisk) were reduced compared with colonies restricted to Carimagua 1, Llanero, or native savanna (control). Growth of the symbiotic fungus isolated from fungus gardens of A. landolti and Atta laevigata (F. Smith) was reduced when grown on agar with aqueous extracts of resistant cultivars compared with growth of the fungus on agar alone. When cultured on agar with aqueous extract of Carimagua 1, fungus growth was equivalent to the control. Growth of the fungus with homogenate of Llanero was intermediate to control and resistant cultivars. Agreement between these measures of resistance to leafcutting ants and their symbiont indicates that resistance in Brachiaria is conferred by plant factors inhibitory to the symbiotic fungus.