African mahogany, Khaya senegalensis, is a high-value timber tree species. Pilot plantings showed that the species performed well in the wet-dry tropical areas of northern Australia, but the yellow looper, Gymnoscelis sp., and the bush cricket, Myara yabmanna, caused severe damage to the trees. The weaver ant, Oecophylla smaragdina, is an effective bio-control agent of a range of insect pests in tropical tree crops and forest trees. To determine whether the ants control these mahogany pests, field experiments were conducted from April 2006 to January 2009 at two sites near Darwin, Australia. The fraction of the total number of trees damaged per year was 4.2-32.4% by yellow loopers and 0-10.4% by bush crickets on trees with weaver ants, but 25-70.4% by yellow loopers and 25-100% by bush crickets on trees without the ants. The damaged trees with weaver ants were attacked on fewer occasions than damaged trees without the ants. Significantly fewer trees with weaver ants were damaged per monitoring occasion (2.1-8.4% at Berrimah Farm and 0.5% at Howard Springs by yellow loopers; 0-2.2% by bush crickets) than trees without the ants (24.2-51.3% at Berrimah Farm and 5.0% at Howard Springs by yellow loopers; 18.1-40.7% by bush crickets). Data suggested that weaver ants were effective bio-control agents of the two pests.