Weaver ants, Oecophylla smaragdina (Hymenoptera: Formicidae), as biocontrol agents on African mahogany trees, Khaya senegalensis (Sapindales: Meliaceae), in the Northern Territory of Australia

Abstract

African mahogany, Khaya senegalensis, is a high quality timber tree that grows well in the wet-dry tropical areas of Australia. Most trees grown in the latter regions are branched at lower levels on the trunk (a symptom known as ‘low-branching’), which limits timber production per tree. Inferring that it may be caused by herbivorous insects, we sought to establish whether low-branching can be reduced by the presence of predatory weaver ants, Oecophylla smaragdina. Two field experiments on young African mahoganies were conducted at two sites in the Darwin area of Australia from 2006 to 2008. Each experiment had two treatments: trees with weaver ants and trees without the ants. Pest damage was found to be the most important correlate of the multiple branching, and weaver ants were effective in limiting the formation of multiple leaders. In 3-year-old mahoganies at Berrimah Farm, the percentage of trees with pest-caused multiple leaders was 4.8% in the weaver ant treatments (WAT) but 45.5% in the treatments without the ants (TWWA). In 2-year-old mahogany coppices at Howard Springs, no trees developed multiple leaders in WAT, but 22% developed such leaders in TWWA. The average pest damage on flushing shoots at both sites was significantly lower in WAT (1.8–2.5%) than in TWWA (22.9–24.9%).