The effect of weaver ants Oecophylla smaragdina on the shoot borer Hypsipyla robusta on African mahoganies in Australia

1. African mahogany Khaya senegalensis is a high-value timber tree species widely grown in central Africa, south-east Asia and northern Australia. Pilot plantings show that the tree grows well in the wet-dry tropical areas of northern Australia, and the shoot borer Hypsipyla robusta (Lepidoptera: Pyralidae) is a potential pest of the tree. The weaver ant Oecophylla smaragdina is an efficient biocontrol agent in some horticulture crops. To investigate whether the ants control shoot borers, field experiments were conducted at two sites near Darwin, Australia from April 2006 to January 2009.

2. In the weaver ant treatments, the overall percentage of trees damaged by shoot borers was 0–2.7% at Berrimah Farm and 0–4.2% at Howard Springs, and the damaged trees were attacked once only. In the treatments without weaver ants, however, 9.9–52.1% trees were damaged at Berrimah Farm, and 6.3–64.6% at Howard Springs, and the damaged trees were generally attacked more than once.

3. At both sites, significantly fewer trees on each monitoring occasion were damaged in weaver ant treatments than in treatments without weaver ants.

4. The mean percentage of overall flushing shoots damaged by the pest at both sites was significantly lower in weaver ant treatments compared with treatments without weaver ants.

5. Fewer shoots were damaged per damaged tree in weaver ant treatments compared with treatments without weaver ants.

6. The data obtained suggest that weaver ants were effective biological control agents of the shoot borer, and that the ants can be used to manage the pest on African mahogany trees.