Interaction between ants and membracid bugs decreases growth and seed set of host plant bearing extrafloral nectaries.

This study describes an experimental analysis of a five-fold plant-insect interaction, with the primary aim of determining the net effect of insect-insect interactions on the host plant. The participlants are the extrafloral nectary-bearing host plant Acacia decurrens; the resident sap-sucking membracid treehopper Sextius virescens; the ant Iridomyrmex sp. which attends both EFN and treehoppers; external herbivores including grasshoppers and gall wasps; and predators and parasitoids of the membracids. The main results are as follows. 1. The membracids alone decrease plant growth and seed set. 2. In the absence of membracids, the ants defend the plant against external herbivores, and their presence increases plant growth and seed set. 3. The interaction between ants and membracids exerts an overall negative effect on plant growth and seed set. This has not been demonstrated for any system previously and is the main result of the study. 4. Ant attendance does not increase membracid populations or feeding rates significantly, or reduce mid-season mortality. 5. Ant attendance decreases late-season membracid mortality significantly. This has been shown previously for Lepidoptera and nymphal Homoptera only. 6. The mechanism of the interactive effect in (3) is apparently that the honeydewsecreting membracids attract the ants away from the EFN and decrease the efficacy of the plant's ant defence.