Indirect control of the invasive yellow crazy ant through biological control of its  mutualist scale insect on Christmas Island

Gabor Neumann *1, Peter Green 2, Dennis O'Dowd 3

1 La Trobe University, PO BOX 612, Christmas Is, WA 6798, Australia
2 La Trobe University, University Drive, Wodonga, 3690, VIC, Australia
3 Monash University, Victoria 3800 Australia

The invasion of the yellow crazy ant (Anoplolepis gracilipes) on Christmas Island, Indian Ocean, threatens the conservation value of its unique island rainforest by triggering an ecosystem “meltdown”.  Mutualism with introduced honeydew-producing scale insects, especially the lac scale, Tachardina aurantiaca (Hemiptera: Kerriidae), appears key to the ant’s success.  Although initial suppression of the crazy ant using toxic bait has been successful, repeated use of toxic chemicals in maintenance management in remote natural areas has proved difficult to sustain.  Development of biological control programs on islands, and of invasive scale insects in particular, have proved especially successful.  Biological control of T. aurantiaca may provide a self-sustaining system to indirectly control crazy ants by denying them resources provided by these honeydew-secreting mutualists. Initial on-island research on feasibility is promising:  (i) T. aurantiaca does not appear to be controlled by natural enemies. Widespread surveys across island rainforest have detected a single primary parasitoid species, Marietta sp. (Hymenoptera: Aphelinidae), a primary/secondary parasitoid that attacks only a small percentage of male lac scale and does not appear to provide effective control of T. aurantiaca. Nevertheless, potential hyperparasitism by Marietta sp. must be taken into consideration during agent selection.  (ii) Further intensive surveys have detected no native or endemic scale insect species which should minimize the potential for non-target effects of parasitoid introduction for T. aurantiaca.  The search for effective natural enemies of T. aurantiaca in Southeast Asia, its putative area of origin, has begun.

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