April 2005. A recent outbreak of the introduced scale insect, Pulvinaria urbicola, on native Pisonia grandis trees at Palmyra Atoll, Line Islands. The scales are causing widespread tree death on the atoll. Outbreaks of the same scale on Pisonia have also recently occurred on three different islands around northeastern Australia, in the Seychelles, on Rose Atoll in American Samoa, and apparently also in Tonga. The question posed was, Is anybody familiar with other examples of this phenomenon in the Pacific or Indian Oceans? Any ideas/insight into what might be triggering these scale outbreaks on this particular tree?
One suggestion provided involved a putative association with ants:
The possibility that an invasive ant like the yellow crazy ant, Paratrechina longicornis, are farming the scale insects leaps to mind as a possibility.
An example of this sort of interaction is between an ant they are calling the brown crazy ant (possibly the same species) and scale in the Congo – see http://www.scienceinafrica.co.za/2004/july/ant.htm. The interaction certainly seems similar – the ants “farm” the scale insects for their honeydew, a waste product of the scale insect but a food to the ant.
On Christmas Island (Indian Ocean) researchers proved that Paratrechina longicornis was farming the scale insects and that there was significant defoliation of the upper forest canopy as a result. For the Christmas Island story see www.deh.gov.au/parks/christmas/fauna/crazy.html.
From ISSG’s GISD database entry on this species: Mutualism between A gracilipes and honeydew-secreting Homoptera can cause population outbreaks of these generalist herbivores and lead to canopy dieback. For the full GISD entry see: www.issg.org/database/species/ecology.asp.
Crazy ants are tramp species and, unfortunately, are already on a number of Pacific islands, although their impacts are not yet well documented in the way they have been on Christmas Island.
In response, it was said that the scales are indeed being tended by large numbers of several ant species, including Pheidole megacephala. It is becoming apparent that this phenomenon is widely distributed. The ants are contributing to and reinforcing the scale outbreak through protection, causing a positive feedback cycle, but it seems that there might be some other factor that triggers the outbreak to begin: on some Australian islands, the scale has been present (with ants) for years and has not increased in abundance, while nearby islands undergo scale outbreak and suffer tree death.
Pisonia is, presumably, native to the localities mentioned as it is found between 240oS and 240oN in the Indo-Pacific Region (Smith & Papacek, 2001).
However, the scale insect Pulverina urbicola is an introduced species. “The scale is probably a native of the West Indies which has become distributed through the central American-Southern pacific Region. Its means of arriving on islets like North East Herald and Coringa can only be speculative, but seabirds are a strong possibility.” (Smith & Papacek, 2001).
Therefore, even native ant populations could get out of balance if they found a new food source in the invasive scale insects, if the scale insects arrived on the island without their native controls. “In spite of its wide host range and distribution, P. urbicola is not regarded as a serious pest. This is because it is normally controlled by two or three parasitoids, and also by predatory coccinellids (ladybeetles) like Cryptolaemus montrouzieri (Mulsant)… Total destruction of its plant host by an insect like P. urbicola is an unusual, but not unique phenomenon. Insect pests are almost always in balance with their host with population checks applied by weather, competition and natural enemies. Homoptera like P. urbicola are particularly susceptible to biological control by hymenopterous parasitoids (encyrtids and aphelinids). Scale insects can greatly multiply in their absence (if weather and food resources are ideal). This situation occurs usually when the pest insect is transferred or imported to a new region or country without its natural enemies, i.e., it becomes an exotic pest. A similar form of imbalance can be induced with careless pesticide use which kills natural enemies but not the pest insect.” (Smith and Papacek, 2001).
But there could also be a number of introduced species interacting and impacting on the trees, compounding the problem. In the case of Christmas Island, there may have been two invasive species involved – the crazy ant and the scale insect, although there are different scale insect species involved. “Population outbreaks of the cryptogenic lac insect Tachardina aurantiaca (Kerridae) and introduced Coccus celatus (Coccidae), in particular, are associated with crazy ant supercolonies (O’Dowd et al, 2001)” implies that at least one of the scale insect species is introduced. In that case, the ants were targeted for control.