Climate variability, biological control and an insect pest outbreak on Australia's Coral Sea islets: lessons for invertebrate conservation

Distant tropical coral sand islets in the Coral Sea have remained isolated from major human interference since their relatively recent inception. As a result they have highly impoverished faunas and floras consisting only of species capable of long distance dispersal. Despite this bias, they have established some degree of stability or equilibrium. In the 1990s, it became apparent that a scale insect, Pulvinaria urbicola Cockerell and at least one species of attendant ant, Tetramorium bicarinatum (Nylander) were undergoing a population explosion on two of these islets causing damage to Pisonia grandis, a tree important as a nesting site for sea birds. The same phenomenon as in the Coral Sea, (Coringa Herald group), was recorded about the same time in the Capricorn group of islets (Great Barrier Reef), on Palmyra Atoll and Samoa in the Pacific and in the Seychelles (Indian Ocean). Control measures, the application of systemic insecticides, poisoning of attendant ants and introduction of biological control agents were applied to some islands. Pest numbers subsequently fell, often within months, even where no control measures were applied, suggesting that the population decline was a natural phenomenon on some sites and not the result of recent invasions. It is suggested that climate variability is likely to be a contributing factor. Rising sea surface temperatures that reduce prey available to sea birds so causing a lower nitrogen input to soils during nesting activities, together with drier conditions, are likely to have put Pisonia trees under stress. Stressed trees mobilise nutrients making them more vulnerable to attack by herbivores. If climate variability is an indirect cause of the pest outbreak, it is important to carefully assess the benefits and disadvantages, both environmentally and economically, of any control measures, especially when biological control agents are concerned, the effects of which are irreversible.