Managing scale insect outbreaks in the Capricornia Cays conserving biodiversity through pest management Since 1993, infestations of scale insects have Using a variety of methods — primarily release affected pisonia forests on three of Central of native ladybirds, a scale predator, and baiting Queensland’s coral cays in the Capricornia Cays of introduced ants which interfere with the scale National Park. First affected was Tryon Island, predators — QPWS brought both infestations where most of the pisonia forest was ultimately under control, and the forests recovered. Today, destroyed, as natural predators failed to a revegetation program is underway on Tryon overcome the scale insect as expected. When Island, and research and monitoring are ongoing John Augusteyn scale outbreaks occurred in 2006 on Wilson across the cays. QPWS’s work has implications and Heron islands, the Queensland Parks and for coral cays and atolls elsewhere in the world, Wildlife Service (QPWS) actively managed them, where pisonia forests, already dramatically armed with the lessons from Tryon Island. reduced by clearing, are further threatened by scale insect outbreaks. Why our pisonia forests matter Threats to pisonia Signifi cant and vulnerable Across the world, pisonia forests are threatened Pisonia grandis, a large-leafed woody shrub or by a variety of factors. Outside Australia, pisonia tree is found almost exclusively on Indo-Pacifi c is frequently cleared for plantation agriculture, islands broadly between the Tropics of Cancer development, and the mining of guano (bird and Capricorn, and as far away as islands of droppings used for fertiliser). the Seychelles, Hawaii and eastern Polynesia. In Australia, climate change — possibly increased John Augusteyn In Australia only a little over 190ha of pisonia El Nino (drought) events and rising sea levels grows — and most is on the islands of the Great — may be affecting some forests. Scale insect Barrier Reef. outbreaks have also wiped out or affected a number of pisonia forests in Australia and elsewhere. • At the southern end of the Great Barrier Reef is Australia’s pisonia stronghold — More than forests at stake the Capricornia Cays — where large pisonia Damage to the pisonia forests of the Capricornia forests total about 135ha. Cays has far-reaching consequences. The forests • Other Australian islands where pisonia grows themselves are of international signifi cance, are Christmas, Cocos, Cobourg Peninsula and and permanent loss may threaten survival of Torres Strait islands and Coringa-Herald cays the Pisonia grandis species. Forest decline may in the Coral Sea National Nature Reserve. also affect the stability of the coral cays, and the Pisonia is critical to the ecology of the Capricornia survival of cay-dependant wildlife such as nesting Cays. Developed pisonia forests help stabilise the turtles and seabirds. Pisonia is critical for black cays, protecting them during cyclones. Pisonia noddy populations which nest almost exclusively in Mark Coombe Mark trees provide shelter and breeding habitat for a pisonia, while dense pisonia canopies create open variety of birds. In the Capricornia Cays, pisonia ground for wedge-tailed shearwaters to burrow. supports 75% of all nesting seabirds in the Great People are also affected by pisonia decline: forest Barrier Reef World Heritage Area. The cays are die-offs can limit recreational use, with visitors also important breeding grounds for endangered unable to walk or camp in affected or unsafe areas. sea turtles. The aesthetic impacts of dying or dead forests can dramatically affect tourist enjoyment of what are Australia’s pisonia forests are internationally usually extraordinarily beautiful coral cays. signifi cant as the majority are in protected areas. Pisonia forests in the Capricornia Cays provide About 70-80 percent of our pisonia forests are on island resorts and campgrounds with valuable the islands of the Capricornia Cays National Park protection from the sun and potentially and National Park (Scientifi c). destructive force of cyclonic winds. John Augusteyn Queensland Parks and Wildlife Service Department of Environment and Resource Management Scale insect and pisonia Pisonia forests play a critical role in A balanced relationship the ecology of the Capricornia Cays, The soft scale insect, Pulvinaria urbicola, which are places is a tiny insect (around 2–3mm long) of extraordinary that lives on the leaves of a number of beauty and ecological diversity. plants, including pisonia. ‘Scale’ insects get their name from their scaly protective covers, although in ‘soft’ scales this is usually absent. DERM When the balance goes wrong A global problem Sometimes, however, the number Since the early 1990s, scale outbreaks of scale insects in a pisonia forest are known to have occurred on three explodes, creating an ‘outbreak’. In of the islands of the Capricornia Cays response to the increased food supply, National Park: Tryon, Wilson and Heron. outbreaks are also accompanied by During the same period, scale outbreaks an explosion in the number of ants – have also affected pisonia forests particularly the introduced African elsewhere. The pisonia forest on big-head ant Pheidole megacephala Coringa Island in Australia’s Coral Sea in the Capricornia Cays. was completely destroyed by scale Under the stress of such large numbers insect during the 1990s. Scale has also of scale, pisonia trees respond as severely damaged pisonia forests on if drought-stressed and drop their Bird Island in the Seychelles and leaves. Scale eggs are plentiful and Palmyra Atoll in Hawaii. their highly mobile larvae will move, or be blown or carried on to other Scale insect outbreaks are a threat trees. The original trees start to recover to pisonia forests worldwide. The and form new leaf buds, but are soon work of QPWS in managing scale on recolonised by scale and ants. Pisonia the Capricornia Cays is vital not just trees can go through a number of for the protection of Queensland’s defoliations, over several months or islands, but for cays and atolls a couple of years, before eventually across the world. dying if the outbreak continues. John Augusteyn Soft scale appear as greenish-brown blisters and in outbreak you can see lots of white woolly-like egg masses. Scale insects suck sap from their host plant to survive. They also produce a sweet, sticky substance called ‘honeydew’, which is a food source for many common ant species. Ants attracted by honeydew will farm scale and often interfere with their predators; they may even move scale eggs to new sites. Our current understanding is that this mutually benefi cial relationship between scale and ants is a normal, balanced situation. Scale insects seem to occur naturally throughout healthy pisonia forests in John Olds very low numbers — less than one or two scale to a leaf. For the most part, their Pisonia grandis leaf, African big-head ants Pheidole megacephala, and soft numbers appear to be held in check by scale insect Pulvinaria urbicola. natural factors, especially predators such as native ladybirds and parasitic wasps. John Augusteyn Legend Managing outbreaks in Capricornia Cays North Reef Reef the Capricornia Cays Shoal edge Tryon Island Tryon Island — growing Capricornia Cays National Park North West Island Broomfield Cay a new pisonia forest Capricornia Cays National Park Wilson Island (Scientific) – no access In August 1993, an outbreak of the scale Wreck Island insect was detected in Tryon Island’s pisonia forest. At this time, Tryon’s Sykes Reef Great Barrier Reef World Heritage Area pisonia covered nearly half the island. Tropic of Capricorn Heron Island This was the fi rst known scale outbreak in a pisonia forest of the Capricornia Erskine Island Tropic of Capricorn Wistari Reef Gladstone Cays, and one of the world’s earliest Irving Reef One Tree Island Capricorn Cays National Park records. Expectations were that natural Masthead Island Polmaise Reef predators — ladybirds and parasitic Lamont Reef Scale wasps — would bring the outbreak Fitzroy 0 5 10 Reef under control. At various points the Wilson Island — managing an nautical miles forests appeared to be recovering, Llewellyn 0 51510 outbreak through ants and ladybirds Reef with previously affected trees showing kilometres In May 2006, QPWS rangers discovered new growth. Ultimately the repeated a scale insect outbreak on Wilson Island. Boult Reef scale infestations killed many pisonia, The island’s 2.4ha pisonia forest was in taking 7–8 years before scale numbers Hoskyn Islands the early stages of infestation. subsided. Even after the outbreak had Fairfax Islands ended, the forest did not regenerate as Armed with the lessons from Tryon might have been expected. Today, 90 Island and the previous successful Lady Musgrave Island percent of Tryon Island’s original pisonia predator and parasite releases in the forest has gone. Coral Sea, QPWS responded quickly. In early June ant bait was used to almost All is not lost, however. Once it became eliminate the introduced ants on the clear that the forest could not recover Island. Two weeks later, 5000 native naturally, QPWS began a revegetation ladybirds (Cryptolaemus montrouzieri) program. Trial plantings in 2004 were released. Ladybirds are natural and 2005 indicated a need for more scale predators, frequently seen in low detailed investigations, and a more numbers on the Capricornia Cays. John Olds comprehensive trial began in July 2006. Surprisingly, however, by early August, QPWS staff and a team of volunteers are Native