Tiny wasp helps protect island bird species threatened with extinction

• One of the world’s rarest birds, Wilkins’ Bunting, has been handed a much-needed lifeline by a small species of parasitoid wasp. 

• Only found on the remote Nightingale Island in the South Atlantic, the bunting’s food source was threatened by an invasive alien scale insect. 

• Conservationists navigated an epic 10,000 km journey across land, sea and air to ensure the safe arrival of parasitoid wasps on the island, to control the scale insect and help the bunting.

One of the world’s rarest birds, the Wilkins’ Bunting has been handed a much-needed lifeline by a species of small parasitoid wasp. This was part of a ground-breaking project to biologically control an introduced pest species on one of the world’s most isolated islands. 

Wilkins’ Bunting Nesospiza wilkinsi is endemic to Nightingale Island, part of the Tristan da Cunha Group in the South Atlantic, the world’s most remote inhabited archipelago and part of UK Overseas Territories. The bird eats the fruit of the Phylica arborea, the island’s only native tree, but an infestation of an accidentally introduced invasive scale insect Coccus hesperidum threatened to devastate the island’s forest and with it, put the Wilkins’ Bunting at real risk of extinction. These scale insects secrete honeydew which encourages the growth of a sooty mould on branches, preventing photosynthesis, weakening and eventually killing the tree. The population of buntings was hit further when huge storms wreaked damage to the forest in 2019. 

Surveys in 2017, before the storm, found there were only around 120 breeding pairs of the bird remaining. A group including the RSPB, CABI, FERA and the Tristan da Cunha Government came together and developed a plan to save the buntings. This was to target the invasive scale insect, set up a tree nursery to boost the number of fruiting trees and improve island biosecurity to avoid similar occurrences in future. 

Trevor Glass, Head of Tristan Conservation Department, said: “We hadn’t realised quite how damaging the scale insects were on Tristan, affecting many plants as well as Phylica. No one knew what a scale insect was before this project, but the community are now very aware of them.”

Entomologists set out to try and find a way to deal with the scale insects. A break-through came when a small parasitoid wasp Microterys nietneri was found to be successful in controlling the scale insect, without having any adverse impacts on other species. But the crucial next step meant getting the wasps to the island - a journey of more than 10,000km whilst Covid restrictions were still in place. 

Dr Norbert Maczey, an entomologist at CABI, said: “The wasps faced an epic journey. Firstly, a flight from London to Cape Town, in a cool bag followed by an enforced stay in a hotel room as part of a staff member’s Covid quarantine. Next came a week-long boat journey to Tristan with temperatures sometimes dropping below zero.  Finally, there was a further boat trip to Nightingale Island. It seemed like luck and time was against us but some of the wasps made it.” 

With less than 10% of the wasps surviving the journey of almost a month by land, sea and air, a first release of wasps on Nightingale Island took place in April 2021 followed by additional releases over the next two years. Some of these were successful and a population of wasps started to establish itself. 

Norbert Maczey said: “Despite the weather on Tristan frequently being cold, wet and windy, the wasps established surprisingly quickly, parasitism rates of the scale insects are currently high, and the trees are already showing signs of recovery.” 

The buntings have benefitted from the wasps’ success. Surveys in February this year showed that despite losing approximately 80% of the forest, there is still an estimated 60-90 pairs of Wilkins’ Bunting on Nightingale. Although the population has reduced, forest recovery, and replanting as a result of this project, has likely contributed towards halting this decline. By controlling this damaging scale insect, we are confident that numbers of buntings should stabilise and will have a chance to recover over the next few years. The future certainly looks a lot brighter for this Critically Endangered bird.    

David Kinchin-Smith, the RSPB’s UK Overseas Territories Project Manager, said: “This project shows what can be achieved in turning around the fate of a threatened species. Steely determination, ecological expertise and a large helping of luck have all contributed to the success of this work, but hopefully we, and the wasps, have given the buntings a much-needed lifeline.”