World Firsts for the Cook Islands

Iain Paterson releasing mites on African tulip tree

The Cook Islands are the focus of a 5-year biocontrol project, funded by the New Zealand Ministry of Foreign Affairs and Trade, aimed at bringing some of their most rampant weeds under control. A workshop was held in 2009, where a range of Cook Island experts prioritised which weeds were suitable as biocontrol targets using a scoring system that considered weed importance and the predicted cost and likelihood of successful biocontrol. Funding was then secured to implement biocontrol for the top seven weed species, and to do further work to explore the appropriateness of targeting an eighth species, peltate morning glory (Merremia peltata). There are conflicting views on whether this invasive vine is native or introduced to the Pacific, and a molecular study is seeking to resolve this. Results to date suggest peltate morning glory may have been introduced from the North Pacific to the South Pacific, but more samples of this vine are needed from across its range to confirm this.

"The development of a comprehensive biocontrol programme was necessary to prevent further deterioration of native forests and disruption to agricultural systems in the Cook Islands, and to prevent one bad weed simply being replaced by another," said Quentin Paynter, who is leading the work. "For example, three invasive vines species are widespread and commonly occur together, so we needed to tackle all three at the same time."

As is often the case with these projects, there have been one or two unexpected challenges pop up. Surveys for potential release sites in April 2015 revealed there had been confusion between giant reed (Arundo donax), which it turns out is quite rare on Rarotonga, and elephant grass (Pennisetum purpureum), which is more widespread. "We then suggested that giant reed would be a good candidate for eradication using manual control rather than going down the biocontrol route", explained Quentin. However, the work sourcing giant reed agents for Rarotonga, and maintaining colonies of them in containment at Tamaki, has not gone to waste. Giant reed is an up-and-coming weed in New Zealand, particularly up north, and so the Northland Regional Council took the opportunity to seek approval from the Environmental Protection Authority (EPA) to release the gall-forming wasp (Tetramesa romana) and a scale insect (Rhizaspidiotus donacis) here. The EPA granted this permission in January, and releases of the wasp can get underway soon.

Recently there have been some promising signs that the ambitious Cook Islands project will pay dividends. Red passionfruit (Passiflora rubra) is one of the most vigorous invasive vines there, but biocontrol has not been attempted against this target anywhere else in the world before now. "There is always a higher risk of failure with novel programmes and I initially thought that red passionfruit would prove to be an impossible target given the level of funding at our disposal",said Quentin. "However, Heliconius butterflies have been extremely well studied and are commonly kept in butterfly houses for their good looks. Published literature indicated that some Heliconius species do not attack the edible passionfruits (which belong to a different Passiflora subgenus to P. rubra) and might therefore attack the unwanted red passionfruit while not harming closely related desirable species. The red postman butterfly (Heliconius erato cyrbia), which is native to Colombia and Ecuador, was therefore considered. We were able to obtain this subspecies from Heliconius Butterfly Works – a commercial supplier for butterfly houses, which reduced costs enormously."

Host-range testing confirmed that the red postman does not attack edible passionfruits and permission to release it on Rarotonga was then obtained. This attractive butterfly was subsequently released there in August 2016 and is already firmly established and spreading. "I'm feeling confident that it will have a major impact on red passionfruit as larval feeding severely damaged plants in containment. They are quite wasteful feeders and commonly chewed through whole stems," said Quentin. It is hoped that the butterflies can be eventually released on other islands such as Atiu, which has an internationally important remnant makatea forest that is being invaded by red passionfruit.

The other novel programme being developed for the Cook Islands is for African tulip tree (Spathodea campanulata). An eriophyid mite (Colomerus spathodeae) was selected as the first agent for this plant, which is considered to be a major problem throughout the Pacific region and elsewhere – indeed African tulip tree is listed as one of the world's 100 most invasive species. The mite, which was recently approved for release in the Cook Islands, was sourced from Ghana by Dr Iain Paterson (Rhodes University). The mite mainly attacks the new foliage, severely distorting it and thereby stunting the growth of this tree.

Iain hand-delivered the fragile critter to our containment facility in Auckland, where a back-up culture is being maintained. He then travelled with Quentin to Rarotonga, where he successfully released the mites onto plants growing in a shade house at the Ministry of Agriculture (MoA) and made some preliminary field releases. Iain stayed on in Rarotonga for a few more days to oversee further field releases around the island. "The mite gets to work very quickly, and we already know that most of the initial releases have taken well," said Iain. Furthermore, host-range testing of a second agent, a leaf-mining beetle (Paradibolia coerulea), is at an advanced stage. Testing to date strongly indicates that it is host-specific, so the beetle may also be introduced to Rarotonga during the current programme.

Strawberry guava (Psidium cattleianum) is another ominous invader in the Cook Islands, as well as one to watch here in New Zealand. This woody shrub is native to Brazil and was introduced to the Pacific region in the 1800s. It displaces native plant communities and is dispersed mainly by birds and pigs, which find its fruit irresistible. Strawberry guava is considered to be one of the worst weeds in Hawai'i, so considerable research has gone into biocontrol options already. A promising scale insect (Tectococcus ovatus) was approved for release in the Cook Islands, and following successful rearing of it at the MoA facility in Rarotonga has recently been released onto strawberry guava plants on the island. The scale insects cause galls on the leaves and stunt the growth of the plant. "We found galls on plants in the field during this last visit, so establishment looks likely," said Quentin. "This trip we made multiple additional field releases of this insect along the southern section of the cross-island walk where strawberry guava is abundant and invading native forest and fernlands," he added.

The aptly named mile-a-minute vine (Mikania micrantha) is still rampant, however, after initial attempts to establish the rust (Puccinia spegazzinii) were unsuccessful. "This has been a challenging plant to work on. Although it grows rampantly in the field, it is attacked by all manner of pests when grown in a shadehouse, and staff at the MoA have had difficulty maintaining plants and the associated rust culture adequately. We therefore tried a different approach during the latest visit by conducting direct field releases using plants that were inoculated multiple times in containment in New Zealand. Multiple inoculations should extend the time over which spores will be maturing, thereby increasing the chances of sporulation when conditions in Rarotonga were most suitable for the rust," said pathologist Chantal Probst.

"To meet phytosanitary requirements, the roots of the plants had to be washed bare and the plants inspected, which we arranged to have done the day before departure from New Zealand to minimise stress on the plants. On arrival in Rarotonga, the plants were re-potted and placed in a mist-chamber at the MoA facility as soon possible and allowed to recover overnight," she explained. This tactic was obviously successful, because some plants had perked up and were displaying mature rust pustules the day after they arrived, and after a few days immature pustules had begun to form on the younger growth. "We planted the infected vines at multiple field sites covering a range of different environmental conditions and hopefully the field releases made during my trip will finally see the rust establish and take off now," said Quentin. This rust is reported to have had a noticeable impact on mile-a-minute in Papua New Guinea and Vanuatu only a few years after release there, so we remain hopeful it will also do good things in the Cook Islands.

A rust fungus (Puccinia xanthii) aimed at cockleburr (Xanthium pungens – part of the Xanthium strumarium sp. agg. complex, and known as Noogoora burr in Australia) has proven a little easier to establish in Rarotonga. Cockleburr is an annual herb that has a widespread geographical range throughout the Pacific, growing well in sandy habitats. This particular rust fungus, which successfully keeps Noogoora burr under control in Australia, was released in Rarotonga in 2015. Survey work undertaken during them latest visit revealed that the rust has established at two sites on opposite sides of the island. "If the rust has the same impact on Rarotonga as it does in Australia, then cockleburr should cease to be a major problem in Rarotonga, once this pathogen begins to reach outbreak levels," predicts Quentin.

The final target weed under consideration is grand balloon vine (Cardiospermum grandiflorum), which is native to South America. Two agents – another rust fungus (Puccinia arechavaletae) and a weevil (Cissoanthonomus tuberculipennis) – were identified as having potential, but at this stage only the rust fungus has been advanced. "This plant is also a big problem in South Africa, so most of the host range testing has already been completed for the fungus, giving us a head start," explained Quentin. A shipment of the rust fungus from South America arrived early in the New Year following a rather prolonged transit, so there were concerns about how viable the material would still be. "Thankfully inoculations have worked and we have a small number of pustules beginning to develop on plants in the Auckland containment facility. We just need to test one non-target plant to check it is not susceptible to the rust and then, assuming we are happy with the results, apply to release it in Rarotonga," he said. While good progress has been made in this project, there is clearly much more to do, including establishing agents on islands other than Rarotonga in the Cook Islands group. A separate environmental impact assessment must be made ahead of releasing any new agent on any other island. "Obviously this is quite a big undertaking, so we'd like to confirm that the agents released on Rarotonga are successful before applying to release them on other islands," Quentin concluded.

If you might be able to help provide some of the much-needed additional samples of peltate morning glory (Merremia peltata) from across its current distribution please get in touch!