Delivering a blow to Cook Islands weeds
Published: 16 August 2016 - by Quentin Paynter
Landcare Research scientists are assisting the war on weeds not only in New Zealand but also its Pacific neighbour, the Cook Islands.
The Crown Research Institute has been working closely with the Cook Islands Ministry of Agriculture, and locals, after having compiled a list of key weeds to target across the country.
Among those identified was the red passionfruit, or Passiflora rubra, that is one of a suite of invasive vines that interfere with agriculture and smother native plants, threatening native forests. Red passionfruit is difficult and expensive to control using conventional control methods, but a striking red, black and white South American butterfly appears a promising solution.
The butterfly, Heliconius erato cyrbia, commonly known as the red postman, was recently released in Rarotonga by Landcare Research scientist Dr Quentin Paynter. Extensive testing was done prior to its release to make sure it didn’t pose a threat to edible varieties of passionfruit.
It is the latest weed biocontrol agent to be introduced to the Cook Islands and it’s expected to deliver a long-term and affordable solution to controlling the weed.
Dr Paynter took over around 77 adult butterflies, which were released in a breeding cage at the Cook Islands Ministry of Agriculture headquarters in Rarotonga. Landcare Research sourced the butterflies from Ecuador, conducted host tests to make sure they didn’t pose a risk to other plants, and reared them at its containment facility in Auckland.
Dr Paynter said the larvae of the butterfly damaged the weed by feeding on the stem tips during infancy, moving onto the mature leaves as they developed, and lastly the green stems before they became a butterfly.
“We don’t know if they’ll do enough damage to kill the plant outright or simply reduce its competitive ability. My gut feeling is the caterpillar feeding will greatly reduce the competitive ability of Passiflora rubra – hopefully knocking it back enough to make it a minor plant, rather than a smothering invader, but we’ll just have to wait and see,” he said.
While in Rarotonga, Dr Paynter trained Cook Islands Ministry of Agriculture staff how to mass rear the butterfly for further releases and how to monitor its spread and success. The butterflies took to their new home and were laying eggs on food plants in the breeding cage within a few hours of being introduced. After three days, so many eggs had been laid that it was decided to release 50 of the adult butterflies into the field.
Once the butterfly has established at the initial release sites Dr Paynter said there were plans to eventually release it at other islands, in particular Atiu, which has a globally recognised remnant of makatea forests that is threatened by invading red passionfruit.
The butterfly is the third biocontrol agent Landcare Research has introduced in the Cook Islands to tackle weeds. Two rust fungi were introduced to attack mile-a-minute weed (Mikania micrantha) and cocklebur (Xanthium pungens) late last year. Further introductions of these fungi are planned to ensure they become established.
Dr Paynter also took over the eggs of a scale insect that appeared a strong biocontrol agent for strawberry guava (Psidium cattleyanum), which will also be propagated at the Ministry of Agriculture before being released later this year.
Work is also well underway on biocontrol agents for the African tulip tree, with a release of the first agent being planned for November this year.
Other weeds Landcare Research will target in the Cook
Islands for better control are Noogoora burr (Xanthium strumarium), grand balloon vine (Cardiospermum grandiflorum), giant
Landcare Research’s weed biocontrol work in the Cook Islands is funded by New Zealand’s Ministry of Foreign Affairs.