Like any other part of Rarotonga, along the back road, we see these creeping plants growing on our hedges, growing on our banana trees and covering our main forests.
Simo’s farm, located at the Are Metua backroad in Matavera, is next to what seems like a vacant land that is covered with invasive weeds.
These include mile-a-minute, the grand balloon vine, red passionfruit, strawberry guava, African tulip tree and Cocklebur.
For grower Danny Mataroa mile-a-minute seems to be a problem around the island, but his particular concern is how these weeds and other invasive plants are affecting the natural forests in the innermost part of the island.
Mataroa says it’s not a major problem for those living in the area close to the main road but it is for those who see these plants creeping onto the natural forest, the trees and plants denied sunlight and dying.
He does acknowledge, however, that mile-a-minute plant are useful for medicinal purposes.
According to a 2009 Landcare Research New Zealand report, much of the Cook Islands’ natural habitats and agricultural land are being threatened by invasive weeds, many of which were brought to the island for their ornamental value, as edible fruit, or as a timber species.
The weeds they say are threatening native biodiversity and many traditional cultural practices, but also the sustainable development of the island group. In particular, a suite of woody vines (Cardiospermum grandiflorum, Merremia peltata, Mikania micrantha and Passiflora rubra) are smothering trees, causing massive deforestation and replacing the native forest with impenetrable vine thickets.
So, with funding from the New Zealand Ministry of Foreign Affairs and Trade, a Landcare team scoped out a six-year plan for developing weed biocontrol in the Cook Islands.
Fifty-two weeds were discussed, but nine were rejected from further consideration because they had some desirable properties, such as being edible fruit or cultural important.
Two species, the giant sensitive plant (Mimosa diplotricha) and lantana (Lantana camara), were also removed from the list because they are already well-controlled by biocontrol agents that have been introduced to the Cook Islands.
The remaining 41 plants were ranked either “hot”, “warm” or “cold” depending on how important they thought it was to control the plant.
Ministry of Agriculture biologist and entomologist Dr Maya Poeschko confirms that six invasive weeds that have negative impact on agriculture and the natural forest habitats have been identified as priority targets for biocontrol.
In a report, Dr Maja says the control agents have been used around the world and they are healthier for the environment rather than using chemical to eliminate the risk of harming other plants.
The control is highly cost-effective because once established the agents provide continuous long-term weed suppression without further intervention.
The agriculture ministry is working with the National Environment Service and Cook Islands Natural Heritage to ensure that the biocontrol agents are dispersing to all area of the island where the weeds are a problem.
Dr Maja says the agents do not eliminate the weed completely but if successful they can reduce weed infestation to an extent that greatly benefits agriculture and allow the native ecosystem to recover.
These biological agents have only been released in Rarotonga. The ministry says many of them could be redistributed to control weed infestations on other islands however, permission and consultation would be undertaken before doing so.
Mile-a-minute: A South American rust fungus was released four years ago in Rarotonga to control the mile-a-minute vine and Dr Maja says this has been well established throughout the island. The agent does not eliminate the weed completely so the traditional uses such as herbal medicine can be maintained. The fungus does not attack other plants.
Cocklebur: A North American rust fungus was released at the same time to control Cocklebur. The agent has worked in Australia. It can be spotted on the leaves and stems within two weeks of infection, appearing to have brown patches.
Red Passionfruit: A population of the red postman butterfly was brought from Ecuador and released in Rarotonga in 2016 to control this passionfruit vine. According to the Ministry of Agriculture the red passionfruit does not produce edible fruits.
African Tulip tree: Gall mite from Ghana was introduced in 2016 to control the tulip tree. This is one of the world’s most invasive plants and a major threat to Rarotonga’s native forests.
Strawberry guava: A Brazilian scale insect was released to control the strawberry guava recently. Trials conducted by the ministry recently indicate that the Brazilian scale can greatly reduce the vigour and ability of the plant to continue growing.
Grand balloon vine: A South American rust fungus puccinia arechavaletae was released in 2017 to control the grand balloon vine that has been present since 1929, smothering native habitats and crops.