Weed succumbs to biocontrol in PNG, shows long term study

Kunibert Tibil, District Administrator, Kavieng, New Ireland, holding dead stems of chromolanea with galls (photo: W.Orapa)

Smallholder farmers in Papua New Guinea continue to benefit from the introduction of a biocontrol agent of one of the world’s worst weeds through a project funded by the Australian Centre for International Agricultural Research (ACIAR).
By 1998, the weed chromolaena (Chromolaena odorata) had invaded many subsistence farms in PNG, smothering crops such as taro, cassava and paw paw, along with plantation crops such as coconuts, oil palm and cocoa.
Today, where farms and roadsides were once choked with the weed, other useful plants are growing, and landholders, having saved countless hours of weeding, have more time to grow more produce for food and sale.

Gall fly effective

From 1998 to 2007, three biocontrol agents were introduced into PNG through the ACIAR project led by Michael Day of the Queensland Department of Employment, Economic Development and Innovation in association with Warea Orapa and Ingu Bofeng of the PNG National Agricultural Research Institute (NARI).
The gall fly (Cecidochares connexa) proved to be the most successful agent introduced.
It was released at over 350 sites in all provinces where chromolaena occurred and established at 300 sites. Following establishment, it spread naturally to a further 350 sites.
“The research showed that, as the number of galls created by the flies increased, branches of chromolaena died, reducing biomass and flower and seed production,” said Mr Day.
At the completion of the project, socioeconomic surveys of nearly 200 landowners found over 80% of respondents thought there was much less chromolaena than before the gall fly was released.
About 70% said they were benefitting from the control of the weed, with half saying their weeding times were reduced, and control costs had fallen 45%.
ver 60% said their crop yields and income had increased as a result of the control of the weed.

Long-term benefits

Five years later, an adoption study recently published by ACIAR (http://aciar.gov.au/publication/CP45) found that the benefits of the project are continuing.
“Information on chromolaena distribution and the status of the gall fly is still being used by NARI officers to both check whether the gall fly is present and to conduct opportunistic releases if the gall fly is not present,” said Mr Day.
“Landowners report that the gall fly has made a substantial difference and some are still moving the insect to new areas.
"Other landowners are now reducing the level of burning or leaving patches of chromolaena to ensure populations of the gall fly are maintained.
“Nearly 70% of landowners who had chromolaena on their farms reported some benefits as a result of the gall fly.
"The major benefit has been the reduced weeding time required to maintain food gardens, allowing landholders to increase the size of their blocks, thus increasing yield and income,” Mr Day said.

Regional differences

While chromolaena has been significantly reduced in most provinces where it was present, there is variation depending on land use and climate according to Mr Day.
“In New Ireland, where we first introduced the gall fly, there has been very good control and socioeconomic impact studies suggested that the landholders are benefiting,” he said.
“In the drier provinces, such as Morobe, control is slower and less complete.
"In West New Britain, which is considerably wetter than most other provinces, control has not been as good and chromolaena remains a problem because the gall fly needs sunny days with temperatures over 30 °C to mate.”
The benefits of this biocontrol project will continue to flow through the community, with the gall fly suppressing populations of chromolaena, spreading naturally throughout areas where the weed is present, and being relocated by landowners into new areas as the weed spreads.

International benefits

Through various international workshops and publications, the news of the outcomes of the chromolaena biocontrol project in PNG has resulted in other countries also introducing the gall fly.
Mr Day reports that the gall fly was introduced into East Timor in 2005 and Thailand in 2009.
An application has recently been submitted to introduce the gall fly into Kenya.
In addition, China, Taiwan and Palau have all expressed interest in importing the agent.
Controlling chromolaena in neighbouring countries to Australia helps farmers increase food security in those countries, reduces the risk of the weed spreading to other neighbouring countries such as the Solomon Islands thus reducing the risk of further spread into Australia, and increases Australia's expertise in weed control.