Biological control of weeds in Papua New Guinea

By ANNASTASIA KAWI and WAREA ORAPA of NARI

Siam weed affected and its growth stunted by Gall Fly - an example of successful bio-control of invasive weeds in PNG

Weeds cause serious obstructions to land use systems worldwide.

Many introduced weeds are serious impediments to agriculture productivity by causing significant production loss and threat to food security.

These impediments pose immense challenges to farmers and other land users.

 Managing these weeds is a critical defy in any attempt to get the maximum output.

And Papua New Guinea is no exception!

While plantation agriculture and some subsistence or semi-subsistence farmers generally use physical and chemical control measures to reduce the negative impacts of weeds in PNG, the use of bio-control methods has been significantly effective in managing some introduced and invasive weed species.

Bio-control, or biological control, of weeds is defined as the use of host-specific natural enemies such as herbivorous insects and mites or disease causing plant pathogens for the regulation of the population of weeds.

Papua New Guinea agriculture is still reliable on manual labour for weeding.

Cultural methods are also used to suppress weeds.

The use of herbicide and manual means (such as hand-pulling) is practical only in very limited situations such as small subsistence food gardens.

 In smallholder semi-subsistence farming situations, it becomes necessary to employ chemicals with large numbers of labour.

 These conventional methods of control are not practical as they are costly, time-consuming and often labour-intensive.

In natural systems where farming is not important, but weeds are a threat to ecosystems or the survival of important native species of fauna and flora, such control measures are not feasible at all.

Bio-control is seen as the only sustainable and cost-effective means to control introduced and invasive weeds, both in production areas (agricultural, forestry and fisheries) and natural areas (natural ecosystems such as rivers and rainforests).

Once released and established in an area, a bio-control agent can take two to six years before the benefits are measured.

When it works, bio-control is permanent, cheap and self-sustaining, requiring very little or no intervention in the long-term as the weed and bio-control agent reaches a point where they regulate each other’s population.

In PNG, there have been 15 bio-control agents introduced for weed control compared to 42 species of parasitoids for insect pest control and four against snail pests.

Generally, weed bio-control has been more successful in terms of establishment and control of the target weeds compared to the effectiveness of bio-control agents used against arthropod pests and snails.

The high level of success of weed bio-control maybe attributed to the fact that successful host-specificity research was done elsewhere before importing into PNG for local use.  

Some textbook examples of successful weed bio-control in PNG include the successful control of salvinia (Salvinia molesta) and water hyacinth (Eichhornia crassipes) in the Sepik River, the recent control of Siam weed (Chromolaena odorata) in New Ireland, Sandaun and East New Britain and the dramatic decline of the broomstick weed (Sida ) in the Markham Valley and Central province.

In the Sepik River case, almost 250 sq km of water surface was covered with the floating fern salvinia, directly impacting the daily livelihoods of river dependent villagers in the late 1970s and early 1980s.

People were not able to travel using canoes and even motorised boats suffered from continuous entangling of the outboard motor and heavy fuel consumption in heavily-infested situations. Fishing for protein became restricted and tourist access to backwater villages was denied due to the thick blankets of floating salvinia and the much larger water hyacinth in the 1990s.

 With the introduction and release of a tiny weevil called Cyrtobagous salviniae, from the Amazon basin in South America where Salvinia originates from, the weed population crashed from the high 250 sq km to a negligible 2 sq km within two and half years!

Life returned to normalcy for the people living along the river and others such as tourists.

A lot of awareness and publicity was made to the people along the Sepik River and other affected areas in PNG have acknowledged the importance of biological control. 

Similarly, the introduction and establishment of gall fly and Arctiid Moth have contained the Siam weed.

Currently, National Agriculture Research Institute (NARI) is implementing a bio-control programme against a major agricultural weed known as ‘Mile-a-minute’ (Mikania micrantha) in PNG and Fiji.

Funded by the Australian centre for International Agricultural Research, the collaborative programme involves the Secretariat of the Pacific Community (Fiji), Queensland Department of Primary Industries (Australia), and PNG’s Cocoa and Coconut Institute and Oil Palm Research Association.

The overall objective is to introduce bio-control agents to suppress the growth and presence of Mile-a-minute in order to minimise its impact on food security, income, and to increase national and regional capacity to undertake future biocontrol programmes against weeds.

One of the major activities of the project is to increase awareness of the bio-control to the farming communities and the general public.  

The bio-control agent used is a rust forming fungus called Puccinia spegazzinii which was supplied to PNG and Fiji by the Commonwealth Agriculture Bureau International, UK, after having collected it from Eastern Ecuador in South America and testing in London.

 The rust has been released in 14 lowland provinces in PNG since early 2009.

Scientists are working with communities in observing the progress.

Subsistence farmers and the commercial plantation sector can anticipate a positive outcome as the control materialises in the near future.

 Biological controls have been proven to be cost-effective and sustainable means of managing weeds for agricultural and land use systems and eventually enhance greater food production and improved livelihoods.