Taro beetle management in PNG and Fiji

Fiji, Papua New Guinea

Mr Aleki Sisifa
Phone: 679 3379214 or 679 3370733 ext. 214
Fax: 679 3370021
Email: Alekis@spc.int

• CSIRO Entomology, Australia
• Ministry of Agriculture, Sugar and Land Resettlement, Fiji
• National Agricultural Research Institute, Papua New Guinea

$853,855

01/01/2002 - 31/12/2005

01/01/2006 - 31/12/2007

Dr T K Lim

Taro is the preferred staple in Pacific communities. One of its main pests is the taro beetle, which damages the corm (an underground stem resembling a bulb) of the plant and creates entry points for secondary pests. The taro beetle causes about 30 per cent yield loss in taro-producing countries such as PNG and Fiji. Taro production is a labour-intensive crop which is grown on a small scale in farming communities. The spread of the taro beetle in the Pacific is a threat to taro exporters and their revenue, and it also has an environmental impact because farmers abandon infested taro gardens and move on to clear established forests for new gardens.

For Australia, the use of fungi such as Metarhizium as mycoinsecticides is attractive because fungi are specific, natural and often give persistent control in the soil. However, the use of mycoinsecticides in Australia has been slow because of the lack of suitable products and the high costs. To improve this situation, more research is needed for better understanding of strain selection, mass production, formulation and application strategies. Better control practices would reduce taro beetle damage in farmers' fields, restore the supplies of taro as a major staple and revive the trade in quality taro in infested countries.

This project aimed to develop biological controls for the taro beetle, and also investigated the combined action of pesticide control and bio-control. It aimed to implement any new methods for taro beetle management in environmentally sustainable cropping systems in Papua New Guinea (PNG) and Fiji.

This project relied upon bio-control methods that were identified during a previous project - testing the fungus Metarhizium anisopliae (Ma), and the virus Baculovirus oryctes (OrV).The researchers used a direct application method (on infested crops) and an indirect application method (on host plants and in breeding habitats) to apply different rates of Ma to various infested taro fields. They monitored the crops, assessed the damage to the beetle populations, and determined the rates of infection with Ma. They also collected and identified a reported widespread strain of a highly pathogenic Ma in the PNG highlands, assessed its mortality and infection rate, and compare it with other strains.
The team developed a low-cost production method for Ma, using rice as a fungal-growing substrate and a new chemical sterilisation method that can be used in countries that do not have access to sterilisation plants.
The scientists released the biocontrol agent OrV in taro plots and surrounding areas under various environmental conditions to find out if more frequent releases of the virus increased its persistence and led to reductions in taro damage and beetle populations. They also improved the currently used polymerase chain reaction (PCR) technique to diagnose the infection with OrV and transferred this technology and knowledge to local staff.
The team conducted chemical control tests in Fiji and PNG to optimise current pesticide techniques, and tested them for compatibility with biocontrol methods in PNG.

In total ACIAR provided six years of project funding (four years for research and two years for participatory research) for the management of taro beetles in Papua New Guinea and Fiji. The commissioned organisation, Secretariat of the Pacific Community (SPC), also used funds from the European Union's project on 'Plant Protection in the Pacific' to extend the activities in Kiribati, New Caledonia, Solomon Islands and Vanuatu, countries also facing menace of the taro beetles.

Taro beetles in the two ACIAR project countries caused losses of up to $A40 million in PNG and about $FJ1 million in Fiji. In Vanuatu and Solomon Islands it was virtually impossible to grow taro without beetle damage. In Fiji, taro growing for commercial purposes shifted to outer islands to get quality taro for export. This increased production cost and transportation problems. In PNG, taro growing was only possible by clearing new land from virgin forests.

In the first four years of the project, the project team conducted extensive laboratory and field experiments to evaluate bioagents and insecticides; these were selected from the Pacific Regional Agriculture Project, where initial studies had been done but no conclusive results obtained.

The results of these studies showed that Metarhizium anisopliae (Ma) when applied to soil in the taro planting holes gave about 30% of the marketable yield of taro corms. Although, the beetle mortality rates were high due to Ma infection, the infected beetles took longer to be killed. As a result damage to corms still occurred. Insecticides imidacloprid when applied to soil in the planting holes at the time of planting and three months after planting resulted in marketable yields of taro corms of up to 90%. Bifenthrin applied in the same way as imidacloprid also gave similar results. Imidacloprid used in low dosages with Ma also gave good control of the beetles, but not as high as when used alone. Residue analysis was also conducted which showed no trace of Bifenthrin in harvested taro corms. Imidacloprid was recorded below maximum residues levels in harvested corms.

Based on these results the team drew up recommendations on dosages levels, frequency and methods of application of the insecticides imidacloprid and bifenthrin and safety in their use, and other taro-growing practices. These were demonstrated to taro growers at Farmer Field Schools in PNG, Fiji, Vanuatu and Solomon Islands. The synergy on low dosages of imidacloprid with Ma was also demonstrated to taro growers. The taro beetle management package of practices was developed and launched at field days in PNG, Fiji and Vanuatu.

The project results are bringing confidence in taro growing communities in PNG, Fiji, Vanuatu and the Solomon Islands. There has been an increase in the sale of the recommended insecticides and more taro is now growing in the beetle-infested areas. Growing of taro on flat lands and repeated plantings are now possible, reducing the clearing of virgin forests for taro plantations. Quality taro can now be produced for food and as a cash crop with returns for the taro growers.

The project found an interim solution to the beetle menace, but further work is needed to gain a long-term solution for this persistent pest. The evaluation of new and effective insecticides with lesser environmental effect needs to continue. Other studies needed include evaluation of pheromones, which can play a vital role in dissemination of the Oryctes virus (Orv). Laboratory studies have shown that the virus is very effective in controlling the beetle and can be used in inaccessible areas of the beetle breeding grounds. Evaluations of plant-derived pesticides are also recommended. It is believed that before the advent of pesticides, farmers were using plant extracts to manage taro beetles in their plantations. Cultural practices used by farmers can be harnessed and put together with the modern approaches of pest control into a 'holistic' pest management practice for taro growers.