Chromolaena odorata (Siam weed) - a native of the West Indies - has become one of the world's worst weeds in the wet tropics of Asia and Africa. It is found as far south as Indonesia and Papua New Guinea, and is regarded as a very serious threat to Australia. If introduced to Australia, scientists expect the weed will rapidly colonise much prime agricultural and grazing land in the tropical north, and increase the frequency of bushfires in the dry season.

Investigations by the Commonwealth Institute of Biological Control (CIBC) from 1966-72 identified 225 natural enemies of the weed in the West Indies. Two of these were tried for biological control, and one, the leaf-feeding moth Pareuchaetes pseudoinsulata, gave excellent results in controlling Siam weed in Guam and the Marianas, though it was less successful in the Philippines and East Malaysia. The increasing importance of the weed in many parts of the world now means it merits a more concentrated attack.

Staff at the Alan Fletcher Research Station (AFRS) in Queensland have been concerned about the southward spread of Siam weed since the early 1980s, and were instrumental in having it listed as a prohibited import by the Australian Quarantine and Inspection Service. It is also a Declared Weed Category P2 (to be destroyed wherever found) in Queensland.

The 1st International Workshop on the Biological Control of Chromolaena odorata was held in 1988. ACIAR supported this Workshop by paying the attendance costs of two scientists from the Philippines and of Dr Rachel McFadyen from AFRS, who was an invited speaker because of her experience in investigating insects attacking the weed in the West Indies.

The objectives of this project, to be led by Dr McFadyen, are to:

. introduce the stem-galling tephritid fly Procecidochares under quarantine to Indonesia and the Philippines and carry out host specificity testing of this fly prior to obtaining permission for field release;
. conduct surveys of Chromolaena populations at selected sites in Indonesia and the Philippines before and after field release of Procecidochares, to monitor the establishment of the control agent and its effect on the weed;
. to repeat the above procedures with either the stem-galling moth Mescinia parvula or the stem-boring agromyzid fly Melanagromyza eupatoriella, as judged most appropriate by the project leader at the time; and
. provide experience to Indonesian and Philippine project personnel with the principles and practice of a biological control operation, including methods used for host-specificity testing of control agents.

The earlier CIBC study evaluated suitable control agents. An application will be made to import Procecidochares into Indonesia, for host-testing at the Marihat Research Centre for Estate Crops in Sumatra. The experienced staff at the laboratory have already handled the moth P. pseudoinsulata for the control of the weed in a project supported by the European Community for West Africa and Indonesia. Galls will be collected in Bolivia or Brazil and transported to Marihat, where an AFRS entomologist will supervise the establishment of a parasite-free colony in quarantine. Parasite-free galls will also be sent from Marihat to the Department of Agriculture quarantine facilities in Manila. The AFRS entomologist will then supervise the establishment of a colony at Los Baos. Nucleus colonies will also be sent to Mindanao to enable mass rearing and field releases. Similar procedures will be followed for the second insect selected.

If the project is successful, releases of the insects will be organised in islands of Indonesia and the Philippines. Weed population densities will be measured before and after release of the agents and estimates will be made of economic losses attributable to the weed.

Contact with entomologists working on Chromolaena control in other countries will be conducted both directly and through the International Chromolaena Secretariat, University of Guam.

Successful biological control of Siam weed will remove the need for costly manual or chemical control. Pasture productivity will increase, and, once established, the agents should spread unaided through weed-infested areas. Other interested countries will be able to obtain nucleus colonies. Australia will benefit from the control of the weed in adjacent countries, particularly eastern Indonesia and Papua New Guinea. At present huge quantities of seed are produced in these areas, greatly increasing the chance of the weed's accidental introduction into northern Australia because of regular traffic.