5th December 1999
By Magdon Jayasuriya
Considerable areas in cultivated and barren lands including road-sides in urban Vavuniya are being invaded by an alien weed called "Whitehead, White broomweed, Wormwood or Mugwort". Botanically known as Parthenium Hysterophorus, it is a herbaceous member of the sunflower family (Asteraceae) growing to about one metre in height. Each plant is capable of producing over one thousand small white flowerheads and two to three thousand tiny one-seeded fruits which are capable of dispersal into various localities of the island. Reports indicate that the species has already reached the level of a troublesome weed in agriculture in the affected areas in Vavuniya. Locally it is known as "Indian Weed."
Most troublesome weeds in Sri Lanka are exotic and they have arrived here sometime in the past, mainly through human activities. Once established in favourable habitats, these alien plants spread fast, and in extreme cases, they become invasive even displacing the native species from their natural ranges.
The species is believed to be native to the West Indies, but found in tropical America extending to Florida and Texas in the USA and to Chile and Argentina in South America.
According to some authors the weed was been accidentally introduced to India in 1810, possibly through wheat imports from America. Presently it is an aggressive weed in parts of India, especially in the South.
The first appearance of this species in Sri Lanka was noted near the sites occupied by the Indian Peace Keeping Force (IPKF) mainly at the School of Agriculture and then spreading into the Government Seed Farm and Agricultural Research Centre in Vavuniya. It is suspected that the initial seeds of the weed were internally transported by goats which were brought from India to feed the IPKF. Seeds have apparently survived in dung and germinated in the local fields. However, some believe that the seeds came as contaminants in mustard brought from India to the IPKF Camps. The occurrence of the weed in other IPKF Camp sites elsewhere in the North and East is not yet known.
A striking observation was made during October this year, when a population of this plant was noted in Pallekele in the Kandy District. The plants were luxuriant, reaching 1.5 metres in height and flowering was noted in late October. Some suspected that the initial seeds would have come along with vegetable transport from Vavuniya to Kandy.
The species has many characteristics that qualify it to be a potentially invasive plant in Sri Lanka sooner or later. K. Nagathasan, Principal, School of Agriculture in Vavuniya reports that the local goats and buffaloes lavishly consume the plant while cattle do not eat it. Therefore, the livestock transport could possibly be an effective mode of dispersal of the weed into other parts of the country. The pollen is known to inhibit fruit set in other species while a certain chemical compound in the plant could cause dermatitis.
Immediate control measures should be taken in view of its potential threat to agriculture and biodiversity. Intensive manual weeding and burning during successive seasons until the weed completely disappears from the landscape in all affected areas is recommended.
The writer is Director of the Plant Genetic Resources Centre, Gannoruwa
The Ministry of Forestry and Environment and the Central Environment Authority have already been informed of the known locations of the weed and photography supplied to help in its identification.
It is expected that the Environment Division of each relevant Provincial Council would take action for the immediate control of this weed. The Plant Protection Service of the Department of Agriculture is making strategic plans to control it. The public is requested to be alert to the appearance of this weed and to inform the relevant authorities and assist in its eradication.
Assistance in identification could be obtained from the Plant Genetic Resources Centre or National Herbarium of the Department of Agriculture by sending specimens. Photographs also could be supplied.
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