Some spreading or noxious plants of Madagascar.
AbstractInformation is given on the following species: 1. Mucuna pruriens. Widespread in N.W. Sakalava, up to altitudes of 700-800 m; its irritating hairs may be carried by the wind; cannot be controlled by burning; common in Reunion. 2. Lantana camara var. aculeata. Introduced some time after 1901, now covers thousands of hectares in all the eastern region and in Imerina; eradication appears impossible but further spread could be prevented; is also a weed in Reunion. 3. Rubus mollucanus. Fairly recently introduced from Reunion; appears to be spreading. 4. Agave ixtli. Introduced as a fibre plant during the 19th century, it has spread in the sub-desert areas of the south-west and south, where it was originally planted for protective hedges; further planting took place after 1925 when the populations of Opuntia monacantha were destroyed by the introduced moth, Cactoblastis cactorum; areas formerly occupied by O. monacantha have been colonized by A. ixtli. S. Acanthospermum hispidum. Recently introduced and now widespread along paths, roads and in the neighbourhood of dwellings. 6. Tribulus terrestris, common in drier sandy areas at lower altitudes; the related T. cistoides is less widespread. 7. Clidemia hirta. Seed from Java was sown in a botanic garden near Tamatave circa 1915; it has formed spiny thickets in all the Tamatave region. 8. Erigeron naudini. Of American origin, it has become a widespread arable weed during the last 40 years, but is not of major importance. 9. Cyperus rotundas. Common in rice and arable fields up to an altitude of 1400 m, but not a major weed. 10. Pteridium aquilinum. Invades secondary bush, forming dense stands, particularly in the centre of the island and in the western boundary of the plateaux region. It has spread owing to attempts to control it by burning. 11. Eichhornia crassipes. The most important water weed; introduced circa 1920 from Reunion as an ornamental; in 1928 a law was passed to prevent its spread, but proved impossible to enforce; large manual collection was attempted in 1947 and spraying trials (using 2, 4-D) in 1951-2 gave variable results; despite continuing control measures the weed is now definitely established; a related mud-rooting species, E. natans, is not a weed. 12. Pistia stratiotes. Common on stagnant water at altitudes up to 1200 m.
Eradication campaigns have been undertaken for some years against Mundulea sericea (common in the plateaux region) and M. monantha (in the Ankaratra region) as they are widely used as illegal fish poisons. Derris trifoliata is used for the same purpose in estuaries. Eradication campaigns were carried out in the 19th century against Cerbera venenifera and Erythrophleum couminga, ordeal trees, but they have now been abandoned.-D.B.