Parthenium hysterophorus L., a tropical American weed belonging to the family Asteraceae is an invader noxious weed. It was accidentally introduced into India, more than fifty years ago. The seeds of Parthenium entered our country along with wheat grains when wheat was imported as PL-480 food aid to India. It was first noticed near Poona, Maharashtra in 1951, as an escape and was reported as a new record for India in 1956 (Rao, 1956). Over the years it traveled along the railway tracks and soon spread all over the southern part of peninsula, as the climatic conditions here were favorable for its growth. Its presence was first recorded in Mysore, Karnataka on 23.11.1971 and by the end of 1972; it had invaded the Mysore district (Rao, 1981)
Because of its proliferate nature, absence of competitors, untouched by cattle and goats and inherent property of Asteracea family - exuding chemicals from their roots which inhibit the growth of surrounding plants, Parthenium aggressively spread and colonized. Eradication measures like physical removal, spraying chemical herbicides, biological control methods such as growing another weed Cassis sericea and releasing a specific herbivorous insect pest Zygogramma bicolorata turned out to be futile and became a conspicuous and natural element in our flora.
It is observed that butterflies, Mottled Emigrant Catopsilia pyranthe, Small Salmon Arab Colotis calais, Plain Orange Tip Colotis eucharis, Common Leopard Phalanta phalantha, Common Silverline Spindasis vulcanus and African Babul Blue Azanus jesous are feeding on Parthenium flowers.
It seems Parthenium has reached the pinnacle of its invasiveness in Bandipur National Park, Karnataka, where we observed the tamed elephants of the Forest Department, in the summer of 2000, to eat this plant with evident relish. This phenomenon has been observed four times in the last six years. Even wild elephants may eat this plant, though no visual observation is reported.
These observations indicate that an invading alien plant is being gradually naturalized to a new environment in a short span of time.
K.B.Sadananda and A.Shivaprakash