Parthenium hysterophorus is a species of flowering plant in the aster family, Asteraceae, that is native to the American tropics. Common names include Santa Maria Feverfew and Whitetop Weed. It is a common invasive species in India, Australia, and parts of Africa. P. hysterophorus invades all disturbed land, including farms  pastures, and roadsides. In some areas, outbreaks have been of almost epidemic proportions, affecting crop production, livestock and human health. In India, it is locally known as Congress Grass or Gajar Ghans. It was first present as a contaminant in imported wheat. It can trigger allergies and is a common cause of pollen allergy.
Contact with this plant causes dermatitis and respiratory malfunction in humans, dermatitis in cattle and domestic animals, due to the presence of toxin parthenin. The presence of Parthenium pollen grains inhibits fruit set in tomato, brinjal, beans, etc., and is also responsible for bitter milk disease in livestock fed on grass mixed with its leaves.
A beetle native to Mexico, Zygogramma bicolorata, was first introduced to India in 1984, and has since become widespread and well-established, and is capable of defoliating and killing this weed. P. hysterophorus has been found to be resistant to glyphosate, a popular systemic herbicide, prohibiting its control in this manner. Paraquat (Gramxone) solution is sometimes applied to plants, when the weeds are young. If the population in a cultivated field is light, it can be removed manually.
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