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The introduced Mexican beetle, Zygogramma bicolorata Pallister (Chrysomelidae), has brought about large scale defoliation of the noxious weed Parthenium hysterophorus L. (Asteraceae) in Bangalore and surrounding areas. Studies carried out over a period of two years in two uncultivated parthenium infested horticultural fields showed that defoliation by the beetle can cause up to 99.5% decline in weed density, provided the soil is left undisturbed. Forty different species of plants, formerly suppressed by the weed, were noticed to grow in the areas vacated by parthenium. The studies also indicated that the rate of decline of parthenium and the degree of diversity of succeeding plant species in fallow lands, after defoliation by Z. bicolorata, may vary depending on the duration of weed occupation and the history of land utilisation.