Canada thistle, Cirsium arvense (L.) Scop., is a noxious, widespread, aggressive, perennial weed of rangeland, pastures and agricultural lands across most of the world lands and control of this vigorous weed is difficult as in rangelands and natural areas, chemical control is expensive and not an economical control option. In order to study the efficacy of thistle tortoise beetle (Cassida rubiginosa) for biological control of Canada thistle and determining its effective density (0, 5, 10, 15, and 20 insect per plant), a series of studies were conducted. Shoot length, biomass weigh and number of flowers per plant significantly declined, due to effect of C. rubiginosa feeding damage. Feeding 20 insects per plant were reduced 78% of the biomass and 94% of weed seed production. Increasing the number of insect per plant decreased the number of flowers and eventually decreased the number of seeds per plant. There was significant difference of capitulum s number per plant. The number of capitulums of C. arvense at 10, 15, and 20 larvae per plant reduced 71%, 88%, and 94%, respectively. Host range studies were conducted in 20 crops with two methods of no-choice and multiple-choice, and in 22 weeds through field surveys. The results showed that plant feeding and female ovipositor by green Thistle was observed in Russian knapweed, Common Russian thistle, Bull thistle and Canada thistle. The insect fed on safflower but only about 5% of plant shoots and only in no choice treatments; however this beetle did not deposit any eggs on sunflower. Green thistle beetle could be a promissing classical biocontrol agent for Canada thistle in grasses, natural grasslands, agricultural fields and pastures.

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