| With the rapid development of international trade and global economic integration, the invasion of alien organisms aggravate day to day. Although only a few exotic plant species invade natural communities and develop high population densities, they change species composition, structure and function of local ecosystem and do tremendous harm to the biodiversity. Now biological invasions became one of the most serious threats to global biodiversity, and their influences are second only to habitat destruction. Our country is one of those influenced seriously by alien invasive organisms. Recently The National Environmental Protection Agency of China has publicized firstling 16 kinds of important invasive species, including 9 kinds of invasive plants, and Alternanthera philoxeroides just was one of them. Alternanthera philoxeroides (alligatorweed) is a herbaceous amphibious weed of Amaranthaceae, native to South America.Alternanthera philoxeroides is now found as a serious weed from tropical to warm temperate regions, including the USA, China, India, South-East Asia, Australia and New Zealand. It is thought to have been brought to China during the l930s, and later widely cultivated and spread in southern China as fodder during l950s. Because of its wide range of habitats to invasion and rapid growth, it is difficult to control, so Alternanthera philoxeroides pose serious threats to local biodiversity, ecosystems and social-economic. The invasions of alligatorweed in China have caused considerable concerns. After its invasion to China, alligatorweed took place in a large-scale. Through field observation, we found that the local spread of insect named Cassida piperata Hope not only feed on the alien invasive weed alligatorweed, but also have a large abundance on them. This paper studies effects of alligatorweed on the feeding habits, development and field happening dynamics of Cassida piperata since its invasion. The results are as follows.1. Under natural conditions, there is no significant difference between abundance of the insect Cassida piperata on local host plants (Chenopodium glaucum Linn., Amaranthus spinosus Linn. and Alternanthera sessilis Linn. ) and invasive weed alligatorweed. This shows that alligatorweed has become an important one of its host plants although it's an exotic invasive grass.2. In order to study whether alligatorweed has an influence on the feeding habits of Cassida piperata or results in host-shift since its invasion, we conducted a selective test to research the preferences of adults of Cassida piperata from different geography populations to different host plants. The results indicate that Cassida piperata still preferred to feed and lay eggs on local host plants, especially on the A. spinosus.3. The results of non-selective test also show that host plant has a significant influence on the development of the larval of Cassida piperata. Compared with on alligatorweed, the larval has a faster development rate on local host plants, but the insect also can complete entire life cycle on alligatorweed. Alligatorweed has become one of the important hosts of Cassida piperata. Its appearance expands the host range of Cassida piperata and has a far-reaching impact and significance on the population occurrence of Cassida piperata.
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