Insect Communities Associated with an Invasive Plant, the Common Ragweed, Ambrosia artemisiifolia L., in Western Japan

The insect community, including herbivores, predators and parasitoids associated with the common ragweed was,orvcyed along the banks of the Yodogawa River and the Yasukawa River, Western Japan, in 2001-2005. Ten rhivores and 20 predators (including three parasitoids) were recorded. Of the herbivores, five species, including Iha dominant three herbivores, were exotic, while two species were indigenous. Of these, Lixus acutipennis and tme vvu,v eo/feae were newly recorded as common ragweed feeders. All 11 species of predators were indigenous. The two dominant. herbivores, Ophraella comrnuna and Corythucha marrnorata, were most abundant in August. Mortality of ilia common ragweed was greatest in early September, just after the densities of these two invasive herbivores peaked. As a result, all the plants died in mid September, and only 4.5% of the plants could set seeds. This catastrophic plant death was thought to be caused by heavy infestations of these two exotic herbivores. The growth period of O. communa reared on ragweed leaves infested by C. marmorata, was significantly longer than those reared intact leaves. This result suggests that an indirect interspecific relationship via food plants exists between the two dominant herbivores.