Factors Determining the Host Range of Two Tortoise Beetles, Cassida nebulosa L. and C. piperata Hope (Coleoptera: Chrysomelidae) in Japan

We investigated the relationship between host-plant use by two tortoise beetles, Cassida nebulosa and C. piperata, in a field and their performance under laboratory conditions. Sixteen plant species were grown in the experimental field with C. nebulosa being observed to grow on three chenopodiaceous species (Chenopodium album, C. album var. centrorubrum, and C. ficifolium), while C. piperata grew on the same three chenopodiaceous species plus three amaranthaceous species (Amaranthus blitum, A. hybridus, and A. retroflexus). Host-plant selection in the field was directly correlated to the larval growth under laboratory conditions. Thus, the two tortoise beetles selected plant species that were suitable as larval food under natural conditions. In the laboratory experiments, the survival period of adults and the number of eggs produced showed that adults have wider food ranges than larvae. Therefore, food quality is less important as a limiting factor for adults compared to larvae. In addition, the field observation showed that C. nebulosa laid eggs only on suitable host plant species, whereas C. piperata laid eggs on both suitable hosts and non-hosts.