Abstract
Among three host races of Henosepilachna vigintioctomaculata, adults of two races distributed in Hokkaido show a mass feeding on Panax japonicus which is not their natural host plant. Causes of the mass feeding were analysed. Results suggested that feeding incitant factor(s) are produced by a mass effect, and that feeding stimulant(s) in leaves of the plant make the ladybirds continue to feed. The other race distributed in Honshu did not represent the mass feeding, probably due to insusceptibility to the stimulants. Although the ladybird could grow with the plant, the adults reared with it in larval stage did not show much shift in their food preference.
