Population Dynamics of an Herbivorous Lady Beetle, Henosepilachna niponica, in a Seasonal Environment

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Abstract
(1) A population study of the thistle-feeding lady beetle, Henosepilachna niponica, was conducted from 1976 to 1980 to two sites along the River Ado in central Japan. Life tables were constructed for five generations at the two sites. (2) Two basic population processes were operative: a density-independent survival process causing population fluctuations, and a density-dependent reproductive process generating marked population stability relative to the availability of food resources. (3) Predatory arthropods were the most important cause of mortality of eggs and larvae. A high level of predation was responsible for the low adult density of one of the populations. The predation was restricted to early in the reproductive season. (4) Seasonal adversity (heat stress in midsummer, and autumn flood) decreased the survival of newly-emerged adults but from then on adult survival up to the reproductive stage was more likely to be size- and sex-dependent. (5) Foliage damage of thistle plants by beetle-feeding and leaf senescence was particularly clear after summer. Leaf amino acid and water contents declined from the beginning of the growing season. (6) Seasonal change in the causes of the major mortality factors coupled with deteriorating food resources profoundly affected the demographic features and individual success from egg to the reproductive stage of the lady beetle.