Population field studies on the aphidophagous ladybird beetle Harmonia axyridis (Coleoptera: Coccinellidae):: resource tracking and population characteristics

To clarify functional and numerical responses to temporal and spatial variations of resources (resource tracking), and the population characteristics of the ladybird beetle Harmonia axyridis, I analyzed the results of a 3-year field observation at 24 sites (seven plant species) on eight species of aphids. The seasonal changes in the number of beetles estimated by the Jolly-Seber method were significantly correlated with those of aphids in the total area. The estimated values of population parameters suggested frequent immigration and emigration of the H. axyridis population, although reproductive rates between spring and summer were rather stable all 3 years (1.87-3.49). The staying time and the daily number of adults and eggs at each site were influenced not only by a single factor but also by interactions among time and quantity and quality of the prey. The adult movement showed two patterns, which corresponded with the movement within and between the subpopulations when an assemblage of H, axyridis occurring on the plants of the same species or genus was regarded as a subpopulation. Adult movement intensely occurred within a subpopulation, although the beetles moving between subpopulations had a significantly greater chance to reach the habitat with a high aphid density. The habitats of H. axyridis could be categorized into a suitable habitat for survival and reproduction and a temporal refuge. The results obtained here suggest that H, axyridis, with high ability of prey searching and reproduction, maintains a stable population in heterogeneous and temporal habitats by its resource tracking mechanisms.