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 varia tions of resources (resource tracking), and the population characteristics of the ladybird beetle Harmonia axyridis, I analyzed the results of a 3-yea r field observation at 24 sites (seven plant species) on eight species of a phids. The seasonal changes in the number of beetles estimated by the Jolly -Seber method were significantly correlated with those of aphids in the tot al area. The estimated values of population parameters suggested frequent i mmigration and emigration of the H. axyridis population, although reproduct ive 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 pa tterns, which corresponded with the movement within and between the subpopu lations when an assemblage of H, axyridis occurring on the plants of the sa me species or genus was regarded as a subpopulation. Adult movement intense ly occurred within a subpopulation, although the beetles moving between sub populations 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 s earching and reproduction, maintains a stable population in heterogeneous a nd temporal habitats by its resource tracking mechanisms.