ORIGINAL ARTICLE Popul Ecol (2000) 42:115–127 © The Society of Population Ecology and Springer-Verlag Tokyo 2000 Naoya Osawa Population field studies on the aphidophagous ladybird beetle Harmonia axyridis (Coleoptera: Coccinellidae): resource tracking and population characteristics Received: March 8, 1999 / Accepted: April 25, 2000 Abstract 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 signifi- cantly correlated with those of aphids in the total area. The estimated values of population parameters suggested fre- quent immigration and emigration of the H. axyridis popu- lation, although reproductive rates between spring and summer were rather stable all 3 years (1.87–3.49). The stay- ing 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 cor- responded with the movement within and between the sub- populations 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 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 re- sults 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. Key words Aphid · Polyphagous · Population Dynamics · Predator–prey interaction · Habitat N. Osawa Laboratory of Forest Ecology, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan Tel. +81-75-753-6077; Fax +81-75-753-6080 e-mail: osawa@kais.kyoto-u.ac.jp Introduction Many studies have been performed on the predator– prey interaction, which is one of the most important fea- tures in ecology (Solomon 1949, 1964; Holling 1961; Begon et al. 1986; Kuno 1987). From the aspect of population ecology, the role of predation as a regulatory or disturbing factor(s) of a prey population has been studied through theoretical and empirical researches (Kuno 1987). Life table analysis of field insect populations is closely related to the demands of pest management (Taylor 1984; Bellows et al. 1992). In many animal populations, it is known that resources determine the level of a population (Begon et al. 1986). However, resource tracking, which is defined as functional and numerical responses to temporal and spatial variations of resources, is not fully clarified, especially in the field population of insect polyphagous predators. Furthermore, in previous population studies of insect polyphagous pre- dators, the predator–prey interaction mainly has been ana- lyzed from the aspect of the prey: the population dynamics of a predator, especially the view of the temporal and spatial variations of prey and population characteristics of pre- dators in a natural population, have not been taken into consideration. Therefore, the population dynamics and pro- cesses should be investigated from the aspect of the preda- tor, which is needed for a further clarification of the predator–prey interaction. Some predators, for example, birds, expand their food niche when the preferred prey type becomes scarce and shrink it when the prey becomes abun- dant (Korpimäki 1986). Moreover, breeding densities of predatory bird populations catch fluctuations of prey with- out a time lag (Korpimäki 1985; Korpimäki and Norrdahl 1991). Thus, vertebrate predators change their behavior and life history strategies in relation to the dynamics of the prey population. However, previous studies on polyphagous in- sect predators have not analyzed the temporal and spatial dynamics of the predator–prey interaction, especially in the field. It is important to clarify the resource tracking of insect predators: i.e., how insect predators utilize prey in tempo-