Population Dynamics of Tephritis conura Loew (Diptera: Tephritidae): Determinants of Density from Three Trophic Levels

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Abstract
(1) The complex of endophytic insects in flower heads of Cirsium heterophyllum (L.) Hill (Cardueae) was dominated by the tephritid fly Tephritis conura Loew (Diptera: Tephritidae). An average of 40-50% of the flower heads in a plant stand with a mean of 8.9 individuals per head were attacked by T. conura. (2) Larval densities of T. conura differed from one plant stand to another. However, within a plant stand annual density fluctuations were small during the 4 years of the study. In spite of high densities in some plant stands, the carrying capacity of the heads was rarely reached. (3) There was no apparent regulation of fly numbers by density-dependent processes during larval development over a wide range of natural densities. Mortalities from parasitoids and predation by microlepidoptera were density-independent. (4) After experimental ''extermination'' of T. conura subpopulations, plant stands were recolonized within one generation. Densities before and after the manipulation were comparable. (5) In natural plant stands levels of infestation were determined every year by colonization processes. Attack rates were mainly influenced by the synchronization of the oviposition period with bud development in a plant stand. (6) Density-dependent ''regulation'' occurred only at high experimental densities and was manifested in adult behaviour during the oviposition period (interactions between searching adults, ability to discriminate against already infested buds, adult dispersal).