An individual based population study of an insect herbivore (Urophora jaceana) and its host plant (Centaurea nigra)

Paul, Roger Philip (1998). An individual based population study of an insect herbivore (Urophora jaceana) and its host plant (Centaurea nigra). PhD thesis The Open University.



Urophora jaceana Hering (Diptera; Tephritidae) is a gall-forming insect of the capitula of Centaurea nigra L. (Compositae). Oviposition occurs at an early stage of flowerbud development. The aim of this thesis is to relate population processes of regulation of fly numbers and phenotypic selection on plant characters to aspects of individual behaviour of Urophora jaceana, through the development of a Dynamic State Variable Model of female fly oviposition choices.

The dynamic model incorporates larval survival functions obtained experimentally, and foraging time budgets estimated from field observations. The model predicts clutch size and super-oviposition at varying egg-loads, age, and host density. Super-oviposition is more likely at low encounter rates with flowerheads, and this prediction was tested experimentally and found to hold. Using a Monte Carlo Simulation, competition for oviposition sites, and the inverse dependence of larval survival on clutch size at high egg densities, emerge as the most likely regulating factors of fly numbers.

The effect of gall infestation on plants was found to be limited to reducing seed production. No density dependent regulation of fly density was discovered by k-factor analysis in the gall formation to pupal phase of the life cycle. The intensity of gall infestation was found to be inversely dependent on day of budding of flowerbuds, and relates to resource availability. Measuring the intensity of selection on three plant characters revealed that gall infestation exerts a directional selection pressure on the day of budding of plants. A simple heritability test suggests that there is an evolutionary response to selection.

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