POLLEN AVAILABILITY, SEED PRODUCTION AND SEED PREDATOR CLUTCH SIZE INA TEPHRITID THISTLE SYSTEM

We develop a simple model explaining clutch size behaviour of Orellia ruficauda on its principle host in North America, Cirsium arvense, Off spring of flies feed solely on thistle seeds and seed production is po llen-limited. Thus, female flies risk reduced offspring fitness when c ommitting large clutches to hosts (female flower heads) occurring in l ocalities where male plants are locally absent. We therefore predict t hat attacked hosts will contain fewer eggs in such localities, a predi ction that is consistent with data obtained in the field: large clutch es are never laid in flower heads in low-pollination localities. Howev er, larvae reared from such low-quality hosts are significantly smalle r on average and will therefore carry smaller egg loads as adults. Sma ll clutches in poor-quality hosts may thus be an expression of lower p er-adult fecundity. Nevertheless, sufficient numbers of large, fecund flies are produced in low-pollination localities to make this last exp lanation less convincing.