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

We develop a simple model explaining clutch size behaviour of Orellia ruficauda on its principle host in North America, Cirsium arvense, Offspring of flies feed solely on thistle seeds and seed production is pollen-limited. Thus, female flies risk reduced offspring fitness when committing large clutches to hosts (female flower heads) occurring in localities where male plants are locally absent. We therefore predict that attacked hosts will contain fewer eggs in such localities, a prediction that is consistent with data obtained in the field: large clutches are never laid in flower heads in low-pollination localities. However, larvae reared from such low-quality hosts are significantly smaller on average and will therefore carry smaller egg loads as adults. Small clutches in poor-quality hosts may thus be an expression of lower per-adult fecundity. Nevertheless, sufficient numbers of large, fecund flies are produced in low-pollination localities to make this last explanation less convincing.