DYNAMICS OF A CEUTORHYNCHUS-NAPI POPULATION IN WINTER RAPE FIELDS

A stem weevil population was studied during three generations from 198 2 to 1985 in a rape agroecosystem. Management of the fields was unifor m, including no use of insecticide. The population size (c. 5 ha) at c rop invasion increased from 27000 (+/- 14600) adults in spring 1983 to 224000 (+/- 30000) in 1985, and reached a maximum of 750000 during em ergence in winter 1984-1985. Realized fecundity varied from 16 to 114. On die average only one third of the emergent adults infested a rape field. Within-field patterns after insect invasion were random in only 3 fields; in die other 10 the proximity of fields sown with rape the previous year (PY plots) explained the patterns. Fields closer to PY p lots were ten fold more infested than fields further away. The contrib ution of each field to the overall population varied greatly each year . Overall population dynamics, dispersal of the stem weevil adults aft er emergence, and isolation of the stem weevil population are discusse d.