Onions in northern Nigeria flower during the dry season when the relative humidity and temperature are unfavourable for insect populations. Selfing onions under paper bag resulted in a seed set of less than one per cent and this is attributed to injury caused by high temperatures within the bag. Attempts to breed Chrysomyia putoria failed, but Musca spp. were found breeding in waste from pig pens. The method of trapping these flies and their use in pollination cages constructed from wire and cloth are described. By this means seed set was raised to 10-20 per cent in plants caged for three days and up to 60 per cent after caging for two weeks.