EFFECTS OF VARIETY AND DATE OF SEEDING SPRING WHEATS AND LOCATION IN THE FIELD ON SEX RATIO OF THE WHEAT STEM SAWFLY, CEPHUS CINCTUS NORT. (HYMENOPTERA: CEPHIDAE)

The sex ratio of the wheat stem sawfly, Cephus cinctus Nort., was affected by date of seeding, variety, and distance from source of infestation but not by diameter of host stem. The results suggest that the sex ratio of the progeny depends on the proportion of adults that are males and hence on the amount of fertilization of the eggs at the time of oviposition; increased percentages of male adults increase percentages of female progeny. Most adults at the beginning of the flight period are males whereas, at the end, most are Females. Most progeny from the earliest-seeded plants, which are infested first, were females, whereas most from the latest-seeded, which are infested last, were males. Red Bobs wheat develops more rapidly than Thatcher and is infested earlier; more female progeny emerged from Red Bobs than from Thatcher. The percentages of female progeny were highest from the margins of the fields evidently because most male adults remained there.The sex ratios of sawflies from different dates of seeding of the resistant wheat Rescue were not consistent with those from the susceptible wheats. The inconsistencies were evidently caused by differences in survival of the individuals in different locations in the resistant stems. In susceptible wheats the first-laid were most likely to survive, whereas in resistant wheats location rather than time of oviposition determined survival.