INVESTIGATING POPULATION DYNAMICS OF THE WHEAT STEM SAWFLY (CEPHUS CINCTUS) (HYMENOTERA: CEPHIDAE) IN CANADIAN AGROECOSYSTEMS
The wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), has been a serious economic threat to wheat production in the northwest Great Plains region of North America for over 100 years. Despite immense research efforts on C. cinctus population dynamics, there remain knowledge gaps. The objectives of this research were to determine how winter and early spring abiotic factors influence: 1) C. cinctus larval mortality and the spatiotemporal variation in the efficacy of larval mortality factors, and 2) adult life history traits of C. cinctus and one of its natural enemies, Bracon cephi Gahan (Hymenoptera: Braconidae). Post-harvest samples were collected from fields in southern Alberta to set baselines for the occurrence of live C. cinctus and larval mortality due to parasitism, fungal infection, and unknown reasons. Fields were re-visited the following spring (once in 2020 and twice in 2021) to measure changes in C. cinctus larval populations. We observed that the number of viable (i.e., living) C. cinctus larvae changed significantly between geographic sites but not over time. This conclusion highlights that overwintering and early season abiotic conditions are unlikely to affect C. cinctus population dynamics. The field-level variation underscores the importance of scouting individual fields if high C. cinctus population densities are expected in a given area. Laboratory experiments were conducted to test the effects of cold temperatures in early spring, that are known to prolong post-diapause development, on adult life history traits (e.g., longevity, body size, lipid content, and egg capacity) of C. cinctus and B. cephi. For C. cinctus, adult longevity and male lipid content significantly decreased when post-diapause development was extended. In contrast, neither B. cephi adult longevity nor body size were affected by the prolonged post-diapause development periods. These results provide novel information on how the post-diapause biology of two important native hymenopterans differ in response to temperature. Altogether, this research shows that while post-diapause abiotic conditions may not increase C. cinctus mortality, cold temperatures that delay C. cinctus adult emergence can greatly impact several adult life history traits that could impact population dynamics. This information will be useful for the development of C. cinctus phenology and population forecasting models.
wheat stem sawfly, agricultural entomology, post-diapause
Master of Science (M.Sc.)