Seasonal development of wheat midge, Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae), in Saskatchewan, Canada

Wheat midge, Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae), was first detected as early as 1901 in western Canada. The first major outbreak in Saskatchewan was recorded in 1983. In 1984, Saskatchewan wheat midge populations were found to be parasitized by the egg parasitoid, Macroglenes penetrans (Kirby) (Hymenoptera: Pteromálidae). This paper describes the effect of temperature on diapause termination, larval and pupal development, and on emergence of field populations of adult wheat midge, S. mosellana, and its parasitoid, M. penetrans, in Saskatchewan. Wheat midge and parasitoid populations were investigated from 1984 to 1985 in five wheat fields where wheat midge populations ranged from 5256 to 43,798 larvae/m2. The appearance of midge larvae in soil samples on May 14 and May 9 in 1984 and 1985, respectively, marked the end of overwintering diapause. Pupae first appeared in samples on June 15 (139 degree-days (DD); >6 °C) and June 10 (125 DD) in 1984 and 1985, respectively. Adult midge and parasitoids began to emerge at all locations on July 6 (411 DD; >6 °C) and on July 9 (447 DD) in 1984 and 1985, respectively. In relation to the pest–host plant synchrony, adults of both wheat midge and parasitoids were present throughout July, with peak midge emergence coinciding both years with the emergence of primary wheat heads from the boot. We observed the first primary heads from about July 12 to 15 at the five sites. Therefore, the relatively long wheat midge emergence has significance because it contributes to damage on the first and second tillers and to a possible increase in populations for the following year. The use of temperature and day-degree accumulation in relation to development is discussed as a predictive tool for management decisions.