FACTORS INFLUENCING THE FALL EMERGENCE OF BESSA HARVEYI (TACHINIDAE: DIPTERA)

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Abstract
Bessa harveyi (Townsend) is a common parasite of the larch sawfly, Pristiphora erichsonii (Hartig), in southeastern Manitoba but it is ineffective in preventing or terminating outbreaks of its host. Bivoltinism acts as a mortality factor in B. harveyi populations because bivoltine individuals do not contribute progeny to the next generation. Heavy mortality due to bivoltinism occurs when the parasite eggs are laid under long-day conditions and the summer is warm and prolonged. Regression analyses show that most of the variation in the incidence of bivoltinism between years and between dates within years can be described by equations involving the incidence of bivoltinism in the previous year, photoperiod, and temperature. The importance of a genetic factor in determining the incidence of bivoltinism was indicated by a strong negative correlation between the incidences in consecutive years. Photoperiod and temperature during development were positively correlated with incidence. The persistence of bivoltinism in Manitoba is postulated to be related to variability in weather and host development plus developmental advantages associated with the bivoltine phenotype.