Overwintering mortality of and host attack by the stem-boring weevil, Mecinus janthinus Germar, on Dalmatian toadflax (Linaria dalmatica (L.) Mill.) in western Canada

Abstract Populations of the weevil Mecinus janthinus Germar (Coleoptera: Curculionidae) were monitored at 13 sites in western Canada during the period 1994–1999 to assess the role of overwintering mortality on the establishment of this biocontrol agent introduced against the weed Dalmatian toadflax, Linaria dalmatica (L.) Mill. (Scrophulariaceae). Results indicated that M. janthinus is intolerant of freezing, as evidenced by adult mortalities of 75–100% occurring in about 30% of the site-years examined and whenever winter temperatures reached ⩽−28 °C. Exposure to low sub-zero winter temperatures explained 70% of the variation in adult mortality for sites in British Columbia. An 8-fold increase in adult mortality occurred in winter 1997/98 at one Alberta site monitored soon after temperatures dropped to ⩽−30 °C. Some inconsistencies in the relationship between winter temperatures and adult mortality at the Alberta site may be explained by the presence of insulating snow cover during the coldest temperatures of winters 1993/94 and 1996/97 and unseasonably cold temperatures during the spring of 1995. Despite the high periodic mortalities suffered by M. janthinus at most sites, the incidence and intensity of weevil attack on its host generally increased with time, thus demonstrating the resiliency of weevil populations.