Emergence patterns and cold tolerance of adult Bathyplectes curculionis (Thomson) were studied in relation to weather conditions that are typical during fall and winter in the Southern Great Plains. Laboratory studies were conducted to confirm that daylength and environmental temperatures prevailing during fall and winter are conducive to termination of diapause, pupal development, and emergence of B. curculionis adults. Mean Julian dates for first detection of the parasite in larvae of Hypera postica (Gyllenhal) were 69 and 72 for two field research sites in Oklahoma. These dates indicate that adults of B. curculionis emerge during winter and are frequently subjected to freezing temperatures as they begin parasitizing the first larvae of H. postica to infest alfalfa, Medicago sativa L., each year. Both laboratory and field studies demonstrate that >80% of adult parasites can survive repeated exposures to daily low temperatures ranging from 0 to −9°C. This level of cold tolerance indicates that the adults can survive during February and March when mean low temperatures are −1 and 3°C, respectively, in central Oklahoma. Although some adults of B. curculionis do emerge during fall and winter in the Southern Great Plains when there are no hosts available, this potential disadvantage to effective biological control is offset somewhat by the cold tolerance of adults, which allows survival during late winter and spring when they parasitize the first larvae of H. postica to infest alfalfa annually.
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