Sheffield et al. 



JESO Volume 139, 2008 



O. lignaria. Tepedino and Torchio (1982) found that smaller than average offspring of 

 both sexes of O. lignaria have a greater likelihood of dying during the winter, and that 

 mortality in females (the larger sex) is higher than in comparable weighted males. In this 

 study, variability was observed within the weight range of both sexes (Fig. 4). In a study 

 comparing winter mortality of two distinct populations of O. lignaria, Rust ( 1 995) found that 

 mortality was significantly higher in bees from Reno, Nevada than those from Logan, Utah 

 when both populations were wintered in the later, more northern location. The differences 

 in mortality in that study (Rust 1995) were almost entirely attributed to the differences in 

 winter temperatures (both average monthly temperatures and extreme minima) normally 

 observed between the two locations. Obviously, winter temperatures are a major factor 

 limiting the geographic ranges of many species of organisms, and local populations of a 

 given species may show some localized differences in cold-hardiness. However, another 

 factor might have been body size (i.e., weight) differences between males and females 

 from both populations as per Tepedino and Torchio (1982). The significantly lighter bees 

 (based on the reported emergence weights) from Nevada had significantly higher winter 

 mortality than those from Utah, but in that study bees from Nevada were trap nested in 

 tubes of 5, 6, 7, 8, and 9 mm diameter versus the uniform 7 mm tubes used in Utah (Rust 

 1995). Bee body size is limited in the smaller sized tubes (Tepedino and Torchio 1989), 

 a characteristic of cavity-nesting species (Roulston and Cane 2000), and this may have 

 contributed significantly to the mortality differences observed. Other factors such as dietary 

 history differences (food plant type and quality) between the two populations also may have 

 contributed to the observed body size differences, as could the time when eggs were laid; 

 size of progeny becomes smaller as the season progresses (Tepedino and Torchio 1989). 

 In the present study, there were significant weight differences between populations (Nova 

 Scotia reared bees were lighter), but this did not seem to influence mortality, as the heavier 

 Utah bees had higher mortality. 



Despite the slight but significant differences in survival observed between 

 populations, high survival was obtained under both controlled and ambient outdoor 

 conditions (Fig. 2), even with the extended pre-wintering period in the Utah population 

 and with minimum outside temperatures approaching the supercooling points of wintering 

 bees. For the time being, O. lignaria must be imported into eastern Canada to obtain 

 adequate numbers for apple pollination. Importation should always be done in the late 

 summer or early autumn when bees have reached the immature non-feeding stages (i.e., 

 prepupal and pupal), but prior to the critical pre-wintering period indicated by Bosch and 

 Kemp (2004). Once bees have reached the adult wintering stage, exposure to elevated 

 pre-winter temperatures should be avoided (Kemp et al. 2004) due to increased winter 

 mortality from excessive fat body depletion (Bosch et al. 2000). Conditions during transit 

 may be variable and temperatures above 5°C may actually increase mortality, or cause bees 

 to emerge prematurely. Consistent or gradually fluctuating temperatures promote survival, 

 while rapid fluctuations in daily minimum and maximum temperatures can be detrimental 

 (Leather et al. 1993). 



Bosch and Kemp (2001, 2003, 2004) stressed the importance of monitoring the 

 populations for determining the development stages prior to wintering, a suggestion even 

 more relevant for importation of bees into a different geographic location. For winter storage 



14 



