Overwintering management of Osmia lignaria 



JESO Volume 139, 2008 



again is earlier than in Nova Scotia. The bees from Utah were not imported into Nova 

 Scotia until late-October/November, as bees had to be at least in the prepupal/pupal stages 

 prior to shipment, and due to delays with importing procedures. The result was a much 

 extended pre-winter storage period for this population compared to the one reared in Nova 

 Scotia. 



Bosch and Kemp (2003, 2004) report significant weight loss and mortality when 

 pre-wintering duration is extensive for Osmia lignaria and the closely related O. cornuta 

 (Latreille), and suggest an optimum range of between 1 and 30 days following adulthood for 

 both species under commercial storage conditions. The pre-wintering period is apparently a 

 critical one for fat body depletion, and placing bees within cold storage within the suggested 

 time frame greatly reduces the rate of loss in O. lignaria (Bosch et al. 2000; Bosch and 

 Kemp 2001,2004). 



The extended pre-wintering period also lengthened the overall wintering period 

 for this population. Kemp et al. (2004) reported that after approximately three months of 

 wintering under controlled conditions, O. lignaria enters a postdiapause transitional period, 

 indicated in that study by increased oxygen consumption rates. Bosch and Kemp (2001, 

 2003, 2004) provide detailed accounts of several aspects of winter storage for O. lignaria 

 and O. cornuta, and for the later species, maximum survival and longevity were found 

 in populations wintered for 90-150 days (Bosch and Kemp 2004). In general, excessive 

 prolonged storage (and/or excessive warm winter temperatures) causes additional depletion 

 of the winter reserves (i.e., fat body) beyond a critical level, as the bees enter the postdiapause 

 transitional period (Kemp et al. 2004). For longer storage periods, colder temperatures (i.e., 

 0°C) are favourable to even slightly warmer ones (i.e., 4°C) (Bosch and Kemp 2003). The 

 depletion of these resources during the postdiapause transitional period occurs at a faster 

 rate than during the winter, and bees held longer than one month beyond the recommend 

 duration show increased mortality (Bosch and Kemp 2001) (bees may even become active 

 while in cold storage at 4°C during the post diapause transitional period - some male O. 

 lignaria have been observed chewing and emerging when they are held beyond the typical 

 release time). Thus, winter survival is influenced mainly by fat body depletion via increased 

 metabolism which can be assessed by measuring respiration rates and weight loss. A similar 

 affect from prolonged storage has been reported in another solitary commercial pollinator, 

 the alfalfa leafcutter bee, Megachile rotundata (Richards et al. 1987), although bees of the 

 genus Megachile overwinter as prepupae. 



Despite the extended pre-wintering/wintering period of the Utah population, 

 survival was higher than expected (>85%) based on findings reported by Bosch and Kemp 

 (2004), but those authors report a similar rate of mortality (16.6%) for bees held at 4°C for 

 210 days (Bosch and Kemp 2003). The slight discrepancy between the findings of that 

 study (Bosch and Kemp 2004) and the present one is probably due to the standard by which 

 survival was measured. Bosch and Kemp (2004) recognized that survival is more than just 

 getting through the winter, and thus measured longevity of emerged bees which gives some 

 indication of the bees ability to fly, feed and mate following the winter. In the present study, 

 bees were considered survivors if they actually emerged, or if inspection of nesting tubes 

 revealed bees which had chewed out of the natal cocoons even if they never emerged from 

 the nesting tube (mainly due to obstruction). 



Body size has been also indicated as a factor affecting wintering success in 



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