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Q: The orchard bees that I purchased were lethargic when they emerged.  Or, many of them died before emerging.  What was wrong?

Compare orchard bees with Megachile rotundata, the alfalfa leafcutting bee.
Questions that remain to be addressed.

A:  Research from the USDA Bee Labs provides an answer (Bosch and Kemp 2000; Bosch et al. 2000).  Vigor of emerged bees depends on their energy reserves.   Energy reserves in bees are stored  in masses of special cells that participate in metabolism, called "fat body".  The fat body reserves provide energy through the winter and into the spring when warm temperatures initiate adult emergence.  If fat body reserves are inadequate in the spring, then the bee will be lethargic. If the bee has enough energy to fly to nectar quickly, it may recover.  However, if the weather is too cold or wet, or if flowers are scarce or too far, the bee may not survive.  Some bees may not have enough energy to chew their way out of the cocoon or nest plug, and will die of starvation in the nest.  The connection between fat body reserves and adult emergence was first suggested by Stephen (1995) for the alfalfa leafcutting bee.  

    One factor that affects fat body reserves is air temperature.  Metabolism in overwintering adult bees is low during cold winter temperatures, so few energy reserves are used.  However, bees are often exposed to warm temperatures in the fall, after they have molted into adults.  If temperatures are too warm in the fall, or are warm for too long, then the recently molted adults may have a high metabolism that depletes energy reserves.  By the spring when air temperatures warm, there may not be enough energy reserves for the bees to emerge.  This is especially true for small bees, particularly males, which are usually smaller than females.  But depletion of energy reserves can happen to any bee.  Bosch et al. (2000); used x-ray analysis of adult Osmia lignaria to qualitatively determine fat body condition.  They found that bees which reached adulthood earlier during the summer and were thus exposed to a longer period of warm temperatures as adults, had depleted fat bodies and survived unfed for a shorter time after emergence (a measure of vigor) than did bees that took longer to develop into adults during the summer.  

    Here's where regional climatic adaptations make a difference in the risk of lethargic Osmia bees.  Bees from southern latitudes and lower altitudes are exposed to longer summers and warmer temperatures than are bees from northern latitudes and higher altitudes.  Local populations in warm climates adapt by requiring increased heat units (accumulated degree days) during the summer to develop from prepupae into pupae and then adults before winter.  This usually occurs as temperatures are falling, so little energy is used in the autumn while the bee is an adult.  Bees from colder climates require fewer heat units to develop into adults because hot summer weather does not last as long in these areas.  These differences appear to have a genetic basis.

    If bees from cold climates are moved to areas with warm climates, they develop into adults earlier in the autumn or late summer then they would in their original habitat.  If they are left at ambient temperatures, they are likely to experience warm temperatures for a substantial amount of time in the autumn (Bosch et al, 2000).  Their metabolism remains high, thus depleting their energy reserves.  

    If bees from warm temperatures are moved to areas with cold climates, they may not be exposed to enough heat units at ambient temperatures to complete development and molt into an adult (Rust 1995).

    Even populations that are locally adapted may experience unusually hot or cold seasons that will either prevent bees from completing their development to adults, or deplete fat body reserves in the fall.

    The USDA Bee Labs in Logan, UT has published a manual with advice on managing the blue orchard bee.  It recommends that you carefully monitor bees during the summer and fall to determine when most of the bees have molted into adults.  This can be done by cutting open a few cocoons around the nipple area with a razor blade. The bees should experience cold temperatures within 45 days of becoming adults.  If ambient temperatures are not cold by this time , the bees should be refrigerated.  Similarly, if ambient  temperatures cool too much for bees to develop into adults, they may need to be moved to a warm place to complete development. 

    If you choose to purchase bees from a climate different than your own, you may need to provide a controlled temperature environment to be sure the bees successfully develop during the summer and emerge as vigorous adults in the spring.  For more details, see Bosch and Kemp (2002).   Bosch et al. (2000) showed that bees moved from a cold climate to a warm climate continued to develop as expected for bees from cold climates in the following generation. We do not know  whether or how quickly these populations might adapt to warm climates after several generations. 

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Megachile rotundata,  the alfalfa leafcutting bee

    Similar principles apply to other twig-nesting solitary bees, including the alfalfa leafcutting bee.  However, most of these overwinter as prepupae rather than as adults, and many have two or more generations in one season if warm temperatures persist for a long season.  For these species, prepupae left in warm temperatures in the fall for too long will use up energy reserves, and may not emerge successfully in the spring.  This is true especially for males.  However, cutting open cocoons in the autumn  to determine when the bees should be moved into cold temperatures is not helpful because we can't distinguish diapausing prepupae from non-diapausing prepupae.  As a general rule, alfalfa leafcutting bees should be moved into cold storage soon after mid September. 

    Peterson et al (1991) released alfalfa leafcutting bees from Manitoba in Manitoba,  Idaho and California.   Survivorship of offspring of the bees from Manitoba was greatest when these offspring were reared locally in Manitoba (90%) and lowest when the offspring were reared in California (75%).  Perhaps this was because Manitoba bees in California experience more degree day accumulations than are needed for initiating diapause. Manitoba bees may therefore risk depleting their fat body reserves in California.   

    The proportion of bees that emerged as second or even third generation instead of going into diapause increased the further south the bees were introduced.   In addition, overwintered bees that had been reared in Idaho or California took longer to emerge the following spring than did bees reared in Manitoba.  This suggests a strong environmental component in the adaptation of development time to local climate for M. rotundata that has not been demonstrated for O. lignaria

    Methods are needed to more accurately determine when bees from different populations should be moved to cold storage to avoid depleting fat body reserves in the fall.  

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Questions that remain to be addressed include:

bulletDo these principles also apply to ground nesting bee species?  
bulletWhat happens to development times and energy reserves when introduced populations of bees mate with local populations?
bulletHow does bee body size (which depends on amount of provisions) affect energy reserves?
bulletFor bees with multiple generations, what determines whether bees enter diapause vs. emerge as a second or third generation bee during the same season?  Do energy reserves play a role?

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A Degree Day Primer

Osmia, the Blue Orchard Bee or Orchard Mason Bee

Bosch, J. and W. P. Kemp.  2002  How to manage the blue orchard bee, Osmia lignaria, as an orchard pollinator.  Sustainable Agriculture Network, USDA, Washington D. C. (order a copy)

Bosch, J.,and W.P. Kemp. 2000. Development and emergence the orchard pollinator of Osmia lignaria (Hymenoptera: Megachilidae).  Environ. Entomol. 29:8-13.

Bosch, J., W.P. Kemp, and S. S. Peterson. 2000. Management of Osmia lignaria (Hymenoptera: Megachilidae) populations for almond pollination: Methods to advance bee emergence. Environ. Entomol. 29:874-883.

Rust, R. W. 1995.  Adult overwinter mortality in Osmia lignaria propinqua Cresson (Hymenoptera: Megachilidae). Pan Pacific Entomologist 71(2): 121-124.

Megachile rotundata, the alfalfa leafcutting bee

Kemp, W.P. and J. Bosch.  2000.  Development and emergence of the alfalfa pollinator Megachile rotundata (Hymenoptera: Megachilidae).  Ann. Entomol. Soc. Am.  93(4): 904-911.

Peterson, S. S., C. R. Baird, and R. M. Bitner.  1991.  Variation in weight and postdiapause development among groups of alfalfa leafcutting bees, Megachile rotundata (F), reared in different locations.  Bee Science.  1(4):230-236.  (get web address)

Stephen, W.P. 1995. How and where were they raised? Critical management considerations in Megachile. 26th Northwest Alfalfa Seed Growers Winter Seed School. 27-35.

For a discussion of development and overwintering of the alfalfa leafcutting bee, see the bee management section of the Pollination Ecology web:
Fall and Winter Management.
Is your live count what emerges?
Alfalfa leafcutting bee management  in Idaho. 
Important degree days for alfalfa seed.

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Updated March 20, 2001.
Copyright 2001, Karen Strickler. All rights reserved.