How to age a bumble bee

“Can you tell how old a bee is?”

This is a question that I frequently hear when people ask about my bumble bee research. This goes hand in hand with another question: “How long do bees live?” As an ecologist, I study how bumble bees relate to one another and their environment. Being able to determine the “age” of my study bees is important. It’s an integral piece of information I collect on each bee, along with forage plant, location, sex, and species.

The short answer about bumble bee lifespan is that it varies quite a lot depending on species, environmental factors, and whether the bee is a queen, male, or worker. Queens have the longest lifespan. In the fall, newly emerged queens go underground for the winter months and then re-emerge in spring to find nest sites, lay eggs and establish colonies. Once their worker daughters take over foraging duties, queens remain in their nests and continue to lay eggs through the growing season. But when fall arrives and hard frosts turn summer flowers brown, all of the colony members die – including the old queens. The exceptions are the new queens who have already gone into hibernation. Therefore, if a queen survives the winter and is successful in founding a colony the next spring, she will live a full year.

Male bumble bees are produced in the colonies from mid to late summer. Newly emerged males spend a few days in the nest consuming food stores without contributing to the workload. After this time, they leave and do not return. They spend their time feeding on nectar and attempting to locate new queens for mating. Unlike males of most other social insects, which typically die soon after mating, male bumble bees are thought to live several weeks.

Most of the research on bumble bee lifespan has been done on the female workers, who are responsible for gathering nectar and pollen and defending the nest. There is no definitive estimate, but their life expectancy ranges from just a couple of weeks in the field to over a month when kept in the lab. Foraging is a dangerous activity, which puts workers at a high risk of predation (e.g., by birds, dragonflies, and spiders) and exposure to severe weather like heavy rain, high winds, and extreme temperatures. It’s also possible that total “work effort” and energy expended are linked to total life span, and that the “hard work = early death” hypothesis may be a driving factor in limiting the lifespan of worker bees. Therefore, keeping a bee in a laboratory setting where there is no predation risk and no need to forage would prolong its life.

Now that we’ve answered the question about how long do bumble bees live, what about telling how old a bee is? Multiple papers have found that bees’ wings degenerate with use and that there is a link between the amount of degeneration and a bee’s age. Through time and use, bees’ wings will become frayed and torn. This increases the wing-loading, which is the relationship between the weight of the bee to the total area of its wings (milligrams per square millimeter). As wing wear increases, the wing-loading measurement increases, which translates to a higher energy cost for the bee. Torn and tattered wings also limit maneuverability, which can increase a bee’s risk of predation.

wing margin

Adapted from Mueller and Wolf-Mueller (1993) (Mueller & Wolf-mueller 1993)

In my research, I use a four-point scale from 0 – 3 to “age” my study bees.

 

0 = perfect wing with intact margin

1 = a couple of nicks, no major wear or tear

2 = wing margin nearly completely tattered

3 = wing margin gone, with large chunks missing

 

perfect wing

Perfect wing margin (age = 0) on this unusually reddish-colored eastern bumble bee (Bombus impatiens). Photo: USGS Bee Inventory and Monitoring Lab

Although the degree of wing wear cannot be directly correlated with a bee’s age in days or weeks, a score of “0” likely represents a very young bee. Likewise, a score of “3” could indicate an older bee nearing the end of its lifespan. As in butterflies, moths, dragonflies, and other insects that spend a lot of time flying, we know there is a positive relationship between wing wear and a bees age. The relationship is not exact however, probably because of environmental variation.

Interestingly, a 1987 paper concluded that there may be a latitudinal effect on bumble bee lifespan with bees in higher latitudes having shorter lifespans. Previous studies noted that foraging bumble bees in Brazil (Bombus morio) had an average lifespan of 36.4 days whereas bees in New Brunswick (Bombus terricola) only lived 13.2 days. One hypothesis for this is that longer days in the summer months mean more time per day spent foraging, resulting in more energy expended per day. Also, cooler temperatures at higher latitudes mean more energy is required to maintain suitable temperatures in the nest. Finally, if bees are forced to fly farther in order to locate floral resources, this can also result in shorter lifespans (hard work = early death) (Goldblatt & Fell 1987).

example of 3

Example of an extreme “3” on the wing-wear scale. Photo: Kalyn Bickerman-Martens

 

How does bumble bee lifespan compare to that of other social insects? Some worker ants can live up to several years and queen ants of the harvester ant species Pogonomyrmex owyheei can live as long as 30 years! Honey bee workers most likely have similar lifespans to bumble bees in the summer but can live inside the hive for up to a few months during the winter, whereas honey bee queens can potentially live five years or more.

 

 

 

References

Cartar, R. V. 1992. Morphological senescence and longevity : an experiment relating wing wear and life span in foraging wild bumble bees. Journal of Animal Ecology 61:225–231.

Goldblatt, J., and R. Fell. 1987. Adult longevity of workers of the bumble bees Bombus fervidus (F.) and Bombus pennsylvanicus (De Geer) (Hymenoptera: Apidae). Canadian Journal of Zoology 65:2349–2353.

Mueller, U. G., and B. Wolf-mueller. 1993. A Method for Estimating the Age of Bees : Age- Dependent Wing Wear and Coloration in the Wool- Carder Bee Anthidium manicatum ( Hymenoptera : Megachilidae ). Journal of Insect Behavior 6:529–537.

Eusociality Information:

http://biology.stackexchange.com/questions/2785/why-do-ants-live-so-long

 

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