“Two-year bee, or not two-year bee? How voltinism is affected by temperature and season length in a high-elevation solitary bee”

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Jessica R. K. Forrest, Regan Cross, and Paul J. CaraDonna (Apr 2019)

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Faced with short and variable summers, mason bee larvae use temperature & birth date to decide on 1- or 2-year life cycle

An adult female Osmia iridis at the same study site, bearing a yellow paint-mark for identification.
(Credit: J. Forrest)

Animals never know exactly what their future holds when it comes to environmental conditions—yet, when faced with alternatives, they often make the correct decision.

For example, some solitary bees can become adults one or two years after hatching from eggs. But how does a bee decide which strategy is best? It’s a tough choice because one-year bees reproduce earlier than their two-year compatriots, but a one-year bee caught mid-metamorphosis when winter arrives will die. Therefore, it’s safer, but less rewarding, to be a two-year bee.

Jessica Forrest became interested in this observation while studying mason bees in the Rocky Mountains, USA. Fellow ecologist Paul CaraDonna shared similar observations, and the two started to ponder what caused the phenomenon. With the help of undergraduate researcher Regan Cross, they set out to determine if temperature or summer season length determines whether bees opt for one- or two-year life cycles. By moving bee eggs from the field to incubators and manipulating temperature and season lengths, the team found that warmer—but not longer—summers, combined with earlier birth dates, increased the frequency of one-year bees.

Study site at Rosy Point, near Gothic, Colorado – note “trap-nests” (mason bee nesting blocks) attached to the aspens – on 4 June 2017.
(Credit: J. Forrest)

With this knowledge in hand, the researchers looked at local climate records, and found that only 7% of summers since 1950 have been warm enough for one-year bees to succeed, explaining the observed scarcity of one-year bees in the study area. As temperatures warm under climate change, suitable conditions for one-year life cycles may become more common; nevertheless, since this one-year strategy is still riskier than the two-year strategy, both life cycles will likely persist. This study shows that bee larvae can integrate temperature cues and information on when in the season they were born to make the right decision about which life cycle to pursue. Just how they are able to do this remains a mystery.


Organisms must often make developmental decisions without complete information about future conditions. This uncertainty—for example, about the duration of conditions favorable for growth—can favor bet-hedging strategies. Here, we investigated the causes of life-cycle variation in Osmia iridis, a bee exhibiting a possible bet-hedging strategy with co-occurring one- and two-year life cycles. One-year bees reach adulthood quickly but die if they fail to complete pupation before winter; two-year bees adopt a low-risk, low-reward strategy of postponing pupation until the second summer. We reared larval bees in incubators in various experimental conditions and found that warmer—but not longer—summers, and early birth dates, increased the frequency of one-year life cycles. Using in situ temperature measurements and developmental trajectories of laboratory- and field-reared bees, we estimated degree-days required to reach adulthood in a single year. Local long-term (1950–2015) climate records reveal that this heat requirement is met in only ~7% of summers, suggesting that the observed distribution of life cycles is adaptive. Warming summers will likely decrease average generation times in these populations. Nevertheless, survival of bees attempting one-year life cycles—particularly those developing from late-laid eggs—will be <100%; consequently, we expect the life-cycle polymorphism to persist.