Abstract
Climate change and increasing average temperatures are now affecting most ecosystems. Social insects such as bumblebees are especially impacted because these changes create spatial, temporal and morphological mismatches that could impede their ability to find food resources and mate. However, few studies have assessed how the colony and life cycle are affected when temperatures rise above optimal rearing temperature. It has become imperative to understand how heat stress affects the life history traits of insect pollinators as well as how changes in life history interact with other traits like morphology. For example, a decrease in the number of foraging workers could be balanced by producing larger workers, able to forage at longer distances and gather more resources. Here, we investigated the impact of temperature on colony production and body size in the bumblebee Bombus terrestris. Colonies were exposed to two temperatures: 25 °C, which is around the optimal temperature for larval development and 33 °C, which is slightly above the set-point that is considered stressful for bumblebees. Although the production of males and workers wasn’t significantly affected by these different temperatures, queen production and reproductive investment were much higher for colonies placed in 33 °C than in 25 °C. We also found that, in agreement with the temperature-size rule, workers were significantly smaller in the higher temperature. The decrease in worker body size could affect resource collection and pollination if their foraging distance and the quantity of food they are taking back to the colony decreases. While in our controlled conditions the bumblebees were fed ad libitum, the decrease of resource collection in field conditions could prevent colonies from producing as many queens as in our study. Together with the decrease of worker body size, our results suggest that elevated temperatures could ultimately have a negative impact on bumblebee colony fitness. Indeed, smaller workers are known to have weaker flight performance which could affect foraging performance and consequently colony development.
Highlights
Over the past few decades, climate change has led to increasingly unpredictable weather patterns (Thibeault and Seth 2014) like heat waves (Perkins-Kirkpatrick and Lewis 2020), and will raise the global average surface temperature by 0.3 °C to 4.8 °C before the end of this century (Pachauri et al 2014)
Together with the decrease of worker body size, our results suggest that elevated temperatures could have a negative impact on bumblebee colony fitness
Global warming affects plant-pollinator interactions, creating potential spatial, temporal and morphological mismatches (Hegland et al 2009; Miller-Struttmann et al 2015; Pyke et al 2016; Gérard et al 2020). These studies do not consider the potential impact of warming on life history traits, such as the number of individuals produced – sexuals – or the timing of their production, which is important for understanding the impact on reproduction, pollination and life cycles
Summary
Over the past few decades, climate change has led to increasingly unpredictable weather patterns (Thibeault and Seth 2014) like heat waves (Perkins-Kirkpatrick and Lewis 2020), and will raise the global average surface temperature by 0.3 °C to 4.8 °C before the end of this century (Pachauri et al 2014). Global warming affects plant-pollinator interactions, creating potential spatial, temporal and morphological mismatches (Hegland et al 2009; Miller-Struttmann et al 2015; Pyke et al 2016; Gérard et al 2020) While informative, these studies do not consider the potential impact of warming on life history traits, such as the number of individuals produced – sexuals (i.e. queens and males in social bees) – or the timing of their production, which is important for understanding the impact on reproduction, pollination and life cycles. Further evidence that elevated temperatures affect bumblebee life cycles comes from the UK, where B. terrestris are becoming increasingly active during the winter (Edwards 2006; Farmer 2006; Hart et al, 2021) In addition to this field-based work, several studies have assessed the impact of rearing temperature on colony development in controlled conditions. We hypothesized that colonies reared at 33 °C may have lower production due to stressful temperature and that the individuals produced will have smaller body size than bumblebees reared at 25 °C, as predicted by the temperaturesize rule (TSR; Atkinson 1994; Angilletta and Dunham 2003)
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