Overwintering temperature and body condition shift emergence dates of spring-emerging solitary bees

Abstract

Solitary bees in seasonal environments must align their life-cycles with favorable environmental conditions and resources; the timing of their emergence is highly fitness relevant. In several bee species, overwintering temperature influences both emergence date and body weight at emergence. High variability in emergence dates among specimens overwintering at the same temperatures suggests that the timing of emergence also depends on individual body conditions. However, possible causes for this variability, such as individual differences in body size or weight, have been rarely studied. In a climate chamber experiment using two spring-emerging mason bees (Osmia cornuta and O. bicornis), we investigated the relationship between temperature, emergence date, body weight, and body size, the last of which is not affected by overwintering temperature. Our study showed that body weight declined during hibernation more strongly in warm than in cold overwintering temperatures. Although bees emerged earlier in warm than in cold overwintering temperatures, at the time of emergence, bees in warm overwintering temperatures had lower body weights than bees in cold overwintering temperatures (exception of male O. cornuta). Among specimens that experienced the same overwintering temperatures, small and light bees emerged later than their larger and heavier conspecifics. Using a simple mechanistic model we demonstrated that spring-emerging solitary bees use a strategic approach and emerge at a date that is most promising for their individual fitness expectations. Our results suggest that warmer overwintering temperatures reduce bee fitness by causing a decrease in body weight at emergence. We showed furthermore that in order to adjust their emergence dates, bees use not only temperature but also their individual body condition as triggers. This may explain differing responses to climate warming within and among bee populations and may have consequences for bee-plant interactions as well as for the persistence of bee populations under climate change.

Solitary Bee Life History Traits and Sex Mediate Responses to Manipulated Seasonal Temperatures and Season Length

“…While the warm temperature treatment did not affect the timing of Osmia emergence, it resulted in increased weight loss. This weight loss was likely due to increased energetic expenditure during fall Kemp, 2003, 2004;Sgolastra et al, 2011;Schenk et al, 2018a) and when emergence was delayed by the early-spring onset treatment. However, only female Osmia were susceptible to this temperature-induced weight loss, suggesting that female Osmia may be more vulnerable to increased pre-emergence energy consumption under climate change compared to males.…”

“…While studies incorporating temperature manipulations in the field impart realism (e.g., Forrest and Thomson, 2011;CaraDonna et al, 2018), controlled laboratory studies are better equipped to elucidate response mechanisms. However, solitary bee responses to temperature have only been investigated in a limited number of managed species (e.g., Kemp, 2003, 2004;Sgolastra et al, 2010Sgolastra et al, , 2011O'Neill et al, 2014;Schenk et al, 2018a), and have primarily isolated responses to a single component of climate change (e.g., extended fall duration, winter temperature, or spring temperature, Kemp, 2000, 2003;Sgolastra et al, 2011;Fründ et al, 2013;O'Neill et al, 2014;Schenk et al, 2018a; but see Bosch and Kemp, 2004). While such studies have provided an important foundation for understanding how temperature influences biological processes in solitary bees, bee life cycles can span several seasons or years (Forrest et al, 2019), and climate change is occurring year-round (Sparks and Menzel, 2002;Alexander et al, 2006;Burrows et al, 2011).…”

“…For solitary bees, responses to climate change will likely be driven in part by the influence of temperature on biological processes occurring prior to emergence. For example, warmer temperatures can accelerate development (Radmacher and Strohm, 2010;O'Neill et al, 2014), reduce stored-energy reserves Kemp, 2003, 2004;O'Neill et al, 2011O'Neill et al, , 2014Sgolastra et al, 2011;Schenk et al, 2018a), increase mortality prior to emergence (Bosch and Kemp, 2003;O'Neill et al, 2011), advance emergence (Forrest and Thomson, 2011), and reduce postemergence lifespan Kemp, 2003, 2004;Bosch et al, 2010). In addition, elevated seasonal temperatures and shorter winters associated with climate change (Sparks and Menzel, 2002;Menzel et al, 2006) may shorten periods of cold exposure, which can disrupt diapause development (Hodek and Hodkova, 1988;Sgolastra et al, 2010) and cause increased pre-emergence energy consumption, increased winter mortality, and delayed emergence Kemp, 2003, 2004;Sgolastra et al, 2010).…”

“…Understanding solitary bee responses to climate change will ultimately depend on interplay among multiple biological mechanisms acting during different developmental stages. In insects and other organisms, the outcomes of this interplay can be complex (Post et al, 2008;Kingsolver et al, 2011), even counterintuitive (Cook et al, 2012), with responses depending on physiological processes associated with life-history traits (e.g., Diamond et al, 2011;Buckley and Kingsolver, 2012;Cook et al, 2012;Sgolastra et al, 2016;Schenk et al, 2018a). First, solitary bee species in temperate zones overwinter either as adults or as prepupae (Stephen et al, 1969).…”

The circadian clock uses different environmental time cues to synchronize emergence and locomotion of the solitary bee Osmia bicornis

“…A daily rhythm of emergence develops when the circadian clocks of bees in a population are synchronized to the 24-hour day and the clocks of all bees are set to the same time. The daily rhythm of emergence should persist in a bee population over a period of several weeks because not all bees emerge at the same date of the year (a certain variation exists within the population) 3 .…”