Factors that affect heat resilience in honey bee drones and their sperm
Climate change is expected to make heatwaves more frequent and severe, threatening fertility in many animals, including insects. Temperatures within the range that can occur during summer heatwaves (35–45 °C) can reduce fertility in red flour beetles (Tribolium castaneum), fruit flies (Drosophila spp.), bumble bees (Bombus spp.), oriental fruit moths (Grapholita molesta), parasitoid wasps (Anisopteromalus calandrae and Aphidius avenae) and honey bees (Apis mellifera).
Heat stress affects many insects, but the possible consequences for honey bees are especially important because of their widespread use in crop pollination. Honey bees tolerate a wide range of climates and have native populations in Africa, Asia and Europe. However, queens and drones are susceptible to heat-related loss of fertility, including reduced sperm viability. Queens can survive heat stress while the viability of sperm stored in their spermatheca declines. Previous experiments reported losses after exposure above 38 °C for more than two hours. Heat exposure can also kill sperm in drones. These losses cannot readily be reversed. Queens mate during a short period early in life and do not replenish their sperm reserves later. Drones produce sperm during development rather than adulthood.
Although drones and queens have similar body masses, drones are more susceptible to heat-related mortality. Honey bee colonies can maintain stable brood nest temperatures, but few studies have measured temperature fluctuations inside colonies during heatwaves. In one California study, temperatures inside colonies exceeded 42 °C when ambient temperatures reached 45 °C and access to water was limited. Temperatures were particularly high at the edge of the nest, where sexually mature drones tend to gather.
Other studies have measured brood nest temperatures as high as 40 °C. Drones might also experience extreme temperatures during bearding, when many bees leave the nest in hot weather and cluster outside, reducing heat production inside. The authors note that this possible exposure has yet to be studied directly. Colony thermoregulation can buffer heat exposure, but drones may still experience heat stress under extreme conditions.
Despite the role of drones in reproduction, little is known about the factors that determine how heat affects their survival and fertility. Two earlier studies found that drones were susceptible to heat-related mortality, but reported different survival rates. McAfee et al. found 50% survival after six hours at 42 °C. Stürup et al. found 23% survival after four hours at the same temperature. Several factors could explain this difference. Drones from different populations may vary in heat tolerance, and traits such as body mass or exposure to other stressors such as infection may also influence survival. In some insect species, heavier individuals are more heat tolerant.
The authors had previously found that drone body mass was positively associated with the number of viable spermatozoa in a drone, suggesting greater investment in reproduction. They therefore considered whether heavier drones might produce sperm of higher quality and greater resistance to stress, including heat. Pathogen infections could increase or decrease heat resilience, depending on the pathogen. Disease can directly reduce individual fitness, or immune responses may compete with reproduction for resources, as observed in other insects. A recent correlational study did not support that trade-off in honey bee drones. Because antiviral immunity and the heat-stress response share some mechanisms, the authors also considered whether some viral infections might improve heat tolerance.
Although upper thermal tolerance varies relatively little among insects in general, heat tolerance in terrestrial ectotherms tends to decline with increasing latitude, from tropical to temperate climates. Previous research found that Saudi Arabian honey bees (Apis mellifera jemenitica) tolerated heat better than European honey bees (Apis mellifera carnica), consistent with that trend. In Colombia, ancestry analyses found more African ancestry in colonies at warmer, lower elevations and more European ancestry in colonies at cooler, higher elevations.
Protein expression patterns in worker honey bees sampled along a latitudinal gradient also show evidence of local climatic adaptation. Whether these patterns extend to reproductive individuals and other populations remains to be tested. Knowing how different lineages respond could help predict their performance as the climate changes. A warmer climate may also affect interactions with pathogens. Heat shock proteins (HSPs) are a conserved group of proteins whose expression generally increases with heat exposure. They help limit protein misfolding and aggregation, and evidence indicates they also contribute to antiviral immunity in honey bees and other insects. Heat exposure and viral infection may therefore interact in honey bee physiology.
McMenamin et al. found that heat exposure and experimental viral inoculation each increased HSP expression. Exposing bees to heat shortly after infection also improved viral clearance. The authors considered whether the reverse might occur, with an earlier infection increasing heat tolerance in drones or their sperm through the dual roles of HSPs. Neither possibility had been tested in drones, which are disproportionately exposed to viruses because Varroa destructor, an important virus vector, preferentially parasitises drone brood.
Little is known about how viral infections affect drone fertility or how infection and heat interact. Historical records show that the number of heatwave days increased most in tropical regions, but temperate regions can also experience extremes. During the 25 June to 1 July 2021 heatwave in western Canada and the Pacific Northwest, temperatures reached 49.6 °C and local beekeepers reported adverse effects, including colony deaths. Beekeepers in high-latitude temperate regions may therefore need heat-management practices they did not previously use. Measures such as year-round hive insulation or shade may help buffer shorter periods of heat exposure.
If managing hive temperatures becomes ineffective or impractical, selecting and breeding for heat resilience could contribute to an integrated response. This first requires identifying how heat resilience relates to population origin and other factors in existing stocks. Southern California was less affected by the 2021 heatwave, but it too has experienced a shift towards more extreme temperatures. The study describes a shift from historically semi-arid conditions (Köppen BSk) towards a hot, dry climate more like BWh. Heatwaves have become more frequent in Southern California since 1950. During the 5–7 September 2020 heatwave in Riverside County, daytime highs reached 47 °C. Genetic analyses found about 38% African ancestry in unmanaged honey bees in Southern California. Given this ancestry and repeated exposure to intense heat, the authors hypothesised that this population might show adaptations to heat stress.
The study examined factors that affect heat resilience in honey bee drones and their sperm. The researchers tested how population origin and body mass related to drone survival and sperm viability after heat exposure. They also tested the effect of viral infection acquired in adulthood on sperm heat resilience.
First, the researchers recorded adult body mass and survival after four hours at 42 °C. In a separate experiment, they compared survival in drones from Southern and Northern California to explore possible local adaptation. Southern California is drier and experiences more extreme maximum temperatures. Finally, they used in vitro assays to examine relationships among body mass, viral inoculation and heat exposure, testing sperm extracted from drones at different temperatures and exposure durations.
The authors hypothesised that
1) heat resilience in drones and their sperm would reflect local climates at their populations of origin, with Southern California drones expected to be more heat tolerant than Northern California drones,
2) heavier drones would be more likely to survive a heat challenge and would produce more heat-tolerant sperm, and
3) viral infection would improve sperm heat resilience because the heat-shock response and antiviral gene expression interact.
Materials and methods: Read the original article
Conclusions
To understand what contributes to heat resilience in honey bee drones and to their reproductive potential, assessed through sperm viability, the authors examined population origin, body mass and viral infection in a series of experiments.
The main findings were as follows.
1) Heavier drones were more likely to survive acute heat exposure.
2) Sperm heat resilience varied widely among drones from different populations within British Columbia.
3) Southern California drones were more likely to survive heat exposure than drones from Northern California.
4) Sperm from heavier drones was no more heat tolerant than sperm from lighter drones.
5) Experimental viral infection in adulthood reduced sperm heat resilience.
The drone survival tests used four hours at 42 °C, conditions that drones might encounter inside or outside hives during extreme heat. Direct field data on drone exposure during heatwaves are scarce. In vitro tests of sperm from British Columbia drones used the same temperature and duration. The broader in vitro temperature and exposure-duration series and the viral infection experiments also used temperatures that may be outside the biologically relevant range. Exposure to 60 °C, for instance, is unlikely in nature.
These experiments were designed in part to identify the limits within which sperm remain viable and to characterise their responses under extreme conditions. Future work could measure the conditions drones experience in the field, including their internal body temperatures, and use those data to guide laboratory experiments.
The higher survival of heavier drones agrees with findings in some other insects, including males of the squash bee Xenoglossa pruinosa. Other studies have found no relationship between size or mass and heat resilience, measured as the critical thermal maximum (CTmax), within or across insect species. These species-specific results suggest that other factors also contribute to heat resilience. Larger individuals take longer to heat up, so heating time may matter more than cooling capacity during short exposures. This alone does not explain why many species show no relationship between CTmax and body size.
The researchers measured wet body mass, which depends on hydration. Body size is a separate measure.Hydration can influence heat resilience. Better-hydrated individuals may have a higher specific heat capacity and a greater capacity for evaporative cooling.Larger individuals have a lower surface-area-to-volume ratio, which can limit cooling. Studying how hydration and body size interact could help explain why the relationship between size and heat tolerance varies among species. Some water may have evaporated between a drone’s death and weighing, lowering its measured mass independently of heat resilience. The authors considered this effect likely to be small because most deaths occurred near the end of the four-hour exposure, when all drones were weighed.
Southern California drones survived the heat challenge more often than Northern California drones, as predicted. Southern California colonies tend to have more African ancestry than colonies in Northern California, which lies outside the hybrid zone. The authors hypothesise that the warmer southern climate and earlier selection in warm environments could have favoured heat resilience. The Southern California colonies in the experiment had feral origins or had bred with the local population for multiple generations. The observed difference reflected both a modest increase in heat tolerance and unexpectedly low survival in the control group, so the authors could not attribute it to ancestry alone.
Larger control groups and tests for concurrent conditions, such as infections, after heat exposure could help explain this pattern. Comparable sperm viability data were unavailable for the California groups and for survivors of different genetic origins. The authors did compare sperm from different sources within British Columbia and found wide variation in heat responses. Beekeepers may have assessed drone age differently, but the range of responses in the University of British Columbia colonies, where age was controlled, resembled the range across other sources in the province. The researchers then
examined possible interactions with viral infection for three reasons. First, drones naturally infected with deformed wing virus type B (DWV-B) had higher sperm viability than drones without a detected DWV-B infection (figure S2 in file S1). Second, earlier work had linked at least one heat shock protein to higher viability of stored sperm. Third, McMenamin et al. had found an antiviral effect of heat exposure. The authors hypothesised that heat shock proteins could help explain these observations by contributing to both antiviral responses and protection against other stressors. They initially considered whether virus-induced heat shock proteins might protect sperm from subsequent heat stress. When they experimentally infected adult drones and then exposed their sperm to heat, they found the opposite. Sperm from infected drones was less heat resilient than sperm from controls. This could reflect a trade-off between sperm maintenance and immunity or a direct effect of infection on sperm quality.
The apparent discrepancy with the positive association observed for natural infection remains unresolved. Possible explanations include how long the drones had been infected before sampling, differences between Israeli acute paralysis virus (IAPV) and DWV-B, and whether infection occurred before, during or after spermatogenesis. Competition for cellular resources is another possibility.
Overall, the experimental data suggest that IAPV infection can exacerbate the loss of drone sperm viability caused by heat. This matters because Varroa destructor preferentially parasitises drone brood and is an important vector of bee viruses. The relatively small effect of infection acquired in adulthood could account for only a small share of the variation in sperm heat resilience observed among sources in British Columbia.
Other factors probably contribute to sperm heat resilience. Genotype is one possibility, but the study did not include genetic tests that could distinguish genetic differences from differences in environment or management. In Drosophila fruit flies, the upper thermal limits of male fertility predict species distributions better than CTmax, suggesting that fertility at high temperatures can be subject to strong selection. The wide geographical range of honey bees may allow more variation in this trait than in other insects. Alternatively, colony thermoregulation may buffer sperm from selection imposed by heat, allowing more variation among individuals. Understanding the sources of variation in drone fertility remains important as beekeeping regions that historically experienced little extreme heat face hotter conditions.
Published 7 February 2025 · PLOS ONE · DOI 10.1371/journal.pone.0317672 · Adapted under CC BY
Authors
Alison McAfee 1,2; Bradley N. Metz 2; Patrick Connor 2; Keana Du 2; Christopher W. Allen 3; Luis A. Frausto 3,4;
Mark P. Swenson 3,4; Kylah S. Phillips 3,5; Madison Julien 3; Zoe Rempel 6; Robert W. Currie 6; Boris Baer 3;
David R. Tarpy 2; Leonard J. Foster 1
1 Department of Biochemistry and Molecular Biology, Michael Smith Laboratories, University of British Columbia, Vancouver, British Columbia, Canada; 2 Department of Applied Ecology, North Carolina State University, Raleigh, North Carolina, United States of America; 3 Center for Integrative Bee Research (CIBER), Department of Entomology, University of California Riverside, Riverside, California, United States of America; 4 Department of Microbiology & Plant Pathology, University of California Riverside, Riverside, California, United States of America; 5 Department of Molecular, Cell & Systems Biology, University of California Riverside, Riverside, California, United States of America; 6 Department of Entomology, University of Manitoba, Winnipeg, Manitoba, Canada
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