Low heat tolerance and high desiccation resistance in nocturnal bees and the implications for nocturnal pollination under climate change

Predicting insect responses to climate change is essential for preserving ecosystem services and biodiversity. Due to high daytime temperatures and low humidity levels, nocturnal insects are expected to have lower heat and desiccation tolerance compared to diurnal species. We estimated the lower (CTMin) and upper (CTMax) thermal limits of Megalopta, a group of neotropical, forest-dwelling bees. We calculated warming tolerance (WT) as a metric to assess vulnerability to global warming and measured survival rates during simulated heatwaves and desiccation stress events. We also assessed the impact of body size and reproductive status (ovary area) on bees' thermal limits. Megalopta displayed lower CTMin, CTMax, and WTs than diurnal bees (stingless bees, orchid bees, and carpenter bees), but exhibited similar mortality during simulated heatwave and higher desiccation tolerance. CTMin increased with increasing body size across all bees but decreased with increasing body size and ovary area in Megalopta, suggesting a reproductive cost or differences in thermal environments. CTMax did not increase with increasing body size or ovary area. These results indicate a greater sensitivity of Megalopta to temperature than humidity and reinforce the idea that nocturnal insects are thermally constrained, which might threaten pollination services in nocturnal contexts during global warming.

A lack of repeatability creates the illusion of a trade-off between basal and plastic cold tolerance

The thermotolerance-plasticity trade-off hypothesis predicts that ectotherms with greater basal thermal tolerance have a lower acclimation capacity. This hypothesis has been tested at both high and low temperatures but the results often conflict. If basal tolerance constrains plasticity (e.g. through shared mechanisms that create physiological constraints), it should be evident at the level of the individual, provided the trait measured is repeatable. Here, we used chill-coma onset temperature and chill-coma recovery time (CCO and CCRT; non-lethal thermal limits) to quantify cold tolerance of Drosophila melanogaster across two trials (pre- and post-acclimation). Cold acclimation improved cold tolerance, as expected, but individual measurements of CCO and CCRT in non-acclimated flies were not (or only slightly) repeatable. Surprisingly, however, there was still a strong correlation between basal tolerance and plasticity in cold-acclimated flies. We argue that this relationship is a statistical artefact (specifically, a manifestation of regression to the mean; RTM) and does not reflect a true trade-off or physiological constraint. Thermal tolerance trade-off patterns in previous studies that used similar methodology are thus likely to be impacted by RTM. Moving forward, controlling and/or correcting for RTM effects is critical to determining whether such a trade-off or physiological constraint exists.

Aged virgin adults respond to extreme heat events with phenotypic plasticity in an invasive species, Drosophila suzukii

Climate warming has increased the frequency of extreme heat events. Alien species usually invade new areas with a low-density population and often have limited mating opportunities due to the unsynchronized emergence of adults. Early-emerging virgin adults often have to wait to mate with later-emerging partners at the cost of aging, which reduces thermal tolerance. To understand the adaptive strategies of virgin males/females versus those of mated males/females in response to heat stress during aging, we conducted a fully factorial experiment to test the basal and plastic heat tolerance (CTmax, critical thermal maximum) of males and females with different mating statuses (virgin and mated) at different ages (5, 10, and 15 days after eclosion) after different acclimation regimes (null, rapid and developmental heat acclimation) in a well-known invasive species, Drosophila suzukii. We found that mating could change the heat tolerance of adults during aging. Mated females had higher basal heat tolerance than virgin females, while mated males had lower tolerance than virgin males. Mating could generally decrease the acclimation capacity (i.e., plasticity of heat tolerance) during aging. Aged virgin adults had a much higher acclimation capacity than aged mated adults. Our findings suggest that phenotypic plasticity of heat tolerance may be a main strategy used by virgin adults to cope with heat events. The phenotypic plasticity of thermal tolerance could increase the invasion success of alien species in new areas by allowing them to rapid respond to local temperature changes.

Investigating reproductive success of the ladybird beetle Harmonia axyridis from the perspective of micropyle variation

Micropyles in insects are small openings that allow sperm entry into, and the number was usually decreased on unfertilized and (or) undeveloped eggs. However, reports showed that Harmonia axyridis, a reproductive success model, deposited similar number of micropyles on undeveloped and developing eggs. Thus, it was confusing whether micropyles in H. axyridis were unaffected. To solve this confusion, two experiments were conducted here. Firstly, virgin female and four different days delayed mating (DDM) experiments were conducted to reveal the effects of fertilization stimulus and delayed-fertilization. Secondly, intercrosses between a light-colored mutant (HAM, an adaptive deficiency) and wild type (HAW) were conducted to further reveal whether there were female and male interactions. We found that (1) eggs produced by virgin and DDM females had significantly less micropyles than control. Even so, more than 18 micropyles were observed on eggs following fertilization and, consequently, egg production as well as hatch rate was not negatively affected by mating delay; (2) number of micropyles was significantly varied among the four reciprocal crosses and virgin HAW female. Specifically, the heterozygous eggs (Cross-D) and wild-type homozygous eggs (Cross-B) respectively had the least and maximum micropyles, and eggs from virgin HAW female had significantly less micropyles compared to those from HAW female (Cross-B or Cross-C), but the number was significantly higher than those from HAM female (Cross-A or Cross-D). These results informed us that the number of micropyles in H. axyridis is plastic but maintaining a high-quantity that offers many benefits, which should contribute to its reproduction success.