Costs of averting or prematurely terminating diapause associated with slow decline of metabolic rates at low temperature

Variation in Thermal Sensitivity of Diapause Development among Individuals and over Time Predicts Life History Timing in a Univoltine Insect

AbstractPhysiological time is important for understanding the development and seasonal timing of ectothermic animals but has largely been applied to developmental processes that occur during spring and summer, such as morphogenesis. There is a substantial knowledge gap in the relationship between temperature and development during winter, a season that is increasingly impacted by climate change. Most temperate insects overwinter in diapause, a developmental process with little obvious morphological change. We used principles from the physiological time literature to measure and model the thermal sensitivity of diapause development rate in the apple maggot fly Rhagoletis pomonella, a univoltine fly whose diapause duration varies substantially within and among populations. We show that diapause duration can be predicted by modeling a relationship between temperature and development rate that is shifted toward lower temperatures compared with typical models of morphogenic, nondiapause development. However, incorporating interindividual variation and ontogenetic variation in the temperature-to-development rate relationship was critical for accurately predicting fly emergence, as diapause development proceeded more quickly at high temperatures later in diapause. We conclude that the conceptual framework may be flexibly applied to other insects and discuss possible mechanisms of diapause timers and implications for phenology with warming winters.

Bigger isn't always better: Challenging assumptions about the associations between diapause, body weight, and overwintering survival

During the winter, animals face limited food availability. Many animals enter dormancy to reduce their winter energy expenditure. Most insects spend the winter in diapause, a state of programmed dormancy. It is often assumed that diapausing insects need nutrient stores to fuel their many months of basal metabolism and must grow heavier than their non-diapause-programmed counterparts. However, the extent to which food limitation affects body weight during overwintering preparation as well as the likelihood and duration of diapause remains unclear. We limited the duration of the feeding period and thus the total quantity of food available to diapause-destined larvae of the pupal-diapausing flesh fly, Sarcophaga crassipalpis, to test how food limitation affects body weight in the context of diapause programming. We also tested the extent to which food deprivation and body weight affect the likelihood and duration of diapause. We hypothesized that diapause-destined larvae grow more quickly and pupariate at a heavier body weight than non-diapause larvae. We also hypothesized that body weight is more dramatically reduced by food limitations when a larva is programmed for diapause. Finally, we hypothesized that larvae with lighter body weight (i.e., food limited) are less likely to enter pupal diapause and also stay in diapause for a shorter duration than heavier, well-fed, individuals. Contrary to our hypotheses that diapausing insects are heavier than their non-diapausing counterparts, we found diapausing pupae weighed less than non-diapausing pupae, especially when larvae received limited food. We found light pupae did not abort their diapause program. In both diapausing and non-diapausing pupae, body weight was positively correlated with simulated winter survival. However, above a weight threshold, body weight no longer affected simulated winter survival in diapausing pupae. Contrary to our predictions and the general consensus in much of the diapause literature, we also found that lighter pupae stayed in diapause longer than heavier pupae. Overall, our results challenge the precept that body weight and diapause are positively associated. The relationship between body weight and diapause is complex and may be affected by the availability of food before and after winter, the availability of high-quality overwintering sites, and the life history of a particular insect.

Temperature dependence of gas exchange patterns shift as diapause progresses in the butterfly Pieris napi

Insects have the capacity to significantly modify their metabolic rate according to environmental conditions and physiological requirement. Consequently, the respiratory patterns can range from continuous gas exchange (CGE) to discontinuous gas exchange (DGE). In the latter, spiracles are kept closed during much of the time, and gas exchange occurs only during short periods when spiracles are opened. While ultimate causes and benefits of DGE remain debated, it is often seen during insect diapause, a deep resting stage that insects induce to survive unfavourable environmental conditions, such as winter. The present study explores the shifts between CGE and DGE during diapause by performing long continuous respirometry measurements at multiple temperatures during key diapause stages in the green-veined white butterfly Pieris napi. The primary goal is to explore respiratory pattern as a non-invasive method to assess whether pupae are in diapause or have transitioned to post-diapause. Respiratory pattern can also provide insight into endogenous processes taking place during diapause, and the prolonged duration of diapause allows for the detailed study of the thermal dependence of the DGE pattern. Pupae change from CGE to DGE a few days after pupation, and this shift coincides with metabolic rate suppression during diapause initiation. Once in diapause, pupae maintain DGE even at elevated temperatures that significantly increase CO2 production. Instead of shifting respiratory pattern to CGE, pupae increase the frequency of DGE cycles. Since total CO2 released during a single open phase remains unchanged, our results suggest that P. napi pupae defend a maximum internal ρCO2 set point, even in their heavily suppressed diapause state. During post-diapause development, CO2 production increases as a function of development and changes to CGE during temperature conditions permissive for development. Taken together, the results show that respiratory patterns are highly regulated during diapause in P. napi and change predictably as diapause progresses.

Host-Parasitoid Phenology, Distribution, and Biological Control under Climate Change

Climate change raises a serious threat to global entomofauna-the foundation of many ecosystems-by threatening species preservation and the ecosystem services they provide. Already, changes in climate-warming-are causing (i) sharp phenological mismatches among host-parasitoid systems by reducing the window of host susceptibility, leading to early emergence of either the host or its associated parasitoid and affecting mismatched species' fitness and abundance; (ii) shifting arthropods' expansion range towards higher altitudes, and therefore migratory pest infestations are more likely; and (iii) reducing biological control effectiveness by natural enemies, leading to potential pest outbreaks. Here, we provided an overview of the warming consequences on biodiversity and functionality of agroecosystems, highlighting the vital role that phenology plays in ecology. Also, we discussed how phenological mismatches would affect biological control efficacy, since an accurate description of stage differentiation (metamorphosis) of a pest and its associated natural enemy is crucial in order to know the exact time of the host susceptibility/suitability or stage when the parasitoids are able to optimize their parasitization or performance. Campaigns regarding landscape structure/heterogeneity, reduction of pesticides, and modelling approaches are urgently needed in order to safeguard populations of natural enemies in a future warmer world.

Seasonal energetics: are insects constrained by energy during dormancy?

In seasonal environments, many animals, including insects, enter dormancy, where they are limited to a fixed energy budget. The inability to replenish energetic stores during these periods suggests insects should be constrained by pre-dormancy energy stores. Over the last century, the community of researchers working on survival during dormancy has operated under the strong assumption that energy limitation is a key fitness trait driving the evolution of seasonal strategies. That is, energy use has to be minimized during dormancy because insects otherwise run out of energy and die during dormancy, or are left with too little energy to complete development, reproductive maturation or other costly post-dormancy processes such as dispersal or nest building. But if energy is so strongly constrained during dormancy, how can some insects - even within the same species and population - be dormant in very warm environments or show prolonged dormancy for many successive years? In this Commentary, we discuss major assumptions regarding dormancy energetics and outline cases where insects appear to align with our assumptions and where they do not. We then highlight several research directions that could help link organismal energy use with landscape-level changes. Overall, the optimal energetic strategy during dormancy might not be to simply minimize metabolic rate, but instead to maintain a level that matches the demands of the specific life-history strategy. Given the influence of temperature on energy use rates of insects in winter, understanding dormancy energetic strategies is critical in order to determine the potential impacts of climate change on insects in seasonal environments.

Transcriptomic evidence indicates that montane leaf beetles prioritize digestion and reproduction in a sex-specific manner during emergence from dormancy

During winter, many organisms conserve resources by entering dormancy, suppressing metabolism and biosynthesis. The transition out of winter dormancy to summer activity requires a quick reversal of this suppression, in order to exploit now-favorable environmental conditions. To date, mechanisms by which winter climate variation affects this transition remains unelucidated. Here we experimentally manipulated snow cover for naturally overwintering montane leaf beetles (Chrysomela aeneicollis), and profiled changes in gene expression during the transition out of dormancy in spring. Upon emergence, beetles up-regulate transcripts associated with digestion and nutrient acquisition and down regulate those associated with lipid metabolism, suggesting a shift away from utilizing stored lipid and towards digestion of carbohydrate-rich host plant tissue. Development of digestive capacity is followed by up-regulation of transcripts associated with reproduction; a transition that occurs earlier in females than males. Snow manipulation strongly affected the ground thermal regime and correspondingly gene expression profiles, with beetles showing a delayed up-regulation of reproduction in the dry compared to snowy plots. This suggests that winter conditions can alter the timing and prioritization of processes during emergence from dormancy, potentially magnifying the effects of declining snow cover in the Sierra's and other snowy mountains.

Cold tolerance and diapause within and across trophic levels: Endoparasitic wasps and their fly host have similar phenotypes

Low temperatures associated with winter can limit the survival of organisms, especially ectotherms whose body temperature is similar to their environment. However, there is a gap in understanding how overwintering may vary among groups of species that interact closely, such as multiple parasitoid species that attack the same host insect. Here, we investigate cold tolerance and diapause phenotypes in three endoparasitoid wasps of the apple maggot fly Rhagoletis pomonella (Diptera: Tephritidae): Utetes canaliculatus, Diachasma alloeum, and Diachasmimorpha mellea (Hymenoptera: Braconidae). Using a combination of respirometry and eclosion tracking, we found that all three wasp species exhibited the same three diapause duration phenotypes as the fly host. Weak (short duration) diapause was rare, with <5 % of all three wasp species prematurely terminating diapause at 21 °C. Most D.mellea (93 %) entered a more intense (longer duration) diapause that did not terminate within 100 d at this warm temperature. The majority of U.canaliculatus (92 %) and D. alloeum (72 %) averted diapause (non-diapause) at 21 °C. There was limited interspecific variation in acute cold tolerance among the three wasp species: wasps and flies had similarly high survival (>87 %) following exposure to extreme low temperatures (-20 °C) as long as their body fluids did not freeze. The three wasp species also displayed little interspecific variation in survival following prolonged exposure to mild chilling of 8 or more weeks at 4 °C. Our study thus documents a remarkable conservation of cold tolerance and diapause phenotypes within and across trophic levels.

Metabolic reserves of diapausing western cherry fruit fly (Diptera: Tephritidae) pupae in relation to chill duration and post-chill rearing conditions

How different macronutrients are utilized at various stages of pupal diapause and the effects of winter length on nutrient reserves remain poorly studied for most insects. Western cherry fruit fly, Rhagoletis indifferens (Diptera: Tephritidae), is a specialist on cherries in higher latitudes or elevations in western North America that exhibits a obligate pupal diapause requiring chilling before adult development can occur. We determined the relationship between metabolic reserves and diapause status in R. indifferens pupae, testing the hypotheses that lipids are the primary reserves utilized during diapause and that long periods of warmth deplete these reserves more than periods of cold. Effects of 0- to 20-week durations at 3°C and subsequent exposure to 23°C and 16:8 L:D (warm rearing conditions) for 0 to 7 weeks on lipid, protein, soluble carbohydrates, and glycogen reserves of R. indifferens pupae were determined. During diapause, lipid reserves were the primary source of energy utilized by R. indifferens, while protein and soluble carbohydrates levels were stable throughout diapause and thus less utilized. At post-diapause, glycogen levels fluctuated the most, indicating that lipid reserves were utilized to produce glycogen to support metabolism for adult fly development. Unchilled pupae did not deplete lipid reserves, unlike chilled pupae, likely because unchilled pupae remained in diapause. Rhagoletis indifferens may have evolved a nutrient utilization strategy typical of rigid diapausing insects in higher latitude environments.

Genomically correlated trait combinations and antagonistic selection contributing to counterintuitive genetic patterns of adaptive diapause divergence in Rhagoletis flies

Adaptation to novel environments can result in unanticipated genomic responses to selection. Here, we illustrate how multifarious, correlational selection helps explain a counterintuitive pattern of genetic divergence between the recently derived apple- and ancestral hawthorn-infesting host races of Rhagoletis pomonella (Diptera: Tephritidae). The apple host race terminates diapause and emerges as adults earlier in the season than the hawthorn host race, to coincide with the earlier fruiting phenology of their apple hosts. However, alleles at many loci associated with later emergence paradoxically occur at higher frequencies in sympatric populations of the apple compared to the hawthorn race. We present genomic evidence that historical selection over geographically varying environmental gradients across North America generated genetic correlations between two life history traits, diapause intensity and diapause termination, in the hawthorn host race. Moreover, the loci associated with these life history traits are concentrated in genomic regions in high linkage disequilibrium (LD). These genetic correlations are antagonistic to contemporary selection on local apple host race populations that favours increased initial diapause depth and earlier, not later, diapause termination. Thus, the paradox of apple flies appears due, in part, to pleiotropy or linkage of alleles associated with later adult emergence and increased initial diapause intensity, the latter trait strongly selected for by the earlier phenology of apples. Our results demonstrate how understanding of multivariate trait combinations and the correlative nature of selective forces acting on them can improve predictions concerning adaptive evolution and help explain seemingly counterintuitive patterns of genetic diversity in nature.