The geographical range structure of the Holly Leaf-miner. III. Cold hardiness physiology

Larval parasitism in a specialist herbivore is explained by phenological synchrony and host plant availability

1. Parasitism is a key factor in the population dynamics of many herbivorous insects, although its impact on host populations varies widely, for instance, along latitudinal and altitudinal gradients. Understanding the sources of geographical variation in host-parasitoid interactions is crucial for reliably predicting the future success of the interacting species under a context of global change. 2. Here, we examine larval parasitism in the butterfly Aglais urticae in south-west Europe, where it is a mountain specialist. Larval nests were sampled over two years along altitudinal gradients in three Iberian mountain ranges, including the Sierra Nevada, home to its southernmost European population. Additional data on nettle condition and adult butterflies were obtained in the study areas. 3. These data sources were used to investigate whether or not differences in parasitism rates are related to the geographical position and phenology of the host, and to the availability of the host plants. 4. Phenological differences in the host populations between regions were related to the severity of summer drought and the corresponding differences in host plant availability. At the trailing-edge of its distribution, the butterfly's breeding season was restricted to the end of winter and spring, while in its northern Iberian range the season was prolonged until mid-summer. Although parasitism was an important source of mortality in all regions, parasitism rates and parasitoid richness were highest in the north and lowest in the south. Moreover, within a region, there was a notable increase in parasitism rates over time, which probably led to selection against an additional late-summer host generation in northern regions. Conversely, the shorter breeding season in Sierra Nevada resulted in a loss of synchrony between the host and one important late-season parasitoid, Sturmia bella, which may partly explain the high density of this butterfly species at the trailing-edge of its range. 5. Our results support the key role of host phenology in accounting for differences in parasitism rates between populations. They also provide insights into how climate through host plant availability affects host phenology and, ultimately, the impact of parasitism on host populations.

The potential global distribution of the papaya mealybug, Paracoccus marginatus, a polyphagous pest

BACKGROUND

The papaya mealybug, Paracoccus marginatus, is a highly polyphagous invasive pest that affects more than 200 plants, many of which are of economic importance. We modelled the potential distribution of P. marginatus using CLIMEX, a process-oriented, climate-based niche model. We combined this model with spatial data on irrigation and cropping patterns to increase the real-world applicability of the model.

RESULTS

The resulting model agreed with known distribution points for this pest and with broad areas where P. marginatus has been reported, but for which no GPS data were available. Our model highlights the potential expansion of P. marginatus into novel areas in Central and East Africa, as well as further expansion in Central America and Asia, as these areas are highly climatically suitable, and have large expanses of suitable crop hosts. It also highlights areas, such as the central and eastern states of the USA as well as the western provinces of China, that are suitable for seasonal invasions of P. marginatus.

CONCLUSION

Our results offer refined resolution on areas with high potential for invasion by P. marginatus. © 2020 Society of Chemical Industry.

The effect of multiple natural enemies on a shared herbivore prey

Natural enemy diversity is thought to be important for effective suppression of herbivores in production systems. Studies investigating the importance of the diversity and composition of the natural enemy complex often use within-year empirical studies or experimental exclusion setups.However, within-year population suppression might not translate in long-term population regulation. Therefore, I used a combination of long-term data collection and an exclusion experiment to investigate mechanisms behind year-to-year population changes and potential effects of disturbance of the natural enemy complex.Using the holly leaf miner study system in Wytham Woods, I find that the dominant predator in the system does not necessarily contribute the most to the reduction in year-to-year changes in mine density or within-patch fluctuations. Using the exclusion experiment, it becomes clear that parasitism later in the prey life cycle can to a certain level compensate for disruption of mortality in the earlier life stage of the prey.Thus, for host suppression in perennial systems the mortality pressure over the whole life cycle is important and disturbance during one part of the life cycle might not necessarily be buffered by mortality in other parts of the life cycle, especially if the natural enemy complex consists of multiple predator guilds.

Cloning and expression of genes encoding heat shock proteins in Liriomyza trifolii and comparison with two congener leafminer species

The polyphagous agromyzid fly, Liriomyza trifolii, is a significant and important insect pest of ornamental and vegetable crops worldwide. The adaptation of insects to different environments is facilitated by heat shock proteins (HSPs), which play an important role in acclimation to thermal stress. In this study, we cloned and characterized five HSP-encoding genes of L. trifolii (Lthsp20, Lthsp40, Lthsp60, Lthsp70, and Lthsp90) and monitored their expression under different thermal stresses using real-time quantitative PCR. Pupae of L. trifolii were exposed to 19 different temperatures ranging from -20 to 45°C. The results revealed that Lthsp20, Lthsp40, Lthsp70 and Lthsp90 were significantly upregulated in response to both heat and cold stress, while Lthsp60 was induced only by heat temperatures. The temperatures of the onset (T_on) and maximal (T_max) expression of the five Lthsps were also determined and compared with published T_on and T_max values of homologous genes in L. sativae and L. huidobrensis. Although L. trifolii occurs primarily in southern China, it has cold tolerance comparable with the other two Liriomyza species. Based on the heat shock proteins expression patterns, L. trifolii has the capacity to tolerate extreme temperatures and the potential to disseminate to northern regions of China.

Measuring thermal behavior in smaller insects: A case study in Drosophila melanogaster demonstrates effects of sex, geographic origin, and rearing temperature on adult behavior

Measuring thermal behavior in smaller insects is particularly challenging. In this study, we describe a new horizontal thermal gradient apparatus designed to study adult thermal behavior in small insects and apply it using D. melanogaster as a model and case study. Specifically, we used this apparatus and associated methodology to examine the effects of sex, geographic origin, and developmental rearing temperature on temperature preferences exhibited by adults in a controlled laboratory environment. The thermal gradient established by the apparatus was stable over diurnal and calendar time. Furthermore, the distribution of adult flies across thermal habitats within the apparatus remained stable following the period of acclimation, as evidenced by the high degree of repeatability across both biological and technical replicates. Our data demonstrate significant and predictable variation in temperature preference for all 3 assayed variables. Behaviorally, females were more sensitive than males to higher temperatures. Flies originating from high latitude, temperate populations exhibited a greater preference for cooler temperatures; conversely, flies originating from low latitude, tropical habitats demonstrated a relative preference for higher temperatures. Similarly, larval rearing temperature was positively associated with adult thermal behavior: low culture temperatures increased the relative adult preference for cooler temperatures, and this response was distinct between the sexes and for flies from the temperate and subtropical geographic regions. Together, these results demonstrate that the temperature chamber apparatus elicits robust, predictable, and quantifiable thermal preference behavior that could readily be applied to other taxa to examine the role of temperature-mediated behavior in a variety of contexts.

Cold truths: how winter drives responses of terrestrial organisms to climate change

Winter is a key driver of individual performance, community composition, and ecological interactions in terrestrial habitats. Although climate change research tends to focus on performance in the growing season, climate change is also modifying winter conditions rapidly. Changes to winter temperatures, the variability of winter conditions, and winter snow cover can interact to induce cold injury, alter energy and water balance, advance or retard phenology, and modify community interactions. Species vary in their susceptibility to these winter drivers, hampering efforts to predict biological responses to climate change. Existing frameworks for predicting the impacts of climate change do not incorporate the complexity of organismal responses to winter. Here, we synthesise organismal responses to winter climate change, and use this synthesis to build a framework to predict exposure and sensitivity to negative impacts. This framework can be used to estimate the vulnerability of species to winter climate change. We describe the importance of relationships between winter conditions and performance during the growing season in determining fitness, and demonstrate how summer and winter processes are linked. Incorporating winter into current models will require concerted effort from theoreticians and empiricists, and the expansion of current growing-season studies to incorporate winter.

No evidence for enemy release during range expansion of an evergreen tree in northern Europe

Plant distributions are dynamic but the role of plant-insect interactions in controlling range dynamics is not well understood. Enemy release, for example could facilitate plant range expansion under climate change. We conducted a transplant experiment with the evergreen tree Ilex aquifolium L. in both the historical and the expanding range in Denmark to study possible effects of geographical position, small-scale distance, and plant types on presence and performance of the monophagous insect leaf-miner Phytomyza ilicis Curtis. The leaf miner was present in the entire range of I. aquifolium in Denmark, and there were no differences in emergence success depending on geographical position. Small-scale distance to existing adult plants influenced the activity of the insect on the transplants, and oviposition density was negatively correlated with distance to adult plants. Plant type had an effect on leaf miner feeding, oviposition and mining, and the native provenance of I. aquifolium supported higher densities than two cultivars. There was no evidence that enemy release facilitates the current range expansion of I. aquifolium.

Organismal climatology: analyzing environmental variability at scales relevant to physiological stress

Predicting when, where and with what magnitude climate change is likely to affect the fitness, abundance and distribution of organisms and the functioning of ecosystems has emerged as a high priority for scientists and resource managers. However, even in cases where we have detailed knowledge of current species' range boundaries, we often do not understand what, if any, aspects of weather and climate act to set these limits. This shortcoming significantly curtails our capacity to predict potential future range shifts in response to climate change, especially since the factors that set range boundaries under those novel conditions may be different from those that set limits today. We quantitatively examine a nine-year time series of temperature records relevant to the body temperatures of intertidal mussels as measured using biomimetic sensors. Specifically, we explore how a 'climatology' of body temperatures, as opposed to long-term records of habitat-level parameters such as air and water temperatures, can be used to extrapolate meaningful spatial and temporal patterns of physiological stress. Using different metrics that correspond to various aspects of physiological stress (seasonal means, cumulative temperature and the return time of extremes) we show that these potential environmental stressors do not always occur in synchrony with one another. Our analysis also shows that patterns of animal temperature are not well correlated with simple, commonly used metrics such as air temperature. Detailed physiological studies can provide guidance to predicting the effects of global climate change on natural ecosystems but only if we concomitantly record, archive and model environmental signals at appropriate scales.

Physiological tolerances account for range limits and abundance structure in an invasive slug

Despite the importance of understanding the mechanisms underlying range limits and abundance structure, few studies have sought to do so. Here we use a terrestrial slug species, Deroceras panormitanum, that has invaded a remote, largely predator-free, Southern Ocean island as a model system to do so. Across Marion Island, slug density does not conform to an abundant centre distribution. Rather, abundance structure is characterized by patches and gaps. These are associated with this desiccation-sensitive species' preference for biotic and drainage line habitats that share few characteristics except for their high humidity below the vegetation surface. The coastal range margin has a threshold form, rapidly rising from zero to high density. Slugs do not occur where soil-exchangeable Na values are higher than 3000 mg kg(-1), and in laboratory experiments, survival is high below this value but negligible above it. Upper elevation range margins are a function of the inability of this species to survive temperatures below an absolute limit of -6.4 degrees C, which is regularly exceeded at 200 m altitude, above which slug density declines to zero. However, the linear decline in density from the coastal peak is probably also a function of a decline in performance or time available for activity. This is probably associated with an altitudinal decline in mean annual soil temperature. These findings support previous predictions made regarding the form of density change when substrate or climatic factors set range limits.

Stage-related variation in rapid cold hardening as a test of the environmental predictability hypothesis

The environmental predictability (EP) hypothesis proposes that rapid cold hardening (RCH) might be common in temperate species incapable of surviving freezing events and which also dwell in unpredictable environments. The kelp fly Paractora dreuxi serves as a useful model organism to test this prediction at an intra-specific level because larvae and adults show different responses to low temperature despite occupying a similar unpredictable thermal environment. Here, using acclimation temperatures, which simulated seasonal temperature variation, we find little evidence for RCH in the freeze-intolerant adults but a limited RCH response in freeze-tolerant larvae. In the relatively short-lived adults, survival of -11 degrees C generally did not improve after 2h pre-treatments at -4, -2, 0, 10, 20 or 25 degrees C either in summer- (10 degrees C) or winter (0 degrees C)-acclimated individuals. By contrast, survival of summer-acclimated larvae to -7.6 degrees C was significantly improved by approximately 37% and 30% with -2 and 0 degrees C pre-treatments, respectively. The finding that summer-acclimated larvae showed RCH whereas this was not the case in the winter-acclimated larvae partially supports the predictions of the EP hypothesis. However, the EP hypothesis also predicts that the adults should have demonstrated an RCH response, yet they did not do so. Rather, it seems likely that they avoid stressful environments by behavioural thermoregulation. Differences in responses among the adults and larvae are therefore to some extent predictable from differences in their feeding requirements and behaviour. These results show that further studies of RCH should take into account the way in which differences among life stages influence the interaction between phenotypic plasticity and environmental variability and predictability.

Impact of extreme temperatures on parasitoids in a climate change perspective

Parasitoids depend on a series of adaptations to the ecology and physiology of their hosts and host plants for survival and are thus likely highly susceptible to changes in environmental conditions. We analyze the effects of global warming and extreme temperatures on the life-history traits of parasitoids and interactions with their hosts. Adaptations of parasitoids to low temperatures are similar to those of most ectotherms, but these adaptations are constrained by the responses of their hosts. Life-history traits are affected by cold exposure, and extreme temperatures can reduce endosymbiont populations inside a parasitoid, eventually eliminating populations of endosymbionts that are susceptible to high temperatures. In several cases, divergences between the thermal preferences of the host and those of the parasitoid lead to a disruption of the temporal or geographical synchronization, increasing the risk of host outbreaks. A careful analysis on how host-parasitoid systems react to changes in temperature is needed so that researchers may predict and manage the consequences of global change at the ecosystem level.

Moving beyond assumptions to understand abundance distributions across the ranges of species

The assumption that species are most abundant in the center of their range and decline in abundance toward the range edges has a long history in the ecological literature. This assumption has driven basic and applied ecological and evolutionary hypotheses about the causes of species range limits and their responses to climate change. Here, we review recent studies that are taking biogeographical ecology beyond previously held assumptions by observing populations in the field across large parts of the species range. When these studies combine data on abundance, demographics, organismal physiology, genetics and physical factors, they provide a promising approach for teasing out ecological and evolutionary mechanisms of the patterns and processes underlying species ranges.

Revisiting Heritable Variation and Limits to Species Distribution: Recent Developments

There are several evolutionary reasons why species distributions are locally and geographically limited, and these mostly revolve around gene flow and levels of genetic variation in populations. While there has been progress in developing models assessing the impact of gene flow, empirical data on hypotheses about factors limiting distributions remain rare. A few recent plant studies have highlighted that levels of additive genetic variance are not limiting, suggesting a role for gene flow. However, some animal studies, mostly onDrosophilaspecies, have refocused attention on heritable variation as an evolutionary reason for distribution limits. There is a need for comparative studies that examine levels of heritable variation across related species groups with broad and narrow distributions, and also across a range of traits including those likely to be under selection at borders. These studies will help to test specific predictions about the relative importance of genetic variance and gene flow in limiting current borders and also help in predicting the impact of future environmental changes on distribution shifts.

Can greenhouses eliminate the development of cold resistance of the leafminers?

Latitudinal patterns for quantitative traits in insect are commonly used to investigate climatic adaptation. We compare the cold resistance of the leafminer (Liriomyza sativae) pupa among populations distributed from tropical to temperate regions, incorporating the thermal overwintering limit of the insect's range. The patterns of cold resistance for northern and southern populations differ. The southern populations significantly increased their cold resistance with latitude, showing a latitudinal pattern independent of seasons, acclimation regimes, and assay methods. In contrast, the northern populations showed no stable patterns; they were always intermediate in cold hardiness between the low-latitude and high-latitude populations within the overwintering limit. Integration of these data with those of the biologically similar congeneric leafminer, L. huidobrensis, suggests that a pattern shift in stress tolerance associated with the overwintering range limit is probably a general adaptive strategy adopted by freeze-intolerant species that have a high-latitude boundary of distribution, but can only overwinter and develop in protected greenhouses in harsh seasons. Considering the widespread availability of greenhouses for overwintering insects in northern China, we speculated that the large-scale existence of thermally-buffered microhabitats in greenhouses might eliminate the development of cold resistance of the leafminer populations. However, results suggest a strong selection for increased cold resistance for natural populations of Liriomyza species at higher latitudes that can overwinter in the field, but not for populations at latitudes above the thermal limit. Thus, habitat modification associated with greenhouses can limit gene flow and reduce cold tolerances even at latitudes above where the leafminers can overwinter in the field.