J. S. Bale

Global Change and Arctic Ecosystems: Conclusions and Predictions from Experiments with Terrestrial Invertebrates on Spitsbergen

Extensive studies on invertebrates from Ny-Alesund, Spitsbergen, Svalbard and more limited data on aphids from Abisko, Sweden, produced the following main conclusions: (1) The population response to raised summer temperatures differed between the above and the below ground species, both in terms of speed and magnitude. (2) Similar animal communities responded differently to similar temperature manipulations on sites with different vegetation cover and composition. (3) For soil animals the between-year and between-site variations in population densities, were greater than the differences produced by the temperature manipulation experiments at any one site in any year. (4) Infrequent extreme climatic events strongly influence long-term trends in population density and community composition. (5) The population response of invertebrates to climate warming is greatest and most rapid at the coldest sites. (6) The spatial distribution of the above ground insect herbivores on their host plant is temperature limited. (7) The numerical abundance of flying predators/parasitoids of the above-ground herbivores is low. (8) The spatial distribution of some predators may be thermally restricted and less extensive than that of their prey. (9) Habitat temperature is the driving variable determining the flight activity patterns of insects. (10) Increased summer temperatures may alter or disrupt the seasonal patterns of insect emergence, particularly in species where the life cycle is cued into the seasonal rhythm. (11) The common species of arctic soil mites and Collembola are well adapted to survive enhanced summer temperatures, providing that moisture is not limited. (12) Water availability during the summer growing period is probably of greater significance than temperature in determining the survival and success of many arctic soil invertebrate groups. (13) Arctic soil microarthropod species are well adapted to survive and operate at subzero and low positive summer temperatures. (14) Freeze-thaw events represent critical points in the life history of the microarthropods. (15) Supercooling points are sometimes poor indicators of the capacity of arctic soil microarthopods to survive low temperatures. From these findings predictions are made as to how high arctic communities will respond to predicted changes in climate.

Effects of experimental temperature elevation on high-arctic soil microarthropod populations

An experiment was conducted to measure the effects of summer warming on the total population densities of soil-dwelling microarthropods in the high Arctic and to compare these results with those from natural between-year and between-site variations. Small polythene tents were used to elevate summer temperatures over 3 years on polar semi-desert and tundra heath in West Spitsbergen, Svalbard, Norway. Soil cores were taken at regular intervals from tented and untented (control) plots and heat extracted for mites (Acarina: Oribatida) and springtails (Collembola). Species present were similar at both sites, but at the start of the experiment total springtail populations were greater at the polar semi-desert whilst oribatid mite densities were equal at both sites. No significant effect of temperature elevation on oribatid mite populations emerged, even after 3 years. By contrast, springtail numbers were significantly lower on tented versus control plots at the polar semi-desert at the end of year 3, but not so at the tundra heath. Collembola numbers declined at both sites during the warm dry midsummers of 1992/1993 and this was most marked at the better drained polar semi-desert site. Over the equivalent period total oribatid mite populations, while relatively more stable, increased significantly at the polar semi-desert as a result of an increase in the number of juveniles. Results are interpreted in the context of the ecophysiological adaptations of oribatid mites and springtails to soil temperature and moisture. The resulting survival characteristics are considered in relation to the temperature and moisture characteristics of the two sites. The experiment demonstrated that year to year variation in climate, interacting with physical differences between sites, produced an equal or greater effect on microarthropod numbers at any one site than the 8–10% increase in “heat availability” (day degrees above zero) resulting from the summer tent treatment. The limitations of the use of tents to elevate soil temperatures are discussed. Comparisons are made with microarthropod population data from other polar and alpine sites.

Difficulties in location and acceptance of phloem sap combined with reduced concentration of phloem amino acids explain lowered performance of the aphid Rhopalosiphum padi on nitrogen deficient barley (Hordeum vulgare) seedlings.

Effects of nitrogen deficiency in hydroponically grown barley seedlings (Hordeum vulgare L.) on the development and reproduction of the aphid Rhopalosiphum padi (L.) (Hemiptera: Aphididae) were investigated.

Exploring plant responses to aphid feeding using a full Arabidopsis microarray reveals a small number of genes with significantly altered expression.

The aim of this study was to determine which Arabidopsis thaliana (L.) genes had significantly altered expression following 2-36 h of infestation by the aphid Myzus persicae (Sulzer). Six biological replicates were performed for both control and treatment at each time point, allowing rigorous statistical analysis of any changes. Only two genes showed altered expression after 2 h (one up- and one down-regulated) while two were down-regulated and twenty three were up-regulated at 36 h. The transcript annotation allowed classification of the significantly altered genes into a number of classes, including those involved in cell wall modification, carbon metabolism and signalling. Additionally, a number of genes were implicated in oxidative stress and defence against other pathogens. Five genes could not currently be assigned any function. The changes in gene expression are discussed in relation to current models of plant-insect interactions.

Effects of temperature on the establishment potential of the predatory mite Amblyseius californicus McGregor (Acari: Phytoseiidae) in the UK

Amblyseius californicus was introduced into the UK in the early 1990s as a biocontrol agent against glasshouse red spider mite Tetranychus urticae. This study investigated the effects of temperature on the establishment potential of A. californicus in the UK in the light of recent reports of their successful overwintering outside of glasshouse environments. The developmental thresholds were 9.9 and 8.6 degrees C respectively using simple and weighted linear regression. Using the day-degree requirement per generation calculated by weighted regression (143 day-degrees) in combination with climate data, it was estimated that up to seven generations would be possible annually outdoors in the UK. Non-diapausing adult females froze at -22 degrees C, with 100% mortality after reaching their freezing temperature. Up to 90% of mites died before freezing after short exposures to low temperatures. Significant acclimation responses occurred; 90% of acclimated individuals survived 26 days exposure at 0 degrees C and 11 days at -5 degrees C (acclimated mites were reared at 19 degrees C, 6L:18D followed by 1 week at 10 degrees C, 12L:12D). Non-diapausing adult females survived over 3 months outdoors in winter under sheltered conditions and oviposition was observed. The experimental protocol used in this study is discussed as a pre-release screen for the establishment potential of other Amblyseius species, and similar non-native biocontrol agents.

Effects of host plant drought stress on the performance of the bird cherry-oat aphid, Rhopalosiphum padi (L.): a mechanistic analysis

Abstract.  1. The growth (increase in height and leaf number) of four grass species was reduced by a −0.5 MPa drought stress, but the performance of an associated herbivore, Rhopalosiphum padi (L.), was not affected consistently. The intrinsic rate of increase of R. padi was reduced by drought stress on three grass species, including Dactylis glomerata (L.), but was unaffected on Arrhenatherum elatius (L.). Therefore, there is no general relationship in the effect of plant drought on an insect herbivore, even among closely related host plant species.

Effects of temperature elevation on a field population of Acyrthosiphon svalbardicum (Hemiptera: Aphididae) on Spitsbergen

A manipulation experiment was carried out on a field population of the aphid Acyrthosiphon svalbardicum near Ny Ålesund, on the high arctic island of Spitsbergen, using cloches to raise temperature. An average rise in temperature of 2.8 deg. C over the summer season markedly advanced the phenology of both the host plant Dryas octopetala and the aphid. Advanced aphid phenology, with concomitant increases in reproductive output and survival, and successful completion of the life-cycle led to an eleven-fold increase in the number of overwintering eggs. Thermal budget requirements in day degrees above 0°C were calculated for key life-cycle stages of the aphid. Temperature data from Ny Ålesund over the past 23 years were used to calculate thermal budgets for the field site over the same period and these were compared with the requirements of the aphid. Each estimated thermal budget was then adjusted to simulate the effect of a +2, +4, and −2deg. C change in average temperature on aphid performance. This retrospective analysis (i) confirms that the life-cycle of A. svalbardicum is well suited to exploit higher summer temperatures, (ii) indicates that the annual success of local populations are sensitive to small changes in temperature and (iii) suggests that the aphid is living at the limits of its thermal range at Ny Ålesund based on its summer thermal budget requirements.

Effect of long‐term and rapid cold hardening on the cold torpor temperature of an aphid

Abstract The effect of long‐term (seasonal) acclimation and rapid cold hardening is investigated on the cold torpor temperature (CTmin) of adult grain aphids, Sitobion avenae, reared at 20 or 10 °C for more than 6 months before experimentation. Rapid cold hardening is induced by exposing aphids reared at 20 to 0 °C for 3 h and aphids reared at 10 to 0 °C for 30 min (acclimation regimes previously found to induce maximum rapid cold hardening). The effect of cooling aphids from the same rearing regimes from 10 to −10 °C at 1, 0.5 and 0.1 °C min−1 is also investigated. In the 20 °C acclimated population, rapid cold hardening and cooling at 0.1 °C min−1 both produce a significant decrease in CTmin from 1.5 ± 0.3 to –0.9 ± 0.3 and –1.3 ± 0.3 °C, respectively. Rapid cold hardening also results in a significant reduction in CTmin of the population reared at 10 °C from 0.8 ± 0.1 to –0.9 ± 0.2 °C. However, none of the cooling regimes tested reduces the CTmin of the winter‐acclimated (10 °C) population. The present study demonstrates that rapid cold‐hardening induced during the cooling phase of natural diurnal temperature cycles could lower the movement threshold of S. avenae, allowing insects to move and continue feeding at lower temperatures than would otherwise be possible.

Extreme adaptive life-cycle in a high arctic aphid, Acyrthosiphon svalbardicum

Abstract. 1 The year‐round biology of a high arctic aphid is described for the first time. 2 The life‐cycle is shown to be genetically determined, and thus markedly different to temperate species where the observed polymorphism is governed primarily by external environmental cues. 3 The fundatrix, which emerges from the overwintering egg, gives birth directly to sexual morphs, a phenomenon previously undescribed in the Aphidinae. This process is essentially prevented in temperate aphids by an endogenous mechanism, the interval timer. 4 In addition to the sexual morphs, the fundatrix produces a small number of parthenogenetic individuals (viviparae) that give rise to a third generation. This last generation consists exclusively of oviparae and males that would increase the number of overwintering eggs provided there is sufficient thermal budget for them to mature and oviposit before conditions become adverse. 5 The position of particular morphs in the birth sequences of the second and third generations maximize the chances of survival in harsh conditions, whilst enhancing the likelihood that individuals from the third generation will add to the number of overwintering eggs. 6 Guaranteed egg production combined with an in‐built flexibility to produce an extra generation in particularly favourable seasons, confer adaptations to the high arctic environment, and ideally suit this aphid to exploit elevated temperatures in an era of climate change.

Rapid cold hardening in the western flower thrips Frankliniella occidentalis.

A rapid cold hardening process is reported in first instar larvae of Frankliniella occidentalis. When larvae are transferred directly from 20 degrees C to -11.5 degrees C for 2h there is 78% mortality, whereas exposure to 0 degrees C for 4h prior to transfer to -11.5 degrees C reduces mortality to 10%. The response can also be induced by exposure to 5 degrees C for 4h or by gradual cooling at rates between 0.1 and 0.5 degrees C min(-1.) The acquired cold tolerance is transient and is rapidly lost (after 1h at 20 degrees C). Rapid cold hardening extends survival times at -11.5 degrees C and depresses lethal temperatures in short (2h) exposures. Rearing at 15 degrees C (12L:12D), (a cold acclimation regime for F. occidentalis), does not protect against the cold shock induced by direct transfer to -11.5 degrees C (which rapid cold hardening does) but does extend survival time at -5 degrees C (i.e. increased chill tolerance) whilst rapid cold hardening does not. The rapid and longer term cold hardening responses in F. occidentalis therefore appear to have different underlying mechanisms.