Genetic differentiation of the green spruce aphid (Elatobium abietinum Walker), a recent invader to Iceland

1 The RAPD method (Random Amplified Polymorphic DNA) was used to investigate genetic diversity of the green spruce aphid, Elatobium abietinum Walker, a pest introduced recently to Iceland. 2 This aphid in Iceland comprised two polymorphic populations, one in the east and the other in the west of the country. The genetic variation between sites within a population was continuous and appeared to be in good agreement with geographical distances. 3 In the eastern population the variation was greater between sites than within sites, whereas in the western population the pattern of variation appeared to be the opposite. This overall greater genetic variation in the eastern population could be due to its having been colonized earlier than the western one. 4 The study also demonstrated a close relationship between the green spruce aphid in Iceland and aphids from Denmark, which agrees with their assumed origin. The differences in introduction time, adaptation and competitiveness between the two Icelandic populations are discussed.     

Performance of the green spruce aphid, Elatobium abietinum (Walker) on previously defoliated Sitka spruce

Abstract 1 The green spruce aphid, Elatobium abietinum, is an important defoliator of Sitka spruce in the U.K. However, it is usual for years in which high E. abietinum populations occur to be followed by a year with low aphid densities. The possibility that the performance of E. abietinum is reduced on previously infested Sitka spruce, and that this is the cause of year‐to‐year fluctuations in population density, was investigated by comparing population development and the growth rate of individual aphids on experimentally defoliated trees. 2 Separate experiments were performed to determine whether aphid performance was reduced either in the autumn immediately after defoliation in the spring, or was reduced in the spring of the next year. Different rates of initial defoliation on trees used to test aphid performance were created by artificially infesting the trees with aphids in the spring before the experiments, and varying the time of infestation. 3 Population development and the mean relative growth rate (MRGR) of individual aphids on previously defoliated and undefoliated Sitka spruce did not differ significantly in the spring of the next year. No differences were observed in the nutrient content of the 1‐year‐old needles of previously defoliated or undefoliated trees at this time. 4 In the autumn and winter immediately after spring defoliation, aphid MRGR was significantly higher on trees that had been heavily defoliated earlier in the season compared with trees that had been lightly defoliated. However, the difference in MRGR decreased over the winter period. Nitrogen, phosphorous and potassium concentrations were 9.4–12.2% higher, at the beginning of the autumn, in the current year needles of heavily defoliated trees than in the current year needles of lightly defoliated trees. 5 The experiments indicate that high populations of E. abietinum in the spring do not induce any defensive mechanisms in Sitka spruce that adversely affect subsequent generations of the aphid. By contrast, the results suggest that high spring densities of the aphid improve the nutritional quality of the current year's foliage for autumn generations.     

Interannual population dynamics of the green spruce aphid Elatobium abietinum (Walker) in France

The hypothesis that similar processes govern interannual dynamics of green spruce aphid in the UK and France, is generally supported by the application of a general discrete model. A simple model based on relatively few parameters was able to closely characterise interannual population dynamics from completely independent aerial and arboreal samples of aphids. Long-term field population estimates of the green spruce aphid Elatobium abietinum (Walker) in France have provided the opportunity to select and evaluate the generality of a model, which was developed in the UK to explain the year-to-year variations in peak abundance of the aphid. The objective was to observe the influence of the local climates and disturbing climate factors on the population densities of the insect in two regions of France. The model uses climate variables and aphid population data from regular samples in the two regions that were investigated. A general discrete model was used to predict aphid population densities. The model performed well in tracking the interannual patterns of population but was less likely to predict absolute population density. To improve predictions, further account would need to be taken of additional site-specific climate variables and the strength of overcompensating density dependence. Nevertheless, it is clear that broadly similar processes are at work in the population dynamics of this insect across its biogeographical range.     

Seasonal changes in the distribution of green spruce aphid Elatobium abietinum (Walker) (Homoptera: Aphididae) in the canopy of Sitka spruce

1 Seasonal changes in the distribution of green spruce aphid Elatobium abietinum (Walker) within the canopy of 20–25‐year‐old Sitka spruce are described based on data from two low‐altitude sites (310–420 m above sea‐level), two mid‐altitude sites (500–550 m a.s.l) and one high‐altitude site (610 m a.s.l). 2 Aphids were counted throughout the canopy on shoots representative of all needle age‐classes present at each whorl of branches. Counts were made during the middle week of each month from September to July for 4 years (1999–2003), and mean E. abietinum densities at each canopy position were calculated separately for each month and for the low‐, mid‐ and high‐altitude sites. 3 During September to November, the highest densities of E. abietinum occurred on 3–4‐year‐old needles on branches low in the canopy. Over the winter and spring, the centre of the aphid’s distribution shifted outward and upward, so that by June of the next year the highest aphid densities occurred on current and 1‐year‐old needles on branches near the top of the tree. 4 The aphid distribution was re‐set each year during July, at the time when the nutrient quality of the host was in decline and E. abietinum populations were decreasing. Aphid densities decreased less on 3–4‐year‐old needles than on current and 1‐year‐old needles, suggesting that older needles were a superior food resource at this time of year and in the autumn. However, other factors, such as higher temperatures in the upper canopy during the summer or differential mortality caused by natural enemies, could also have contributed to the change in distribution. 5 The outward and upward shift in the aphid distribution over the winter period provided no evidence that aphids at positions lower and deeper in the canopy were better insulated from freezing temperatures and had higher over‐winter survival rates. Mean air temperatures at the top and bottom of the canopy during the winter were also found to differ by only 0.1–0.2 °C. 6 The percentage of the total aphids per tree that occurred on current or 1‐year‐old needles varied widely between seasons and between sites. Consequently, sampling programmes designed to estimate total population numbers of E. abietinum have little option but to sample needles throughout the canopy, and at regular intervals during the period when the aphid is abundant.     

Consistency of resistance to attack by the green spruce aphid (Elatobium abietinum Walker) in different ontogenetic stages of Sitka spruce

Abstract 1 The susceptibility of different genotypes of 29‐year‐old Sitka spruce to damage by the green spruce aphid, Elatobium abietinum, was investigated in a progeny trial where aphid damage on individual trees had previously been assessed twice in an earlier stage of ontogenetic development. The progeny trial comprised 14 open‐pollinated families originating from a clonal seed orchard that had been established using mature spruce trees selected for aphid resistance. 2 Previous investigations had demonstrated that resistance was inherited by the offspring, and that differences in resistance between progenies of the individual orchard clones were highly significant. 3 Susceptibility to aphid attack was recorded as the percentage loss of previous year's needles. Differences in susceptibility recorded between the juvenile trees were found to persist after the trees had developed into the closed‐canopy, sexually reproducing stage. Needle loss of the families was significantly less than that of the reference population of Sitka spruce. 4 Hybrids between Sitka spruce and white spruce were defoliated more heavily than pure Sitka spruce, and the difference increased with age. 5 Family heritability of resistance was estimated as 0.60 compared to 0.73 when the trees were assessed in the juvenile stage. The genetic correlation based on family means between damage in the juvenile and sexually reproducing stand was high (0.83), indicating a high consistency of resistance to the aphid over years and ontogenetic stages. 6 A skewed distribution of defoliation indicated that major genes are involved in the expression of resistance, and that the genetics behind resistance has a nonadditive component.     

Interactions between green spruce aphid (Elatobium abietinum (Walker)) and Norway and Sitka spruce under high and low nutrient conditions

1 Green spruce aphid (Elatobium abietinum) is a serious pest of spruce (Picea spp.) in north‐west Europe, causing defoliation of one‐year‐old and older needles. 2 Relationships between population development of E. abietinum, needle loss and tree growth were compared for five pure genotypes of Sitka spruce and mixed‐genotype material of Sitka and Norway spruce, grown under high and low nutrient conditions. 3 Despite wide differences in flushing date between spruce genotypes, E. abietinum populations peaked on the same date on each genotype and on the mixed‐genotype material, irrespective of nutrient supply. 4 Larger aphid populations developed on trees grown under high nutrient conditions than under low nutrients. However, more needles were lost per aphid in the low nutrient treatment and overall defoliation rates in the two nutrient treatments were similar. 5 Total aphid numbers differed significantly between Sitka spruce genotypes within nutrient treatments, but not in relation to bud‐burst or needle terpene content. Reductions in height growth caused by infestation were greater (15–44%), and related to mean aphid densities and defoliation, in the low nutrient treatment, but were smaller (11–27%) and not related to aphid density and defoliation in the high nutrient treatment. 6 Development of E. abietinum populations was similar on Norway and Sitka spruce, but Norway spruce lost fewer needles. However, the effects of infestation on tree growth were more closely related to aphid density and were similar for Norway and Sitka spruce. 7 Infestation caused a decrease in total root dry weight of Norway and Sitka spruce in proportion to the reductions observed in above‐ground growth.     

Influence of the green spruce aphid on defoliation and radial stem growth of Sitka spruce

Premature abscission of older Sitka spruce foliage on trees within forests in Northern Ireland was compared with green spruce aphid population levels 2 to 3 months earlier. Each aphid has a lower relative impact on needle loss at higher aphid population density; nevertheless, more aphids on a tree results in greater rates of needle loss. Evidence from horizontal stem increment sequences suggested that aphid feeding inhibited incremental growth, and that this inhibition might be delayed for up to two field seasons. However, the effect of aphids on stem growth in most years and on most trees seems relatively small compared with the potential of other tree-or year-specific processes.     

The effects of high temperatures on individuals and populations of the green spruce aphid Elatobium abietinum (Walker)

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Damage and performance of the green spruce aphid, Elatobium abietinum on twenty spruce species

Field and laboratory observations of the relationships between the performance of Elatobium abietinum (Walker), Homoptera, Aphididae, and various species of spruce were undertaken from January 1980 to April 1981. The study included sampling for aphids in established field plots of spruce during May and June respectively before and after the migration period in spring. The aphid's performance (weight and mean relative growth rate) at different seasons on pot grown plants of selected spruce species was monitored, covering, in all, 20 species of spruce.Aphid performance was greatest on the North American spruces, especially Picea sitchensis (Bong) Carr and P. mexicana Martinez; the Asian spruces were the least favoured, especially P. glehnii (Schmidt) Mast. Between these two geographical groups the Eurasian spruce species (sensu Wright, 1955) demonstrated an intermediate aphid performance.

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Résumé Les observations dans la nature et au laboratoire sur les performances d'E. abietinum Walker et différentes espèces d'épicéas ont été effectuées de janvier 1980 à avril 1981. Les observations dans la nature comprenaient des prélèvements de pousses d'épicéa pour dénombrer les pucerons avant (mai) et après (juin) la période de migration du printemps. Les performances des pucerons (poids et taux moyen de croissance relative) ont été examinées à différentes saisons sur des plantes en pot sur un total de 20 espèches sélectionnées d'épicéas.Les performances du puceron ont été supérieures sur les espèces d'épicéa de l'Amérique du nord Picea sitchensis et P. mexicana; les espèces asiatiques étaient les moins favorables, particulièrement P. glehnii. Entre ces deux groups géographiques, les espèces eurasiennes (sensu Wright, 1955) ont permis des performances intermédiares chez les pucerons.

     

Flight regulation in the green spruce aphid (Elatobium abietinum)

In Britain the flight of alate parthenogenetic viviparae of Elatobium abietinum was found to occur only in early summer. Data for this study were extracted from suction trap catches and weather records obtained from meteorological stations adjacent to the traps. The duration of the flight period increased with latitude. At 51° N 95 % of the trap catch was taken in 24 days and at 55° N in 32 days. A flight threshold temperature of 12–13°C was deduced from trap catches taken within the forest crop. Displacements in median flight days at various sites in Britain could be related to values of accumulated day-degrees but not to differences in photoperiod. The median flight day can be predicted empirically by accumulating day-degree values over 8°C from 1 January up to a total of 115 day-degrees. The implications of seasonal flight activity on crop reinvasion is discussed.     

Acclimatisation in the green spruce aphid, Elatobium abietinum

Thin layer chromatography separation of 80% ethanol extracts of adult Elatobium abietinum revealed the presence of the polyhydric alcohol mannitol in aphids overwintering outdoors but not in aphids kept permanently indoors at 15°C. After 3 days at 15°C no traces of mannitol were left in overwintering aphids. Mean freezing temperatures of outdoor, unfed instar I nymphs were about 4°C lower than those of unfed instar I nymphs produced at 15°C. Mean freezing temperatures of overwintering adults were considerably higher than those of unfed instar I nymphs and showed no changes associated with time at 15°C following transference indoors. Similarly, mean freezing temperatures of Sitka spruce needles transferred to 15°C did not change. It was concluded that, although freezing was mainly avoided by supercooling, the presence of mannitol lowered the true freezing temperature of aphid haemolymph and, consequently, the actual freezing temperatures of nymphs produced under cold conditions. However, the considerable increase in freezing point temperatures caused by imbibition of plant sap masked these acclimatisation changes in feeding aphids.     

Frequency and intensity of drought stress alters the population size and dynamics of Elatobium abietinum on Sitka spruce

The green spruce aphid, Elatobium abietinum, is the most important defoliating pest of Sitka spruce, Picea sitchensis, in the UK. Populations are expected to increase in response to predicted climate change, therefore placing Sitka spruce under increased risk of widespread and severe defoliation. The effect of spring–summer drought stress on E. abietinum population dynamics and development over multiple years was assessed in a field experiment under five different drought treatments with differing intensities and frequencies. The impact on host tolerance, in terms of needle retention, was also investigated. No differences in the length of time taken to reach peak population size were observed. Despite this, E. abietinum populations were found to respond positively to low amplitude intermittent stress, with high densities maintained for longer after the peak. Spring peak densities of aphids did not differ among drought levels, although an autumn peak was observed consistently over 2 years on trees subjected to continuous severe drought. This suggested advancement in the onset of Sitka spruce dormancy. Aphid infestation significantly increased percentage needle loss under all drought treatments, although no differences were observed between drought levels. An interaction between aphid presence and drought treatment was, however, observed during a second year of drought where no aphids were present. The study herein presented has contributed to the understanding of E. abietinum population responses under a changing climate. The implications for damage to host Sitka spruce are of relevance to forest management strategies, as an increase in drought events are predicted in the UK.     

Effects of Elatobium abietinum on nutrient fluxes in Sitka spruce canopies receiving elevated nitrogen and sulphur deposition

1 In a field acid mist simulation experiment, Sitka spruce (Picea sitchensis) was sprayed with different pollutant treatments: N, NH₄NO₃; S, Na₂SO₄; NS Acid, NH₄NO₃ + H₂SO₄ and control, no spray. Treatment effects on the abundance of the green spruce aphid Elatobium abietinum and honeydew production were assessed. In addition, needles were sampled for phyllosphere micro‐organisms. In a manipulative experiment, shoots were established and maintained as with or without E. abietinum infestation in order to determine the effects of infestation on needle loss and throughfall nutrient fluxes. 2 Aphid numbers were highest during the end of May and early June, with almost twice as many needles infested in the NS Acid treatment compared with the other treatments. Honeydew production was not affected by the treatments. 3 On infested shoots, increasing numbers of yellowing and dead needles were recorded above the throughfall collectors as the season progressed. The numbers of dead needles falling into the collectors were significantly higher beneath infested shoots. There were strong positive correlations between aphid numbers above the throughfall collectors, the number of yellowing and dead needles on the shoots and the number of needles in the funnels of the throughfall collectors. Litter production was more affected by aphid number than by pollutant treatment. 4 Bacteria, yeasts and filamentous fungi were more prolific on infested needles and treatment effects on colony forming units (CFUs) were most pronounced in the NS Acid treatment. 5 Fluxes of inorganic nitrogen beneath infested shoots were generally lower than beneath uninfested shoots. This effect was more pronounced in those treatments that supplied N i.e. N, NS Acid. The combination of aphid infestation and N‐addition exerted the strongest influence on nutrient fluxes. The fluxes of potassium and of organic carbon (DOC) were higher beneath infested shoots in all treatments, through most of the survey period.     

Interannual dynamics of aerial and arboreal green spruce aphid populations

Partial defoliation of spruce by the green spruce aphid Elatobium abietinum (Walker) is a recurrent event in European and, increasingly, North American forests. The patterns of insect abundance on trees have never been satisfactorily described by a numerical model despite considerable knowledge of endogenous and exogenous factors in the population dynamics of the species. Long-term field population estimates of the aphid on foliage provided the opportunity to evaluate such a model. Unlike comparable models for tree-dwelling aphids, this was also applicable to almost completely independent aphid field data derived from the Rothamsted Insect Survey’s nationwide network of suction traps. Although based on relatively few parameters, the model was robust in its predictions of alate aphids geographically remote from the forest in which the original population was estimated. The population maximum, which causes the greatest forest damage, is reached in early summer and can be predicted from knowledge of winter temperature (chill bouts), spring temperature (thermal sum), and interannual negative feedback (density dependence). The model provides confirmation that alate populations of spruce aphids, upon which a number of other extensive studies have been based, are ultimately influenced by similar endogenous and climatic factors and that they are a reasonable proxy for aphids on trees.     

Supercooling and the low-temperature survival of the green spruce aphid Elatobium abietinum

Laboratory investigations into the low-temperature tolerance of the green spruce aphid, Elatobium abietinum, revealed that the insect was killed by freezing. Aphids and host Sitka spruce needles showed similar seasonal changes in supercooling ability. A noticeable increase in this ability occurred between June and October. Aphids were more susceptible to low temperatures when attached to the plant. It is suggested that mortality resulted from ice which formed in the sap of the host needles and spread into the feeding aphids via their mouthparts. Neither the chlorotic banding of needles, caused by aphid feeding, nor needle length affected needle supercooling. Increased duration of exposure increased the probability of freezing of supercooled needles at low temperatures. A small percentage of first-instar nymphs supercooled to much lower temperatures than the remainder of the population. These were newly born nymphs whose high supercooling ability markedly decreased when they began to feed.     

Effects of temperature on overwintering populations of the green spruce aphid Elatobium abietinum

Recorded minimum temperatures of – 7 ^oC or lower noticeably reduced overwintering populations of the green spruce aphid, Elatobium abietinum, in north-east Scotland. It is suggested that, at these temperatures, ice formation in the needles of the host Sitka spruce, caused attached aphids to freeze. Aphid mortality also occurred when maximum temperatures did not rise above + 6 ^oC for prolonged periods, possibly as a result of starvation following an extended chill coma. A diagrammatic representation of the main factors affecting anholocyclic populations of E. abietinum during the winter is presented to emphasize the governing role played by temperature. The balance between mortality and recruitment determines the size of the population at the end of the winter, and this in turn determines the subsequent summer infestation. It should be possible, therefore, to predict aphid outbreaks either from winter temperatures or from the number of aphids present at the end of the winter. Temperature records obtained from integrating thermometers indicated that the inside of the lower crown tended to be the warmest part of the tree during the winter, resulting in greater aphid survival in the lower branches.     

Geographical distribution and host range of entomophthorales infecting the green spruce aphid Elatobium abietinum Walker in Iceland.

Entomophthora planchoniana and Neozygites fresenii caused infection in populations of the green spruce aphid, Elatobium abietinum, in Iceland. On this aphid species En. planchoniana was exclusively found in the western part of Iceland, while N. fresenii was exclusively found in the eastern part of Iceland. This discrete and nearly nonoverlapping geographical distribution correlates with the distribution of two different populations of El. abietinum found in Iceland. On other aphid species N. fresenii, En. planchoniana, Pandora neoaphidis, and Conidiobolus obscurus were documented throughout the country. Transmission experiments showed that Pa. neoaphidis and En. planchoniana could infect the eastern population of El. abietinum, although they have never been found on this population in nature. This strongly indicates that there is little or no interaction among El. abietinum, other aphids, and their respective entomophthoralean fungi in the field. Furthermore, this study is the first to record epizootics caused by N. fresenii and En. planchoniana in the subpolar region.     

A reappraisal of flight regulation in the green spruce aphid, Elatobium abietinum

Alate formation and its importance in the population dynamics of Elatobium abietinum is examined. It is concluded that alate formation is related to high soluble nitrogen levels in Sitka spruce needles. In N.E. Scotland the proportion of the population developing as alatae was related to aphid density. The apparently differing status of alatae in S. England may be related to differences in the physiology of the host caused by heat input. In Scotland alate formation may cause a decline in spruce aphid populations when densities are very high but, at the densities normally encountered in Scotland and Germany only small proportions of the populations become alate. In these areas the population collapses in late June or early July occur in response to low soluble nitrogen levels in the spruce needles. The role of alate adults in the formation of new colonies following migration to uncolonised hosts is discussed.     

Season and drought stress mediate growth and weight of the green spruce aphid on Sitka spruce

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The effect of needle age on the acceptability of Sitka spruce needles to the aphid,Elatobium abietinum (Walker)

Summary A comparison of the feeding behaviour of E. abietinum on current year needles and previous year needles of P. sitchensis revealed that during the summer months the aphids preferentially settled on previous year needles, this response not being evident in winter. Aphids on current year needles in summer took a much longer period of time to commence sap uptake than in winter, intake ceasing following a very short feeding period. On previous year needles sap uptake in December commenced after a shorter feeding period than in June. Analysis of total and soluble nitrogen levels in Sitka spruce needles showed that current year needles had initially higher levels during shoot elongation in May and early June, but that previous year needles had higher levels for most of the remainder of the year. Quantitative analyses of amino acids revealed that in current year needles the levels were generally lower than in previous year needles. Less marked proportional differences were observed between previous year needles in May and in July/August when the needles were unsuitable. Addition of amino acids in solution into cut current year shoots resulted in increased longevity on shoots containing introduced iso-leucine, histidine and methionine and revealed a general imbalance of the amino acids.