Reproduction of an arboreal aphid pest,Elatobium abietinum,is altered under drought stress

Climate change in the UK is predicted to increase both winter temperatures and the frequency of summer drought events. Elatobium abietinum, the green spruce aphid, is the most important defoliating pest of Sitka spruce, Picea sitchensis, a conifer very widely used in British forest industry. This aphid is expected to respond strongly to altered climate, with changes to population densities leading to more frequent serious outbreaks and defoliation. The impact of simulated spring–summer drought on the reproductive performance of E. abietinum was investigated under laboratory conditions. Rates were assessed under five drought treatments of differing frequencies and intensities to characterize the direction of responses under different drought scenarios, and in time‐staggered trials to explore seasonal variation. Variation in the response of reproduction to water deficit was mediated by drought frequency and magnitude. Low‐amplitude, moderate intermittent stress improved reproductive rates, while severe stress, both continuous and high‐amplitude intermittent, had a detrimental impact when compared with observations made on well‐watered controls. Season was also found to modify the response, with improvements to plant nutritional quality under high‐amplitude stress reflected by improving reproduction. Despite this, no differences in rates were found during the autumn, suggesting no advancement in spruce dormancy under drought. Drought stress therefore has the potential to alter E. abietinum population densities, structure and phenology in Sitka spruce plantations, with implications for forest management, damage levels and natural control of the aphid under future altered climate.     

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.     

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

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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.     

Drought intensity and frequency have contrasting effects on development time and survival of the green spruce aphid

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

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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.     

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.     

The effects of host plant on the coccinellid functional response: Is the conifer specialist Aphidecta obliterata (L.) (Coleoptera: Coccinellidae) better adapted to spruce than the generalist Adalia bipunctata (L.) (Coleoptera: Coccinellidae)?

Female Aphidecta obliterata consumed an average of 26.6 ± 5.8 Elatobium abietinum and males consumed an average of 19.9 ± 3.2 Elatobium abietinum, but there was no significant difference in the number consumed between the sexes. In Petri dish trials, the larval stages of A. obliterata and all stages of Adalia bipunctata appeared to exhibit a Type II response to prey density, although A. obliterata adults showed a more linear response to prey density. There was no significant difference between the functional responses of the 3rd instars of the two coccinellid species, but there was a significant difference between the responses shown by the 1st instars, with A. obliterata larvae consuming more than those of A. bipunctata, especially at low densities, suggesting that the two species respond differently to an increase in prey density.At low prey densities, adults and 4th instars of both species exhibited a similar response to an increase in prey density but at higher densities the 4th instars and adult stages of A. bipunctata showed higher attack rates when compared with the same stages of A. obliterata.Adult and 4th instar A. obliterata exhibited Type II functional responses on spruce sections. The 4th instar A. obliterata larva appeared to be a more effective predator than the adult of the species, and was more effective when compared with adult A. bipunctata at lower prey densities but A. bipunctata adults appeared to be a more effective predator at higher prey densities.The host plant affected the rate of consumption by adult A. obliterata as adults on Sitka spruce sections consumed significantly higher numbers of aphids than their counterparts on Norway spruce. This was most noticeable at densities above 16 aphids.The distributions of the two coccinellid species in the olfactometer were significantly affected by the presence of host plant material. Aphidecta obliterata adults were found in significantly higher numbers in the Sitka spruce chambers than the control chambers (those without plant material). Adalia bipunctata adults were found in significantly lower numbers in the Norway spruce chamber than the control chamber.Although A. bipunctata has a higher level of voracity than A. obliterata, the latter is more adapted to the spruce environment and the boom and bust population dynamics of E. abietinum.     

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.     

Damage caused by the Green Spruce Aphid to Norway and Sitka spruce needles

Three-month-old needles of Sitka spruce were less susceptible to Elatobium abietinutn than 15-month-old needles. Symptoms appeared after longer aphid feeding times but only a proportion of damaged needles fell. After short feeding periods symptoms appeared in more Norway spruce needles than in Sitka spruce, whereas longer feeding periods resulted in more needles producing symptoms in the Sitka spruce. The symptoms took 4—6 days longer to appear in Norway spruce, and needle fall followed a longer feeding period than on Sitka spruce. Following 72 h feeding, needle fall occurred more quickly on Sitka spruce than on Norway spruce. The time taken for needle fall to occur was inversely related to the feeding time in Sitka spruce but such a response was not evident in Norway spruce. The results are discussed in relation to the differences exhibited in the probing behaviour of the aphid on the two spruces.     

Differences in the probing behaviour of Elatobium abietinum feeding on Sitka and Norway spruces

The probing behaviour of Elatobium abietinum (Walk.) on the needles of Norway spruce and Sitka spruce was directly related to the stomatal frequency accompanied by heavy deposition of saliva, and multiple branching and divergence of the salivary sheath, all of which were associated with the impenetrability of the endodermis. On Norway spruce these features were very greatly reduced where there was a higher rate of probing coupled with a higher rate of penetration into the phloem, which was not accompanied by early uptake of phloem sap when compared with aphids on Sitka spruce. These differences in probing behaviour are related to damage symptoms exhibited by the host trees.     

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.     

The growth and decline of a population of the spruce aphid Elatobium abietinum during a three year study,and the changing pattern of fecundity,recruitment and alary polymorphism in a Northern Ireland Forest

The population size and structure of the green spruce aphid was followed throughout the spring — summer cycle on the same group of trees in a low-elevation coastal Sitka spruce forest for three consecutive years. The relationship between the pattern of change and the phenology of bud burst, which heralds a marked change in needle sap quality, suggests that yearly differences in the winter temperature regime may affect the duration of the population growth phase and hence the peak numbers attained in late spring.An index of population growth rate was sufficiently sensitive to aphid fecundity during the population cycles of two years to suggest that the changing rate of fertility was the decisive process governing changes in population size. The commitment of aphids to alate development was greater than that recorded elsewhere in Britain but did little to effect population decline since the contribution of alatae to larviposition was substantial while seasonally pulsed.     

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.     

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.     

Genetic diversity of the green spruce aphid (Elatobium abietinum Walker) in north-west Europe

Abstract 1 The Random Amplified Polymorphic DNA (RAPD) method was used to investigate genetic diversity of anholocyclic populations of the green spruce aphid, Elatobium abietinum Walker, in north‐west Europe. 2 The results showed that the aphid in this region was divided genetically into three major groups. Aphids from the British Isles and north‐west France comprised the first group, the second group consisted of aphids from Denmark and Iceland, and the third group consisted of aphids from Norway. 3 The results indicated a significant level of gene flow within and between sites and geographical regions, especially in the British Isles and north‐west France. Lateral migration of the aphid and/or sexual reproduction is likely to have facilitated the gene flow. 4 The implications of these findings on management of the green spruce aphid are discussed.     

The effects of nutrition and density on the production of alate Elatobium abietinum on Sitka spruce

Summary Observations over a period of 10 years showed that, in Northeastern Scotland, alatae of E. abietinum regularly appeared in mid-May, the timing being unrelated to aphid density. The peak number of alatae produced was, however, correlated with aphid density. Following an initially high level the proportion of alatae dropped to virtually nil by mid-June, whilst over the same period the aphid population density increased. Amino acid levels in spruce needles were considerably higher during the period of alate formation than they were at the termination of alate production. It is suggested that a high amino acid level was the main factor controlling the formation of alatae and that population density affected the proportions of these alatae only when nutritional levels were favourable for alate formation.