Population responses of a conifer-dwelling aphid to seasonal changes in its host

Abstract.  1. Current evidence suggests that seasonal changes in spruce needle sap nutrients have a decisive influence on green spruce aphid ( Elatobium abietinum ) population density, but the mechanisms of population change, the roles of development rate, fertility and mortality, and the existence of density-dependent processes, are not clearly understood.
2. Experimental studies of aphid populations were conducted in controlled environments to estimate seasonal patterns in aphid mean relative growth rate, prenatal development, fertility, and mortality. Studies were also made of the effect of aphid crowding on vital rates.
3. Independent of the degree of aphid crowding, seasonal changes in the amino acid concentration of needle sap were tracked by aphid growth rate, fertility (and adult size), but not by rates of aphid mortality. The most pronounced change in vital rates, and the one most likely to drive seasonal population change, was in fertility. Prenatal development time actually became shorter in periods when nutrients were scarce, but the resulting adult aphids were smaller and less fertile than during periods of improved nutrition.
4. Density dependence of vital rates was only observed during mid-summer when nutrients were least available. Mortality, growth rate, and prenatal development were the most strongly density-dependent processes. In contrast, there was no evidence that fertility rates were likely to respond to crowding.
5. There were no important differences between populations reared on small, potted spruce trees and those on plantation trees aged 25 years. This gives confidence that demographic data from a variety of field and laboratory sources could be used to compile data appropriate for population models.     

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.     

Non-linear feedback processes and a latitudinal gradient in the climatic effects determine green spruce aphid outbreaks in the UK

The role of climatic fluctuations in determining the dynamics of insect populations has been a classical problem in population ecology. Here, we use long-term annual data on green spruce aphid populations at nine localities in the UK for determining the importance of endogenous processes, local weather and large-scale climatic factors. We rely on diagnostic and modelling tools from population dynamic theory to analyse these long-term data and to determine the role of the North Atlantic Oscillation (NAO) and local weather as exogenous factors influencing aphid dynamics. Our modelling suggests that the key elements determining population fluctuations in green spruce aphid populations in the UK are the strong non-linear feedback structure, the high potential for population growth and the effects of winter and spring weather. The results indicate that the main effect of the NAO on green spruce aphid populations is operating through the effect of winter temperatures on the maximum per capita growth rate (R_m). In particular, we can predict quite accurately the occurrence of an outbreak by using a simple logistic model with weather as a perturbation effect. However, model predictions using different climatic variables showed a clear geographical signature. The NAO and winter temperature were best for predicting observed dynamics toward the southern localities, while spring temperature was a much better predictor of aphid dynamics at northern localities. Although aphid species are characterized by complex life-cycles, we emphasize the value of simple and general population dynamic models in predicting their dynamics.     

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

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

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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.     

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.     

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.     

The effect of previously infested spruce needles on the growth of the green spruce aphid, Elatobium abietinum, and the effect of the aphid on the amino acid balance of the host plant

The effect of chlorosis induced in needles of Sitka and Norway spruce by the green spruce aphid on growth of the aphid is investigated, and the effect of infestation of the aphid on amino acid levels in Sitka spruce foliage is reported. On both Sitka and Norway spruce green spruce aphids were heavier when reared on chlorotic (previously infested) needles than when reared on green (previously uninfested) needles. The effect was more pronounced on Sitka than on Norway spruce. Infestation of the aphid altered the amino acid balance of Sitka spruce foliage but not the concentration of total amino acids. Possible causes of chlorosis, the influence of individual amino acids on aphid growth, the potential effect of chlorosis on outbreaks of the aphid and the differences in susceptibility of Sitka and Norway spruce to damage by the aphid are discussed.     

The role of grasslands in the yearly life-cycle of Rhopalosiphum padi (Homoptera: Aphididae) in Sweden.

The population density of the bird cherry-oat aphid (Rhopalosiphum padi) was monitored in spring sown barley and in grasslands (leys and pastures) and a suction trap was used to monitor the flight periodicity of the aphids. Emigrants from the primary host (bird cherry) colonised both grass and cereals in spring and a migration from cereals to grasses took place in mid-summer. There was a negative correlation between the sizes of summer and autumn migration. There was a positive relationship between late summer growth in leys and the size of autumn migrations. It is concluded that the size of the autumn migration is mainly dependent on aphid population growth in grasslands during late summer and autumn.     

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.     

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

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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.     

Shoot growth dynamics and size-dependent shoot fate of a clonal plant,Festuca rubra, in a mountain grassland

The relation of the within-season and between-season patterns of shoot growth were compared in a clonal grass with long-lived shoots,Festuca rubra, in a mown mountain grassland. The growth rate of shoot length from spring to summer in a year was almost constant for each shoot irrespective of spring shoot length each year. The annual shoot growth rate from spring to spring was negatively correlated with the shoot length in the first spring. Shoots of different length and age therefore tended to converge over time to a population of identical shoot size, suggesting an equalizing effect of growth pattern on size structure. Shoot size (shoot length and number of leaves) influenced the fates of shoots. Larger shoots showed an increased incidence of both flowering and formation of intravaginal daughter shoots and a decreased incidence of death in the subsequent time period. The fates of shoots were independent of their age. Although the negatively size-dependent springto-spring annual shoot growth rate acted to decrease shoot size variation, the remaining variation within the shoot population was still sufficient to generate different fates of shoots. These fates were not related to the previous life history of individual shoots. There was a significantly positive effect of the shoot size at initiation on its life expectancy. This was mainly attributable to the positively size-dependent survival rate of shoots in the early stage (<1 year old) of shoot life history. Later on (> 1 year old), shoot size had little effect on the survival rate of shoots. Once small young shoots have survived this early stage (< 1 year old) in life history, they can grow vigorously, little affected by competition regardless of shoot size, and converge to a stable size structure of shoots of similar size. Only shoot size in the early stage ( < 1 year old) of life history is important for the persistence of a shoot population.     

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.     

Interactive effects of host resistance and drought stress on the performance of a gall-making aphid living on Norway spruce

Four full-sib families of Norway spruce (Picea abies), of which two were resistant to a galling aphid (Adelges abietis) and two were susceptible, were exposed to drought stress for 2 years. The primary aim was to test the hypothesis that the various life stages of the aphid differ in their response to environmentally induced changes in the host plant. The drought treatment had a significant negative effect on tree growth. This was reflected in gall size which responded in a similar way to the drought stress, thus supporting the plant vigour hypothesis. Drought affected the survival of aphid stem-mothers negatively in susceptible trees but positively in resistant trees. This result was matched by the response of an individual phenolic compound which, contrary to the total phenolic concentration, tended to increase in susceptible trees exposed to drought and decrease in similarly exposed resistant trees. Thus it is possible that this single, as yet unidentified, phenolic compound could be used as a marker of resistance. The performance of surviving stem-mothers, evaluated by measuring the diameter of the wax cover they produced (a correlate of fecundity), was not significantly affected by drought, but aphids on susceptible trees produced more wax than those on resistant trees. The change in gall density over time (analysed separately for each treatment and phenotype) correlated best with patterns of stem-mother survival. This indicates that stem-mother survival, which is closely linked to host plant quality and resistance, may play a significant role in the population dynamics of this aphid. However, drought-stress-induced changes in host plant quality affected survival, and hence gall density, less than the genetically determined level of resistance. The results also support the hypothesis that an insect may respond differently to environmentally induced changes in the host plant at different stages in its development. Received: 15 September 1999 / Accepted: 3 December 1999     

A survey of aphids and aphid parasitoids in cereal fields in Denmark, and the parasitoids' role in biological control

In 1996 and 1997 a field survey of the abundance and species composition of cereal aphid primary and secondary parasitoids in spring barley, winter wheat and durum wheat was conducted in Zealand, Denmark. The purpose was to create a better understanding of the mechanisms underlying aphid–parasitoid dynamics in the field. Such an understanding can be used when developing biological control methods in cereals. In both years aphid attacks in cereals began in late June and never exceeded the economic threshold. In 1996 the first aphids were found in wheat on 26 June; in 1997 the first aphids were found on 24 June on both crops. The highest densities reached in 1996 were an average of six aphids per shoot in winter wheat and one aphid per shoot in spring barley. In 1997 the highest densities reached were 11 aphids per shoot in winter wheat and four aphids per shoot in spring barley. The aphid population collapsed by the end of July to early August in 1996, but it collapsed by mid-July in 1997. The onset and peak of parasitization were delayed in comparison to aphid infestation. Parasitism was 20–60% by the end of the cropping season in spring barley, and 30–80% in winter wheat and durum wheat in 1996. In 1997 parasitism did not exceed 3–11% in barley and was less than 2% in one winter wheat field but more than 40% in the other winter wheat field sampled. In both years most parasitism was due to Aphidiidae (Hymenoptera). The two dominant species were Aphidius ervi Haliday and Aphidius rhopalosiphi De Stefani-Perez. Hyperparasitism began after primary parasitism and increased progressively during the cropping season. The two years were similar in many respects, including for species composition of aphids and parasitoids. The late start of the aphid infestation may have contributed to the high level of parasitization found in 1996, but in 1997 the aphid infestation period was so short that a parasitoid population did not have time to build up.     

Summer survival of the green spruce aphid,Elatobium abietinum,in North East Scotland

Summary In North East Scotland small numbers of all developmental stages of the green spruce aphid survived on Sitka spruce needles during the summer months despite the nutritional inadequacy of these needles for aphid survival following population collapse in early summer. The surviving adults lost weight and fat reserves in response to time and contained low numbers of embryos. No metabolic acclimatization of the respiration rate occurred in response to exposure to different temperatures. Aphid respiration rates during summer were significantly higher than those in winter. Therefore, it was concluded that no summer aestivation occurred. Summer survival was dependent on the ability of a few aphids of all instars to survive on the nutritionally inadequate host, these aphids possibly possessing higher than normal fat reserves or feeding on marginally nutritionally superior trees or shoots.     

Evaluation of induced responses, insect population growth, and host-plant fitness may change the outcome of tests of the preference-performance hypothesis: a case study

The preference‐performance hypothesis predicts that insect preference should correspond to host suitability for offspring development. We studied the pattern of within‐plant preference in the aphid Sipha flava and its consequences for offspring performance on the host‐plant Sorghum halepense, regarding the role of induced responses of plants to aphid feeding. The consequences of within‐plant preference on aphid population growth and host‐plant traits were also evaluated. Our results showed that winged and wingless aphids preferred to settle on mature rather than young leaves. In contrast, aphid individual growth rate was higher on young leaves when compared with mature leaves, suggesting that the outcome of this test rejected the preference‐performance hypothesis. However, the inclusion of the factor ‘previous aphid infestation’ changed the outcome from a maladaptive choice to a neutral one. Thus, individual growth rates of S. flava increased when aphids developed on leaves that had been previously infested. Interestingly, aphid growth rate on previously infested leaves did not differ between young and mature leaves. On the other hand, aphid population reproductive rate was higher and the percentage of winged aphids lower when infestation occurred on mature rather than young leaves. Aphid infestation reduced plant and shoot biomass, and increased leaf mortality. These negative effects on plant traits related to plant fitness were greater when aphid infestation occurred on young leaves. Likewise, whereas infestation on mature leaves did not cause a significant reduction in the number of flowering plants compared with control plants, aphid infestation on young leaves did reduce the number of plants at the flowering stage. Consequently, if both the reproductive rate of aphids in the mid‐term, and host‐plant fitness are taken into account, the results indicate that aphid preference for mature leaves may be an adaptive choice, thus supporting the preference‐performance hypothesis.     

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.