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AIR3-CT94-1883
The Spruce Aphid in Western Europe: Ecology, Status, Impacts and Prospects for Management |
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Proposal No: | AIR3-CT94-1883 |
| Date Prepared: | September 1999 | |
| Source: | Final report 1998 |
Summary
Introduction This Concerted Action focused on the ecology and management of a serious pest on European conifers. The green spruce aphid is chiefly a pest of Sitka spruce where it is grown in the northwestern maritime regions of the European Community. The group set out to studied regional differences in the problems (the aphid, the trees and the climate) and advance research into the economic costs of the pest problem, the way in which the pest can be affected by the environment, and the possible ways in which the problem can be minimised in a sustainable manner by biological controls and by the introduction of tree resistance mechanisms.
Objectives The objectives were:
Activities
This project has brought together European expertise to share current research methods and results. It has focused on the temporal and regional abiotic factors which contribute to different patterns of pest occurrence. It has refined knowledge of the economic costs of the pest problem and made contributions to developing two potentially synergistic pest management strategies. The pest's response to phenotypic and genotypic variability in Sitka spruce has been examined with a view to selecting trees for forest plantation which have the highest pest resistance, given other silvicultural constraints. Other site-related factors which pre-dispose trees to attack have also been explored. In addition, the research programme has investigated the impact and variable occurrence of natural enemies of the pest, and has sought opportunities to enhance the effects of biological controls. The interaction of some of these processes has been modelled in order that the outcome of pest management proposals can be predicted, and so that recommendations will be given an economic context.
Conclusion
The future of agricultural pest management in Europe lies with sustainable control measures that have low environmental impact and rely on a high level of biological understanding. This project will be a contribution to the initiation and development of such a system and be of relevance to other crop protection schemes.
Exploitation
Results are presented in the form of a book, that which summarises progress in the field and the expectations for research and development of an integrated pest management programme in the coming years. The reference for the publication is:
Day, K.R., Halidorsson, G., Harding, S. and Straw, N. (eds.) (1998) The green spruce aphid in western Europe: ecology, status, impact and prospects for sustainable management. Forestry Commission Technical Bulletin 24, HMSO (UK)
Results
Origins and background to the green spruce aphid in Europe Original records of Aphis abietina (syn. Elatobium abietinum) in the British Isles and the defoliation damage that it has caused to its spruce host plants, is reviewed. E. abietinum becomes a pest problem in areas having a mild winter climate where the aphid is anholocyclic; this situation is particularly prevalent in those areas of northwestern Europe that have a maritime climate. In Asia, two other known species of Elatobium occur on conifers, and there have been five other species described from non-conifer hosts, but opinion is that these could be more appropriately placed in other aphid genera. From its original description in Europe, E. abietinum has now been found to be extending into several more distant parts of the world where plantations of spruce have been established. Aphid attacks in plantation forestry in northwestern Europe (Britain, France and Iceland) can be documented particularly in those areas where spruce is not indigenous.
Damage sustained by individual trees Empirical studies on the impact of E. abietinum on Sitka spruce have been conducted in Britain, Ireland, Norway and Iceland, and have involved manipulative experiments on pot-grown plants in nurseries and young trees in the field, and observations on the effects of natural outbreaks in plantations. Differences in methodology, tree age, growing conditions and other factors hinder direct comparisons between studies, but certain general patterns in tree response can be identified. Height increments are reduced, on average, by 10-30% by the end of the season following severe or complete spring/early summer defoliation. Further reductions in height increment may occur in the following 1-3 years. Reductions in stem diameter increment are of a similar magnitude but, in contrast to height growth, occur primarily in the following year or even after a longer period. The effect on diameter increment in the year of attack is usually small. In the oldest trees studied, diameter increments were suppressed for up to 7-8 years. The different response of height and diameter growth is probably caused by a strong direct effect of aphid feeding on leader extension at the time of infestation, followed by a more typical, delayed response of both diameter and height increments to the loss of needles. Fertiliser treatments and S02 pollution appear capable of modifying the impact of E. abietinum, by increasing tree susceptibility to attack, by promoting higher aphid population densities and by altering the tree's growth response to defoliation. However, information on these and other potential modifying factors is limited, and many general patterns in the response of Sitka spruce to infestation by E. abietinum have yet to be established.
Growth costs to plantations: modelling the cumulative effect of defoliation by Elatobium abietinum The process model GROMIT developed by the British Forestry Commission was used to simulate the long-term effects of periodic defoliation by Elatobium abietinum on Sitka spruce (Picea sitchensis). The model calculates total assimilation and allocates carbon according to the pipe-model to predict individual tree growth. Different age classes of foliage are incorporated, and all age classes except the most recent are allowed to be affected by defoliation. A brief description of the model is given, together with simulation results for defoliation rates of 0%, 25%, 50%, 75% and 100%. Defoliation was set to recur every 6 years and stand development was simulated for 50 years.
Defoliation caused an immediate reduction in diameter increment of up to 40%. At the 100% defoliation rate, cumulative yield (stem mass) was reduced by 16%, absolute stem volume was reduced by 20%, at the end of the simulation. As the trees became older and larger, recovery times increased. Light defoliation had little effect on long-term growth. Further simulations showed that single episodes of severe (100%) defoliation early in the rotation, at ages 6 and 12, had negligible effect on final yield. Development of the model will require better parameterisation to describe spruce growth under different site and regional conditions, inclusion of within year susceptibility of tree growth to attack, and modifications to the distribution of defoliation within the crown in a more realistic manner. The model could then be used to predict the effects of different intensities, frequencies and duration of defoliation.
Green spruce aphid population dynamics: effects of climate, weather and regulation The within- and between-year dynamic relationships of spruce aphid populations in plantations of Sitka spruce are reviewed. Indices of aphid abundance have been obtained from suction trap catches and from forests, in the form of aphid counts on foliage. Correlations between trap and forest-based samples increase the confidence that can be placed in traps as a means of long-term aphid surveillance. The pattern of within-year aphid population change is determined by nutrient availability in spruce needles, whereas the timing of the population peak is governed regionally by thermal input. The size of the population peak (and the damage caused) is dependent on the population which survives the winter and can be moderated by natural enemies. Simulation studies also confirm these conclusions and show how European regional differences in aphid population dynamics can be explained by the thermal environment. Year to year differences in population size (and the frequency of "outbreaks") are determined by the severity of winter weather and feedback from the population density in the previous year. Independent studies point compellingly to the role of (overcompensating) density dependence in this system. Various sources of density regulation are discussed. The available evidence suggests that the centrifugal redistribution of aphids (both apterae within trees and alatae between trees), especially adults at high population density, coupled with the loss of needle habitat, could explain much of the dynamic response observed. A growing understanding of spruce aphid population dynamics and the use of flexible simulation models will become useful tools in integrated pest management.
A conspectus of potential natural enemies found in association with the green spruce aphid in Northwest European spruce plantations An account is given of a three year survey of potential natural enemies of Elatobium abietinum from spruce plantations in Denmark, Great Britain, Iceland and Norway. Special emphasis was given to surveys of Sitka spruce plantations in Norway which has similar climatic conditions to that of Iceland. Some 28 species of Coleoptera; 6 species of Diptera; 6 species of Neuroptera; 5 species of Hemiptera; and 9 species of Araneida, all potential aphid predators, were found in association with E. abietinum during this study. Parasitoids and hyperparasitoids numbering 7 species were reared from mummified E. abietinum. Six species of insect pathogenic fungi were found, five of which are newly recorded using E. abietinum as a host; only one species, Neozygites fresenii, was found in all four countries. The distribution of these natural enemies and their significance for regional differences in the development of E. abietinum populations is reviewed and discussed.
The quantitative impact of natural enemies and the prospect for biological control The arthropod natural enemies of the green spruce aphid are little known, rarely studied and poorly understood. The major groups identified in studies in Scotland, England, Northern Ireland, Germany and Denmark are the aphidophagous syrphids, lacewings and coccinellids. Parasitoids have also been recorded as causing some impact on aphid populations although recorded parasitism rates rarely exceed 10%. The most commonly identified predator is the ladybird Aphidecta obliterata, although two other species have been found feeding on green spruce aphid, Coccinella septempunctata and Adalia decempunctata. Despite syrphids being poorly identified and possibly under-recorded, computer simulations indicated that they could be of use in reducing the numbers of the green spruce aphid. The possibility that host plant effects influence predator and parasitoid efficiency was considered. A simple model to explore the possible role of predation in regulating the numbers of E. abietinum was developed and indicated that both coccinellid predators and hymenopteran parasitoids could play an important role in depressing aphid numbers.
Selecting for resistance in genetically defined Sitka spruce Current tree breeding programmes in Sitka spruce aim at improving adaptability, quality and yield and so far do not include resistance to the green spruce aphid. The processes involved in the resistance mechanisms are not fully understood. Indications are given of the importance of secondary compounds and other hypotheses focus on the availability of sap nutrients. Also non-chemical features related to needle structure may be relevant. Differential susceptibility to attacks by the green spruce aphid has not only been demonstrated between spruce species, but also at provenance, family and clonal level. Provenance tests in Northern Ireland have shown significant variation in aphid peak population density between provenances and, although somewhat contradictory, correlations between population density and the original latitude of the seed origin. No influence of latitude was found in Danish trials. Within- provenance variation is very profound, making selection of resistant plant material possible in most provenances, a circumstance which gives excellent prospects for tree improvement programmes. Also at family level, significant differences in susceptibility to the green spruce aphid have been evidenced. The family heritability is high and high gains can be expected from selection. Resistance is obviously inherited by the offspring and appears to be fairly consistent in time; resistance has been demonstrated in offspring more than 30 years after selection for the trait. Examples of significant variation at clonal level are given. It is concluded that the advantages of including aphid resistance in current breeding programmes of Sitka spruce are obvious.
Atmospheric pollution, elevated CO2 and spruce aphids The effects of atmospheric pollutants on agricultural and forest pests have been studied extensively in the last 10 years. This research has included work on several tree-dwelling aphid species, such as Elatobium abietinum and Cinara pilicornis on Picea sitchensis, and Cinara pini and Schizolachnus pineti on Pinus sylvestris. These studies have shown that exposure to SO2 and NO2, consistently enhances aphid performance (as typically measured by relative growth rate). Work on ozone has produced less consistent results. The effects of atmospheric C02 have been studied on a range of tree-dwelling insects including E. abietinum, C. pilicornis, C. pini, Phyllaphis fagi on Fagus sylvatica, and Drepanosiphum platanoidis on Acer pseudoplatanus. Published research on plant-chewing insects has shown that elevated C02 tends to have a detrimental effect on insect performance. Research on aphids, however, has produced less consistent results, although spruce aphids appear to perform less well on plants exposed to elevated rather than ambient C02. It is concluded that future research effort on atmospheric pollutants and elevated C02 should concentrate more on aphid pests and study the combined effects of factors such as C02 , drought, temperature, plant nutrient status, and pollutants such as ozone as well as focusing on the consequences of the results obtained for the abundance and damage caused by forest pests.
Prospects for sustainable management of forests to minimise the green spruce aphid problem in Europe The green spruce aphid, Elatobium abietinum, is now recognised as a significant component in the plantation forestry system of western Europe. Its real status as a forest pest, that is its potential to reduce the productivity of the forest environment, has remained in question until recently despite its evident capacity to cause widespread and periodic defoliation-in most areas where Sitka spruce is grown. The evolution of its pest status can generally be attributed to two conditions. Firstly, planting of Sitka spruce, one of the most susceptible hosts, has increased in recent years. Secondly, the climate of areas conducive to high growth potential in Sitka spruce, is particularly favourable for aphid survival and population development. Research in Europe has demonstrated that severe infestations of E. abietinum have a significant and protracted effect on spruce growth, Recent studies have characterised this response for young trees, but the response of older trees in plantations is less well documented and may be different. Modelling studies have started to use the empirical information available to estimate impacts on final tree sizes and yields and these suggest that growth losses might be greater during the latter half of the forest rotation. Model simulations also indicate that repeated attacks over the life of the crop are capable of reducing timber yields significantly.
The green spruce aphid can be killed by pesticides, indeed this is an option used by Christmas tree growers, but it is not considered a sustainable means of regulating the pest nor is it likely to be good for the integrity of a forest plantation environment, even in the short-term. The management of the aphid in future will rely more heavily on developing resistance in forest trees through tree-breeding or silviculture, and combining this with augmentation of biological controls. A framework for achieving cost-effective management will be built on flexible models for the aphid-tree system and a better knowledge of the impacts of the insects on tree growth. The effects of climate change, and pollution are external factors that deserve continued study, given the long-term nature of forest economics. A final consideration is the genetic basis for aphid bionomics and how this affects the pest problem in existing and new parts of its range.
There is a need to combine a knowledge of all these elements to produce practical recommendations for IPM (Integrated Pest Management) to be applied at the European scale. A range of recommendations have arisen from this project, but for the most part the activity has enabled a harmonisation of activities and provided an international impetus for future research which will achieve an IPM goal.
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Updated
by CPL Press:
03/07/2007
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