Improving resistance of spruce forests to the green spruce aphid

Keith Day, University of Ulster
Collaborative European Research and Development Project FAIR 3 CT94 1792

The green spruce aphid is a widespread pest of Sitka and other spruces in North-West Europe (Day et al. 1998). In years such as 2002, the insect causes most needle leaves to drop prematurely from the tree and the tree fails to grow to its full potential. The joint-investigators set out to answer several principle questions. What affect does the aphid pest have on plantation forests and their productivity ? Is it serious enough to merit investment in management ? Are there ways in which the levels of pest population can be reduced below an acceptable economic threshold ?

Research programme coordinated by the University of Ulster.

Partners: Imperial College London, Cardiff University, Forest Research Agency UK, Universite Joseph Fourier Grenoble, Royal Veterinary & Agricultural University Copenhagen, Norwegian Forest Research Institute, Iceland Forest Research Station.

Results show decisively that there is genetic variation in resistance to the green spruce aphid among Sitka spruce trees (Jensen et al. 1997). Identified spruce clones supported smaller aphid populations and lost fewer needle leaves through their effect on aphid fertility and development. The resistance mechanism is based on antibiotic properties of the plant, including anatomy of the needle leaf, and not on plant secondary chemistry. There is a high degree of spatial and temporal consistency of needle leaf retention (tolerance) for Sitka spruce clones challenged by aphids over a ten year period, and a substantial degree of heritability (Harding et al. 2003). Two markers linked to resistance genes were found, and could be used to develop aphid resistance in breeding programmes (Skov & Wellendorf 1999, 2000).

Problems caused by high pest population density are often associated with mild winters, during which the anholocyclic aphid can reproduce. In this project we turned our attention back to the intrinsic insect-plant relationships which underpin population dynamics. This enabled us to model population processes as they are affected by potential management tools (silviculture, biological controls) and as they affect plant and forest growth. Results emphasized the importance of seasonal processes in the host-plant, which are tracked by aphid reproduction, and overcompensating density dependent aphid mortality in mid-summer, which results in the “see-saw” effect. Interestingly, overcompensation doesn’t seem to result from the effects of high aphid population density on plant quality (Williams et al. in prep). The see-saw effect means that a high aphid population in one year is usually followed by an unusually low population in the next, whether or not the winter temperature has been severe (Day et al. 2004).

Brown lacewing larvae (Hemerobius spp.) prey on aphids. Ladybird larvae (Aphidecta obliterata) are effective predators. An egg of the predator Anthocoris nemorum. Rhagonycha lignosa prey on aphids. Many aphids succumb to Entomophthorales infections.

Pathogenic fungi (Nielsen et al. 2000, 2001) and several insect predators were identified as jointly having a decisive effect on aphid populations. The results of such interactions could be modelled and verified by field data and laboratory experiment (Kidd et al. in prep.). Representatives of the key natural enemy groups were present in most plantation forest areas, although their abundance varied locally Field studies suggest that there would be potential for enhancing natural enemies through management and that reductions in biodiversity and natural enemy frequency would be detrimental to the forest economy.

Molecular differences between aphids from different regions. Gel image of DNA samples from Iceland (1&2) and Ireland (3). Banding patterns made by 12 primers indicate bands shared by Icelandic populations and not detected in the Irish population.

Genetic analysis of aphids from the European regions where they are most abundant, identified several groups with significant gene flow between them (Sigurdsson et al. 1999; Halldórsson et al. 2004). Despite its indiscernably low level of sexual reproduction, the aphid has a pattern of genetic diversity that would allow it to adapt to climatic change or resistant plant material, although there is little evidence that ecotypes with aggressive adaptations have so far developed (Halldórsson et al. 2001).

Growth of trees

Serious loss of older spruce needles and damage to breaking buds.

A wide range of field and laboratory work has explored the effects of aphids on tree growth. Loss of needles from Sitka (closed circles) and Norway spruce (open circles) challenged by aphid populations. (a) high nutrient treatment (b) low nutrient (Straw & Green 2001).

All experimental studies have confirmed the close relationship between aphid numbers and needle loss – so the resistance differences between spruce genotypes is based on their effects on aphid performance rather than simply greater tolerance to attack. However, growing conditions also influence needle loss, which calls for caution in interpreting spatially-separated tests of resistance. An unexpected result is that aphids cause a reduction in needle size the year after attack – together with the effects of needle loss and changing canopy structure during plantation development, this leads to a complex relationship between aphid populations and tree growth (Straw et al. 2000, 2002; Straw 2001; Straw & Green 2001). The big question is, how much growth is lost as a result of aphids ? Tree growth simulation models, validated against field and experimental data, suggest that the impact of aphids will increase as trees grow older. Tree growth was charted for a range of pest levels, but a moderate rate of attack is predicted to result in more than 7% reduction in annual timber volume increment. Models of the accumulated effects of successive aphid outbreaks on plantations suggest a reduction of gross income from harvest yield of 2-4%, on a discounted basis. Higher pest levels may reduce financial returns by up to 17%. Any of these figures strongly confirm the potential gains to be realised from maintaining or enhancing natural enemy controls or resistance characteristics against the aphid.

Conclusion

Englemann spruce killed by aphids in the Pinalenos mountains, Arizona.

In conclusion, the aphid is economically important and its effects can be remediated by introducing the most economically favourable measures for plantation growth of Sitka spruce. Recent invasion of the green spruce aphid in the “sky islands” of Arizona is killing Engelmann and blue spruce because high population densities occur in autumn (on all needles) rather than in summer. Tree mortality and needle fall is posing a real fire risk. In Alaska, recent problems on indigenous Sitka spruce are attributed to climate change. There are now several manifestations of the aphid problem in different parts of the world, but what we have learned in Europe is certain to shed light on management solutions elsewhere.

References
Day, K.R. (2002) The green spruce aphid – a pest of spruce in Ireland COFORD Connects – Silviculture and Forest Management No. 4, COFORD, Dublin.

Day, KR., Armour, H., and Docherty, M. (2004) Population responses of a conifer-dwelling aphid to seasonal changes in its host. Ecological Entomology, 29 (5): 555-565.

Day,K.R., G.Halldórsson, S.Harding and N.Straw (1998) The Green Spruce aphid in Western Europe: ecology, status, impacts and prospects for management. Forestry Commission Technical Paper, 24 HMSO, London, 105 pp.

Halldórsson, G., Sigurdsson, AE, Thórsson, TH, Oddsdóttir, E.S., Siurgeirsson, A. and Anamthawat-Jónsson, K. (2004) Genetic diversity of the green spruce aphid (Elatobium abietinum Walker) in north-west Europe. Agricultural and Forest Entomology, 6: 31-38.

Halldórsson, G., Docherty, M., Oddsdottir, E.S., & Day, K.R. (2001) The performance of different populations of the green spruce aphid (Elatobium abietinum Walker) at different temperatures. Icelandic Agricultural Sciences, 14: 75-84.

Harding, S., Roulund, H., and Wellendorf, H. (2003) Consistency of tolerance to attack by the green spruce aphid (Elatobium abietinum Walker) in different ontogenetic stages of Sitka spruce. Agricultural and Forest Entomology, 5: 107-112.

Jensen, J.S., Harding, S., Roulund, H. (1997) Resistance to the green spruce aphid (Elatobium abietinum Walker) in progenies of Sitka spruce (Picea sitchensis (Bong) Carr.). Forest Ecology and Management 97: 207-214.

Kidd, N.A.C., Day, K.R., Docherty, M. and Leather, S.R. (2004) Simulation models describing seasonal dynamics of spruce aphid populations (Elatobium abietinum): performance and sensitivity. in prep.

Nielsen, C., Eilenberg, J., Harding, S., Oddsdottir, E. & Halldórsson, G. (2000) Entomophthoralean fungi infecting the green spruce aphid (Elatobium abietinum) in the North-western part of Europe. IOBC WPRS Bulletin, 23: 131-134.

Nielsen, C., Eilenberg, J., Harding, S., Oddsdottir, E., and Halldòrsson, G. (2001) Geographical distribution and host range of Entomophthorales infecting the green spruce aphid Elatobium abietinum Walker in Iceland. Journal of invertebrate Pathology, 78: 72-80.

Sigurdsson, V., Halldórsson, G., Sigurgeirsson, A., Thórsson Æ.Th., and Anamthawat-Jónsson, K. (1999) Genetic differentiation of green spruce aphid (Elatobium abietinum Walker), a recent invader to Iceland. Agricultural and Forest Entomology, 1 (3): 157-163.

Skov, E. and Wellendorf, H (1999) Tracking resistance genes against the green spruce aphid. Proceedings of the 25th Biennial Southern Forest Tree Improvement Conference, New Orleans, Louisiana, USA, July 10-14, 8-17.

Skov, E. and Wellendorf, H. (2000) RAPD markers linked to major genes behind field resistance against the green spruce aphid Elatobium abietinum (Walker) in Picea sitchensis (Bong.(Carr.)). Forest Genetics, 7: 233-246.

Straw, N.A., Fielding, N.J., Green, G. and Price, J. (2000) The impact of green spruce aphid, Elatobium abietinum (Walker), and root aphids on the growth of young Sitka spruce in Hafren Forest: effects on height, diameter and volume. Forest Ecology and Management, 134 1-3: 97-109.

Straw, NA, Fielding, NJ, Green, G, and Price J. (2002) The impact of green spruce aphid, Elatobium abietinum (Walker), on the growth of young Sitka spruce in Hafren Forest, Wales: delayed effects of needle size limit wood production. Forest Ecology and Management, 157: 267-283.

Straw, N.A. (2001) The impact of green spruce aphid (Elatobium abietinum (Walker)) on young and mature spruce plantations. In RI Alfaro, K.Day, S Salom, KSS Nair, H Evans, A Liebhold, F Lieutier, M Wagner, K Futai, and K Suzuki (eds) Protection of World Forests from Insect Pests: Advances in Research. IUFRO World Series, Vol. 11 , IUFRO Secretariat, Vienna, pp 29-36.

Straw, N.A. and Green, G. (2001) Interactions between green spruce aphid (Elatobium abietinum (Walker)) and Norway and Sitka spruce under low and high nutrient conditions. Agricultural & Forest Entomology, 3: 263-274.

Williams, D.T., Straw, N.A. and Day, K.R. (2004) Performance of the green spruce aphid, Elatobium abietinum (Walker) on previously defoliated Sitka spruce. In prep.

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