A few weeks ago I was contacted by a researcher from the One Show. They were interested in the possibility of doing a festive piece about what people bring into the house with them on Christmas trees with the idea that George McGavin would shake a Christmas tree over a piece of white paper and tell the audience all about the insects that fell out; a typical media “how gross nature” is piece.
The researcher was somewhat disappointed when I told her that being winter that there would be relatively little hiding in the tree, especially if it was a commercially reared cut tree bought from a garden centre or other retail outlet. Cut Christmas trees in the UK tend to be harvested from October onwards so the chances are that your tree has lain about for at least a month before you bring it into your house and by that time, any sensible winter active herbivore has long departed for fresher trees. Although conifer trees have a large number of insect species associated with them, most of them spend the winter either off the tree or as inactive eggs hidden under the bark or as eggs actually laid inside the needles e.g. the pine sawfly Neodiprion sertifer. You would probably find a few opportunistic spiders and possibly some mites and bark lice, but not much else unless you had a potted tree or one that had only recently been felled. The other thing that would influence what you would find is of course what species of tree you had bought. Gone are the days when the Christmas tree and Norway spruce (Picea abies) were one and the same. I guess my caveating and pessimistic reply proved too much for the researcher as I never heard back from her.
The one insect I had waxed lyrical about was of course an aphid, the green spruce aphid, Elatobium abietinum to be precise. There are a number of aphid species that make a living on spruce trees, some of them quite large and spectacular such as the greater black spruce aphid, Cinara piceae, but like most aphids, they overwinter as eggs (Leather, 1992).
The greater black spruce aphid, Cinara piceae (Photograph courtesy of http://influentialpoints.com/Gallery/Aphids_on_spruce_Picea_in_Britain.htm)
The green spruce aphid, E. abietinum or Elatobium as it is commonly known, (there is only one species in the genus), overwinters in the UK and most other parts of the world, as an adult or immature stage (nymph) (Nicol et al., 1998).
The adult is small, green and inconspicuous, and quite difficult to see unless you are actually looking for them.
The green spruce aphid, Elatobium abietinum and nymphs.
The green spruce aphid is a native of Europe and normally attacks Norway spruce. They avoid current year needles as these tend to be distasteful to them (the chemistry of young spruce needles is pretty nasty and makes them unsuitable hosts for the aphids) and feed on the previous year and older needles. Spruce needles, even older ones, are not particularly nutritious, so the aphid injects a toxic material in its saliva that makes the needles more nutritious by encouraging nitrogen mobilisation (Kloft & Erhardt, 1959). Their populations build up during the spring and towards the end of May and beginning of June, they take flight and the trees seem relatively free of aphids (Bevan, 1966). As they are so small, they are most obvious after they have gone, either by the damage they cause, premature senescence of the needles as shown in the photograph above, premature needle drop or by the presence of a large number of ladybird larvae. When I worked for the Forestry Commission as an entomologist, I quite often received phone calls from distressed foresters who had sprayed the blue beetles damaging their spruce trees!
Although they are difficult to find during the summer months they are still there; this summer collapse of singe-host aphids is quite common (Karley et al., 2004). In the autumn, Elatobium populations begin to build up and as they do not overwinter as eggs, they are able to continue reproducing through the winter months (Powell & Parry, 1976). Sitka spruce, Picea sitchensis, the most commonly grown conifer in the UK, is a native of North America and as such has very low resistance to Elatobium and displays an almost hypersensitive response to the toxic saliva produced by the aphid.
If it is a particularly mild winter then the spruce trees are likely to show severe signs of damage by June and July. After several mild winters spruce trees may end up with only current year needles present, which has a severe effect on their growth and appearance.
Branches of Sitka spruce with only current year needles present after a severe Elatobium abietinum infestation
Sitka spruce trees showing discoloured needles after attack by Elatobium abietinum.
It may be small, inconspicuous and not worth a TV appearance, but Elatobium abietinum is now a pest with a world-wide distribution and an international reputation.
Bevan, D. (1966). The green spruce aphis Elatobium (Neomyzaphis) abietinum Walker. Scottish Forestry 20, 193-201.
Karley, A. J., Parker, W. E., Pitchford, J. W. &Douglas, A. E. (2004). The mid-season crash in aphid populations: why and how does it occur? Ecological Entomology 29, 383-388.
Kloft, W. & Ehrhardt, P. (1959). Unterschungen uber Saugtatigkeit und Schadwirkung der Sitkafichtenlaus, Liosomuphis abietina (Walk.), (Neomyzaphis abietina Walk.). Phytopathologie Zeitzschrqt 35, 401 – 410.
Leather, S. R. (1992). Aspects of aphid overwintering (Homoptera: Aphidinea: Aphididae). Entomologia Generalis 17, 101-113.
Nicol, D., Armstrong, K. F., Wratten, S. D., Walsh, P. J., Straw, N., Cameron, C. M., Lahmann, C. & Frampton, C. M. (1998). Genetic diversity of an introduced pest, the green spruce aphid Elatobium abietinum (Hemiptera: Aphididae) in New Zealand and the United Kingdom. Bulletin of Entomological Research 88, 537-543.
Powell, W. & Parry, W. H. (1976). Effects of temperature on overwintering populations of the green spruce aphid, Elatobium abietinum. Annals of Applied Biology 82, 209-219.
Sullivan, C.R. (1965) Laboratory and field investigations on the ability of eggs of the European Pine Sawfly, Neodiprion sertifer (Geoffroy) to withstand low winter temperatures. Canadian Entomologist, 97, 978-993
During the 1980s when ‘Acid Rain’ was very much in the news, Elatobium damage was often mistaken as a symptom of acid rain in the UK.