Green spruce aphid (Elatobium abietinum) is an important defoliating pest of Sitka spruce (Picea sitchensis). This page briefly describes the biology of the species, the risk it poses to Sitka spruce trees in Britain, and its identification. There is no statutory requirement to report sightings of green spruce aphids to the plant health authorities, but if serious damage has occurred or you have concerns, enquiries can be submitted through Tree Alert.
All information current as of April 2020.
Picture: Donald Owen, California Department of Forestry and Fire Protection, Bugwood.org
Green spruce aphid adults are between 1-2mm long, and are pale to dark green in colour. In the UK, adults are almost always the apterae (wingless) form. Identification of green spruce aphid infestation is easiest through the symptoms (defoliation, chlorosis of needles, etc.) rather than of the actual insect. It is exceptionally rare, and only in very specific situations, that green spruce aphid feeds on any conifer other than spruce.
Cinara is a genus of aphids that may be confused with the green spruce aphid, they also feed on conifers and may be found on the same trees as green spruce aphid. However, Cinara are larger (5-6mm) and dark brown/black in colour. Where green spruce aphids feed on the needles for nutrients, Cinara feed on the bark and can be much more noticeable in amenity situations. Advice on dealing with Cinara aphids can be sought through Tree Alert.
The risk posed/ symptoms
Green spruce aphids feed on the needle leaves, taking nutrients from the sap that were stored in the leaves over winter for new bud growth. The quantity of nutrients taken from each individual tree is relatively minor and does not adversely affect the tree’s health. The true effect from a green spruce aphid infestation, is from the toxins that are secreted from each aphid as it feeds. These toxins cause chlorotic spots and bands to form on the needles (yellowing due to a lack of chlorophyll), see picture below. With continual feeding, these patches coalesce, eventually resulting in the leaf turning brown, dyeing and falling from the shoot. In the right numbers, as seen in an outbreak year, this can cause severe defoliation, see picture below.
Defoliation not only reduces the photosynthetic capacity of a tree, but also results in a decrease of remaining leaf space the tree has for storing nutrients for the next year’s new shoot growth. Overall, although green spruce aphid induced defoliation results in a relatively low instance of mortality, the loss of needles affects growth of the tree by reducing leader extension and volume increment. Infestation by green spruce aphids also makes trees more susceptible to other causes of mortality, such as bacterial and fungal attacks or drought.
Indicators of green spruce aphid infestation:
- Older needles develop pale mottled discoloration during the late winter months and into spring (chlorosis; pictured below).
- In late spring and into summer, individual or groups of trees show a loss of needles.
- New tree growth isn’t affected, so there should be a visible contrast between the new green tops and new shoots in the lower canopy of the tree against the older brown defoliated branches.
Picture: Chlorosis caused by green spruce aphid - Petr Kapitola, Central Institute for Supervising and Testing in Agriculture, Bugwood.org.
Picture: Mid-rotation Sitka spruce showing extremely heavy Elatobium defoliation, having lost 95% of the canopy. The green shoots prove the tree is still alive, and it should eventually recover - Max Blake, Forest Research.
Green spruce aphid is found on trees all year round in the UK’s climate. This is thanks to the UK’s mild winter temperatures, allowing green spruce aphids to continually birth live young throughout the year (anholocyclic life cycle), rather than including an overwinter egg phase, which is employed in regions with colder winter temperatures. However, it is only really during a specific part of the year, when spruce trees are preparing to flush new needles, that spruce aphid can cause the defoliation they are well known for.
In maritime regions, such as north America and north-west Europe (including the UK), green spruce aphids (Elatobium abietinum) become active between March and May. Populations begin to increase as the temperature and day length rise, spurring the spruce to translocate nutrients stored within the older needles into buds for new growth. The aphids feed and breed on the older needles, causing them to develop chlorotic yellow banding, as toxins in the aphid’s saliva begin to build up, before turning brown and falling off from the shoot. Once the new tree buds have flushed and shoot extension begins, the nutrient quality the aphids have been feeding on diminishes and the aphid population growth will slow. After spring, with a decline in sap quality and continued rising temperatures, green spruce aphid populations will be largely killed off. A reduced population will then persist into autumn, where numbers can increase slightly. As winter descends, numbers will either be further killed off, or in the case of successive mild winters, where temperatures do not fall below -8oC, over winter mortality will be low, which provides a bigger starting population for the following spring.
The cyclical effect of green spruce aphid on spruce trees is largely dependent on its maximum population density in any given year. In boom years where population sizes are at their maximum, the aphids cause acute needle defoliation. This reduces the needle space available to the tree for storing the nutrients which would otherwise accumulate over summer in preparation for needle flushing in the next spring, slowing down the growth of the tree.
Local predator and parasite populations boom with the increased number of aphids and carryover to the next year, as well as the reduction in needle-held nutrients for the aphids, will also heavily impact the aphid populations, causing this post-boom year to result in a population crash. A decrease in green spruce aphid numbers provides the affected trees with time to recover and re-grow lost needles, eventually increasing the available food source for green spruce aphids. After a couple of years, a mild winter may once again trigger a green spruce aphid peak – these outbreak years occur roughly every 3-6 years in Great Britain. As peak years never occur back-to-back, trees will seldom be killed solely by green spruce aphids, however the loss of new growth and frequency in attacks, make green spruce aphids an important background pest, whose effects may be exacerbated by climate change.
In the UK, the last green spruce aphid outbreaks were reported in 2012 and 2015, reports from Scotland and south west England indicate that 2019 was another peak ‘outbreak’ year.
Picture: Elizabeth Willhite, USDA Forest Service, Bugwood.org. Spruce beginning to flush and showing early symptoms of Elatobium.
Origins and spread into Britain
Green spruce aphids are native to European spruce forests and particularly in association with Norway spruce (Picea abies). Having coevolved with green spruce aphid, Norway spruce shows less of a reaction to their presence in comparison to other species of spruce, such as Sitka spruce (Picea sitchensis). Green spruce aphid populations have spread from continental Europe following human movement and introductions of spruce as ornamental and forestry trees and can now be found throughout Australia, New Zealand, Chile, North America and the UK.
Green spruce aphids have a strong dispersal ability, though if they arrived in the British Isles under their own steam or were introduced alongside translocated spruce plants for amenity and forestry purposes is known. Unusually, the aphid was described as a new species to science from material in London (Walker, 1849), rather than from tree material in its native range. The spread and effect of green spruce aphid on spruce trees was not clear until after the expansion on conifer forests in the UK in the 1900’s, where it was able to feed on both its native Norway spruce and the naive Sitka spruce.
Green spruce aphids are pests of spruce trees (Picea). This includes its native host the Norway spruce (P. abies), although infestation does not greatly affect this species due to their shared evolutionary history.
However naïve species of spruces, which have not evolved alongside green spruce aphids, are greatly affected by the toxins secreted when the aphid feeds. This includes most American spruces: Sitka spruce (P. sitchensis) in UK forestry plantations and Colorado blue spruce (P. pungens) in amenity situations.
Green spruce aphids have also been found on other conifer trees, including various species of firs (Abies). However, this has only been seen when these trees have been in close proximity to spruces, and when the population has peaked during an outbreak year. Where green spruce aphids have been found on non-spruce trees, the damage has been limited.
Control / management
Green spruce aphids feed on the nutrients within the needle leaves between April – May. By the time symptoms begin to appear by June (chlorosis of leaves and defoliation), the damage is already done, and the aphids will be long gone, experiencing their summertime decline. As such, chemical control is not recommended, as it will be ineffectual and is outweighed by the negative effects to non-target wildlife.
Research has shown that increasing the biodiversity of natural aphid predators across the landscape is the most promising way to protect trees against infestation and limit the damage of an outbreak year. This can be achieved by creating diversity in the landscape by using alternative silviculture methods, such as mixed aged stands, this creates diverse habitats that can support more species and thus more predators.
You can read about this research in more detail on our green spruce aphid research page.
Picture: Sitka spruce planted with noble fir (Abies procera, blue needles) showing moderate Elatobium defoliation – Max Blake, Forest Research.
Carter, C. I. (1972). Winter temperatures and survival of the green spruce aphid Elatobium abietinum (Walker). Forest Record, 84:10 pp
Straw, N. A., Halldórsson, G., & Benedikz, T. (1998). Damage sustained by individual trees: empirical studies on the impact of the green spruce aphid. The green spruce aphid in western Europe: Ecology, Status, Impacts and Prospects for Management, 15-31.
Straw, N.A., Fielding, N., Green, G., Price, J. & Williams, D. (2011) Defoliation and growth relationships for mid-rotation Sitka spruce attacked by the green spruce aphid, Elatobium abietinum (Walker) (Homoptera: Aphididae). Forest Ecology and Management 262, 1223–1235.
Straw, N. A., Williams, D. T., Fielding, N. J., Jukes, M., Connolly, T., & Forster, J. (2017). The influence of forest management on the abundance and diversity of hoverflies in commercial plantations of Sitka spruce: the importance of sampling in the canopy. Forest Ecology and Management, 406, 95-111.
Straw, N. A., Bladon, F. M., Day, K. R., & Fielding, N. J. (2019). The effects of high temperatures on individuals and populations of the green spruce aphid Elatobium abietinum (Walker). Agricultural and Forest Entomology, 21(1), 69-78.
Walker, F. (1849). Description of aphides. Annals and Magazine of Natural History, 3, 301-302.
Williams, D. T., Straw, N. A., & Day, K. R. (2005). Performance of the green spruce aphid, Elatobium abietinum (Walker) on previously defoliated Sitka spruce. Agricultural and Forest Entomology, 7(2), 95-105.