Adelges abietis

Identification & Distribution:

The Adelges abietis gall on spruce, known as a 'pineapple gall', (see first picture below) is ellipsoidal with its length less than 1.5 times the width and usually about 15-20 mm in length. The spruce needles on the gall are shorter than normal and are a slightly paler green than on a normal shoot. The slits to gall chambers are often orange-red or deep pink before opening. There are often several galls together at the base of adjacent shoots, and plant growth often continues beyond gall. The gall is initiated by the pseudo-fundatrix (not pictured) which is yellowish-green to light green with 5-segmented antennae. The offspring of the pseudo-fundatrix which live inside the gall (see second picture below) are a yellowish orange and are densely waxed. The gall chambers open in August-September to release the gallicolae.

The Adelges abietis gallicola (see third picture above) is yellow with 5-segmented antennae and five pairs of abdominal spiracles. The eggs laid by the gallicolae hatch to give 'crawler' which overwinter close to buds, and then mature in spring to the wingless pseudo-fundatrices

The pineapple gall adelgid is mainly found on Norway spruce (Picea abies), but it can also occur on other Picea species including sitka spruce (Picea sitchensis) and white spruce (Picea glauca). Unlike many of its genus, Adelges abietis does not host alternate, and only has two morphs: the apterous pseudo-fundatrix, and the alate gallicola. Sexual forms are unknown. Whilst its original primary host was presumably spruce, the pseudo-fundatrix does not hatch from a sexually-produced egg. Adelges abietis is distributed throughout Europe, and is also found in Morocco, India and North America.

Biology & Ecology:

In spring the wingless female pseudo-fundatrices deposit pale yellow eggs in batches of about 50, partly covered with white waxy threads. The eggs hatch in late spring to give nymphs which feed on the stem at the base of needles. This produces a localised swelling which then develops into a pineapple gall and completely encloses the developing nymphs.

A young developing gall is shown in the first picture above. The second picture above shows the gall (cut in half) contains numerous chambers, within which are groups of pinkish-orange nymphs. These mature to winged gallicolae (see picture below) which emerge from the gall in late summer.

These gallicolae may migrate to other spruce trees or (very often) just oviposit on the same tree. Most appear to not actually oviposit but to die in situ with the eggs in their abdomens (see both pictures below).

Large numbers of dead, egg-laden, females can accumulate on the needles (see picture below).

These eggs hatch to give nymphs known as crawlers (see picture below).

Apart from winged forms, the crawlers are the only active form of adelgids. The crawlers overwinter close to buds, and then mature in spring to the wingless pseudo-fundatrices.

Fidgena et al. (1994)studied the within and between crown distribution of Adelges abietis on young white spruce (Picea glauca) at three sites in Canada. Within a branch, the majority of galls were found on lateral shoots. Gall densities were highest on mid-crown branches of open-grown trees. After crown closure, most galls were found in the upper crown, above the point of branch overlap. Gall distributions were always strongly clumped within trees. Inter-tree distribution of galls followed a negative binomial distribution, indicating a high degree of aggregation among trees. It was concluded that a stratified random sampling plan using 20 lateral shoots of a mid-crown branch as a sampling unit would be adequate for monitoring Adelges abietis populations.

McKinnon et al. (1999) evaluated the performance of Adelges abietis on young, open-grown trees of white spruce whose growth rates had been increased through fertilization or decreased through root-pruning. Gall densities were highest on unfertilized trees and on mid-crown branches. Reduced galling success on fertilized trees was largely due to higher overwintering mortality of first-generation nymphs. Although short shoots were more abundant, intermediate length shoots had the most galls. It was speculated that Adelges abietis lacks the necessary resources for successful gall formation on small shoots and is unable to produce a stimulus large enough to induce gall formation on large shoots.

Björkman (2000) looked at how host resistance and drought stress affect the the performance of Adelges abietis on Norway spruce (Picea abies). Drought-stress-induced changes in host plant quality affected adelgid survival, and hence gall density, less than the genetically determined level of resistance.

Other aphids on same host:

Adelges abietis has been recorded from 13 Picea species.

Blackman & Eastop list about 170 species of aphids as feeding on spruces (Picea) worldwide, and provide formal identification keys.

• Blackman & Eastop list 28 aphid species as feeding on Norwegian spruce (Picea abies) worldwide (Show World list). Of those aphid species, Baker (2015) lists 16 as occurring in Britain (Show British list).

• Blackman & Eastop list 31 aphid species as feeding on Sitka spruce (Picea sitchensis) worldwide (Show World list). Of those aphid species, Baker (2015) lists 16 as occurring in Britain (Show British list).

• Blackman & Eastop list 38 aphid species as feeding on white spruce (Picea glauca) worldwide. (Show World list). Of those aphid species, Baker (2015) lists 14 as occurring in Britain: (Show British list).

   

  Damage & Control:

  Young Norway spruce trees are sold as Christmas trees, so extensive galling, especially with old dead hard galls (see photo below), can make the trees unmarketable.

  

  Gambrell (1931) noted that spruce gall aphid is an important enemy of Norway spruce in New York nurseries and discussed its control using various oils, lime-sulfur, soaps and nicotine. Cameron et al. (1973) describe methods of chemical control of nymphs and galls of Adelges abietis on young Norway spruce trees in Canada.

  The emphasis nowadays is on genetic resistance of Picea trees to adelgids. Mattson (1998) demonstrated that Norway spruce varies widely among individuals in susceptibility to Adelges abietis. The genetic and environmental contributions to this variation were estimated in a polyclonal spruce plantation in France. Aphids infested 98% of the clones, but infestation levels were highly variable, ranging from an average of <1 to 95 galls/branch per ramet per clone. Neither altitude nor forest region of clonal origin made a significant contribution to variation in tree resistance. Instead, most of the variation was due to individual tree genetics. There was no consistent relationship between tree infestation by Adelges abietis, and the co-occurring Adelges laricis.

  Flaherty et al. (2010) evaluated the effect of clone (susceptible vs resistant), shoot length, crown level and gallicola density on the performance of Adelges abietis after the gall was induced. Gall development success was inversely related to shoot length, and was higher on the susceptible clone than on the resistant clone. Galls were also larger on the susceptible clone than on the resistant clone.

  Acknowledgements We especially thank the UK Forestry Commission Bedgebury Pinetum for their kind assistance, and permission to sample. Whilst we make every effort to ensure that identifications are correct, we cannot absolutely warranty their accuracy. We have mostly made identifications from high resolution photos of living specimens, along with host plant identity. In the great majority of cases, identifications have been confirmed by microscopic examination of preserved specimens. We have used the keys and species accounts of Blackman & Eastop (1994) and Blackman & Eastop (2006) supplemented with Blackman (1974), Stroyan (1977), Stroyan (1984), Blackman & Eastop (1984), Heie (1980-1995), Dixon & Thieme (2007) and Blackman (2010). We fully acknowledge these authors as the source for the (summarized) taxonomic information we have presented. Any errors in identification or information are ours alone, and we would be very grateful for any corrections. For assistance on the terms used for aphid morphology we suggest the figure provided by Blackman & Eastop (2006).

  Useful weblinks

  References Björkman, C. (2000). Interactive effects of host resistance and drought stress on the performance of a gall-making aphid living on Norway spruce. Oecologia 123, 223-231. Full text Cameron, E.A. et al. (1973). Insecticidal control of Adelges abietis in Pennsylvania. Journal of Economic Entomology 66(3), 811-812. Abstract Fidgena, J.G. et al. (1995). Intra- and inter-crown distribution of the eastern spruce gall adelgid, Adelges abietis, on young white spruce. The Canadian Entomologist 126(05), 1105-1110. Abstract Flaherty, L. et al. (2010). Post-gall induction performance of Adelges Abietis (L.) (Homoptera: Adelgidae) is influenced by clone, shoot length, and density of colonising gallicolae. Ecological Entomology 35(1), 9-15. Abstract Gambrell, F.L. (1931). The spruce gall aphid (Adelges abietisJournal of Economic Entomology 24(2), 355-361. Abstract Mattson, W.J. et al. (1998). Genetic and environmental contributions to variation in the resistance of Picea abies to the gall-forming adelgid, Adelges abietis (Homoptera: Adelgidae). General Technical Report - North Central Research Station, USDA Forest Service No. NC-199 pp. 304-314. Abstract McKinnon, M.L. et al. (1999). Influence of tree growth rate, shoot size and foliar chemistry on the abundance and performance of a galling adelgid. Functional Ecology 13, 859-867. Full text